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26Russian Journal of Nematology, 2016, 24 (1), 33 - 48
Effect of planting and irrigation practices on nematode reproduction, root galling, plant growth and yield of two Asian lowland rice varieties infected by the rice root-knot nematode Meloidogyne graminicola
Pa Pa Win1, Pyone Pyone Kyi1, Zin Thu Zar Maung2, Yi Yi Myint3 and
Dirk De Waele4, 5' 6
'Plant Protection Division, Department of Agriculture, Ministry of Agriculture and Irrigation, Bayint Naung Road, West
Gyogone, P.O. Box 1011, Insein, Yangon, Myanmar Agricultural Nematology Laboratory, 256 Giltner Hall, Michigan State University, 48823, East Lansing, MI, USA 3Department of Plant Pathology, Yezin Agricultural University, Yezin, Myanmar ^Laboratory of Tropical Crop Improvement, Department of Biosystems, Faculty of Bioscience Engineering, University of Leuven (KU Leuven), Willem de Croylaan 42, B-3001, Heverlee, Belgium ®Unit for Environmental Sciences and Management, North-West University, Private Bag X6001, 2520, Potchefstroom, South Africa 6Crop and Environmental Sciences Division, International Rice Research Institute (IRRI), DAPO Box 7777, Metro Manila, Philippines
e-mail: dirkdewaele@pandora.be
Accepted for publication 25 April 2016
Summary. A screenhouse experiment was conducted to examine the effect of planting and irrigation practices at the time of planting on the reproductive and damage potential of Meloidogyne graminicola on rice. In the experiment the effects of two irrigation (early irrigation and delayed irrigation) and two planting (direct seeding and transplanting) practices on the damage and yield loss potential of M. graminicola on two commonly cultivated Asian lowland rice varieties (Thihtatyin and Yatanartoe) were evaluated. Both rice varieties were less susceptible in direct seeded rice that had early irrigation. For both varieties, the highest Mf-eggs (Multiplication factor) were observed in transplanted plants that had delayed irrigation. For both varieties, the highest number of second-stage juveniles and eggs per root unit and per root system, and Mfeggs were observed in transplanted plants compared with direct seeded plants, irrespective of irrigation practice. The highest root galling index (7.4) was observed on transplanted delayed irrigation plants of variety Thihtatyin, while the lowest root galling index (1.8) was observed on direct seeded, early irrigated plants of variety Yatanartoe. Reduction in most of the plant growth and yield variables measured caused by M. graminicola was observed in transplanted plants of both rice varieties grown under delayed irrigation. Tolerance of both rice varieties was observed in direct seeded rice that had early irrigation. Variety Yatanartoe was more tolerant than variety Thihtatyin. For variety Yatanartoe, grain yield loss caused by M. graminicola was observed only in transplanted rice that had delayed irrigation (30.8%). For variety Thihtatyin, grain yield loss was observed in both direct seeded rice and transplanted rice (40 and 37%, respectively) under delayed irrigation and in transplanted rice (41.3%) under early irrigation.
Key words: damage, delayed irrigation, direct seeded, early irrigation, intermittent flooding, multiplication factor, plant growth traits, permanent flooding, sensitivity, susceptibility, tolerance, transplanted, yield-contributing traits, yield loss.
During the past decade, the rice root-knot nematode Meloidogyne graminicola Golden & Birchfield, 1965, has emerged as the most damaging Meloidogyne species on Asian rice (Oryza sativa L.) (De Waele & Elsen, 2007). This sedentary endoparasite has been found in all South Asian and Southeast Asian rice producing countries surveyed so far (Jain et al., 2012) and can infect all types of
rice production systems, including lowland and upland, irrigated and rainfed, and deepwater rice (Bridge et al., 2005). Second-stage juveniles (J2) usually invade rice roots in upland conditions (i.e., free draining, without surface water accumulation) just behind the root tip (Rao & Israel, 1973). They cannot invade rice in flooded conditions but quickly invade when infested soils are drained (Manser,
1968). Females develop within the roots and eggs are laid mainly in the cortex (Roy, 1976). The J2 of M. graminicola can remain in the maternal gall or migrate intercellularly through the aerenchymatous tissues of the cortex to new feeding sites within the same root (Bridge & Page, 1982). Meloidogyne graminicola induces swellings and galls throughout the root system. Infected roots become swollen and hooked, a symptom that is characteristic for this nematode species.
The population build up of plant-parasitic nematode species on rice is influenced by many factors including agronomic practices (Ramakrishna & Sharma, 1998), climate change and urbanisation, resulting in less water being available for growing agricultural crops, and higher labour costs. The development of early-maturing rice varieties and improved nutrient management techniques along with increased availability of chemical weed control methods have encouraged many farmers in South and Southeast Asia to switch from transplanting nursery-grown rice seedlings to direct wet seeding followed by irrigation (Bouman et al., 2002; Tuong & Bouman, 2003; De Waele & Elsen, 2007; Farooq et al., 2011).
In Myanmar, rice is cultivated under rainfed and irrigated lowland conditions, rainfed upland conditions and in deep water. Most rice is grown in the lower Ayeyarwady River Delta area in the southern part of Lower Myanmar. Manual transplanting of 1-month-old seedlings from flooded nurseries into puddled or flooded soil is the traditional method of rice crop establishment in the lowland rice ecosystem in Myanmar. However, direct wet seeding by broadcasting pre-germinated (in plastic bags) seeds is increasingly being adapted by many rice farmers in the irrigated lowland rice ecosystem during the dry summer due to changes mentioned above.
Based on the results of an intensive survey of 450 rice fields, Win et al. (2011) suggested that the prevalence of M. graminicola in lowland rice fields in the summer-irrigated lowland rice agro-ecosystem in Myanmar was not only caused by the water regime (i.e., flooded vs upland conditions), but also by a combination of direct wet seeding, delayed irrigation, rice monoculture, high cropping intensity and the use of high-yielding rice varieties.
At a time when different agronomic practices are being developed to alleviate the water shortage problem, especially in summer-irrigated lowland
rice production, it is important to determine the influence of water regime and planting practices on the damage potential ofM. graminicola on rice. The importance of water regime on the prevalence of M. graminicola has been demonstrated by several authors (Plowright & Bridge, 1990; Prot & Matias, 1995; Tandingan et al., 1996; Soriano et al., 2000; Win et al., 2015a) but information on the importance of the other agronomic practices mentioned above is lacking.
In our study, a screenhouse experiment was carried out to evaluate the effect of two planting practices and two irrigation practices at the time of planting on the reproductive, damage and yield loss potential of M. graminicola on two commonly cultivated lowland Asian rice varieties. Our study is complementary to the study of Win et al. (2015a) who examined the effect of different water regimes on the reproduction, damage and yield loss potential of lowland and upland Asian rice varieties grown in two soil types infested by M. graminicola.
MATERIALS AND METHODS
The experiment was conducted at the campus of the Plant Protection Division, Yangon, from December 2010 until March 2011 (i.e., during the summer-irrigated rice growing season). The variety Thihtatyin was included in the experiment because it is the most commonly cultivated rice variety in the summer-irrigated lowland rice ecosystem under direct seeding and delayed irrigation in Myanmar. It also had the highest prominence value of M. graminicola, and the highest root galling indices during the nematological survey conducted in 2009 (Win et al., 2011). The lowland variety Yatanartoe was included in the experiment because it is commonly grown in the summer-irrigated lowland rice ecosystem after transplanting and under early irrigation in Myanmar. It had the lowest prominence value of M. graminicola and the lowest root galling indices (Win et al., 2011).
Preparation of plants. Seeds of the two varieties Thihtatyin and Yatanartoe were obtained from the Myanmar Rice Research Centre, Hmawbi and the Rice Division, Department of Agricultural Research (DAR), Yezin, Nay Pyi Taw. The characteristics of the two lowland Asian rice varieties are listed in Table 1. The seeds were first soaked in water overnight and pre-germinated on wet paper in Petri dishes at room temperature for 3 days.
Table 1. Characteristics of the lowland rice varieties included in the experiment.
Rice variety Original name Crop cycle (days) Plant height (cm) Grain yield (t ha-1) Variety type
Thihtatyin Yatanartoe IR 13240-108-2-2-3 Thai 1-9-3E 115 120 85-95 120-135 5-6 4.6-6.2 HYV1 HYV1
1 HYV: a high-yielding variety.
Nematode inoculum. The nematode inoculum and the tray method (Whitehead & Hemming, 1965) used to extract the J2 from the roots were the same as described by Win et al. (2015a).
Nematode inoculation, treatments and experimental set-up. Three-day-old pre-germinated seeds and 21-day-old seedlings established in a nursery were planted singly in 17-cm-diameter * 22-cm-high pots containing 1,500 ml of sterilised clay loam soil (Win et al., 2015a). The soil in the pots was saturated (i.e., 100% of the soil pore volume filled with water) at planting and at nematode inoculation.
At direct seeding of the 3-day-old pre-germinated seeds or transplanting of the 21-day-old seedlings, six plants of each variety were inoculated with 3,000 M. graminicola J2 per plant (Win et al., 2015a). Six plants of each treatment of each rice variety were not inoculated and acted as control plants. Immediately after inoculation of the seeds and seedlings, early and delayed irrigation were applied as follows: i) direct seeding followed by early irrigation: i.e., light irrigation (a thin layer of water) added immediately after seeding and nematode inoculation until the seedlings were 1-week-old; after that time flooding (2-5 cm of water) was applied; ii) transplanting followed by early irrigation: i.e., flooding started immediately after transplanting and nematode inoculation; iii) direct seeding followed by delayed irrigation: i.e., flooding was applied starting 6 days after direct seeding and nematode inoculation; iv) transplanting followed by delayed irrigation: i.e., flooding started 6 days after transplanting and nematode inoculation. The flooding (2-5 cm of water) was maintained until the tillering stage of the plants by daily watering as necessary. After that, the plants were intermittently flooded 3 times per week until plant maturity (harvest), except maintaining the water 1 week at the flowering stage of the plants to simulate the same water regime and the same irrigation practice as applied in the summer-irrigated lowland rice farmer's fields. The pots were placed in a
screenhouse at an air temperature ranging from 26 to 38°C.
All combinations of rice variety, planting practices, irrigation practices and nematode inoculum were laid out in a split-split-split plot design with six replications. The two rice varieties (Thihtatyin and Yatanartoe) were considered as the main plots; the two planting practices (direct seeding and transplanting) as subplots; the two irrigation types (early and delayed irrigation) as sub-subplots; the two nematode inoculum levels (0 and 3,000 J2 pot-1) as sub-sub-subplots.
Assessment of plant growth, yield-contributing traits and yield. The experiment was terminated when the panicles of each rice variety were mature and ready to harvest. At harvest, the plants were carefully removed from the soil, and the following three plant growth and six yield-contributing traits measured: plant height, fresh root weight, dry shoot weight, number of tillers per plant, number of panicles per plant, number of filled grains per panicle, percentage filled grains per plant, filled grain weight per plant and weight of 1,000 filled grains. Yield was estimated according to Yoshida (1981) and Win et al. (2015a) at 14% moisture content.
Assessment of Meloidogyne graminicola population densities and severity of root galling. At harvest, the nematodes were extracted from the rhizosphere soil (one 100 ml soil sub-sample) and from the roots (one sub-sample of 3 g roots), and counted as described by Win et al. (2015a). The number of eggs in the root sub-samples was also determined as described by Win et al. (2015a). The population densities of M. graminicola were calculated as the number of J2 (100 ml soil)-1, J2 (g root)-1, eggs (g root)-1, J2 (root system)-1 and eggs (root system)-1. The final nematode population density was calculated as the number of J2 in the soil in the pots (1,500 ml) + J2 per root system. The nematode multiplication factor (Mresgs) was calculated as the final J2 population density/3,000 J2.
The severity of galling (root galling index) was visually assessed for each up-rooted plant by rating the percentage of roots with root tip galls per root system on a 0-10 scale according to the rice root-knot rating chart of Bridge & Page (1982).
Analysis of data. The data were analysed with the STATISTICA 11.0 software (StatSoft Inc., Tulsa, USA). Prior to analysis of variance, plant growth, most yield-contributing traits and yield data, and nematode population densities were log(x+1) transformed, while percentage of filled grains per plant and root galling indices were arcsin(x/100) transformed to meet the assumptions of ANOVA (i.e., normality and homogeneity of variances). The Shapiro-Wilk test was used to examine whether the dependent variable was normally distributed within groups while the homogeneity of the variances of the groups was tested with the Levene's test. The outliers were determined by calculating the standardised residuals falling outside the range from -2 to +2. When the assumptions for ANOVA were met, the data were analysed by using ANOVA. Oneway ANOVA with Tukey's HSD test or t-test were performed for mean comparisons of the measured variables. Mean numbers are shown in the tables, after back-transforming the data, to facilitate interpretation.
RESULTS
Meloidogyne graminicola population densities.
Meloidogyne graminicola reproduced and multiplied on both rice varieties under all planting and irrigation practices (Table 2).
For variety Thihtatyin, no significant effect of either planting or irrigation practice was observed on the soil population density (J2 (100 ml)-1). For variety Yatanartoe, plants that had been transplanted and delayed irrigated had a significantly (P < 0.05) higher (on average 15 times) soil population density compared with plants that had been transplanted but had early irrigation.
For both varieties and for both planting practices, the J2 root population density was significantly (P < 0.05) higher (1.4 to 2 times for J2 (g root)-1; 1.9 to 4.1 times for J2 (root system)-1) in plants that had delayed irrigation compared with plants that had early irrigation. For variety Thihtatyin, no significant effect of planting practice was observed on the J2 root population density except for the number of J2 (root system)-1, which was 3.2 times higher (P < 0.05) in plants that had been transplanted and had early irrigation compared with plants that had been direct seeded and had early irrigation. For variety Yatanartoe, a significant (P <
0.05) effect of planting practice was observed on the J2 root population density that was for both irrigation practices significantly (P < 0.05) higher (5.1 to 5.8 times for J2 (g root)-1; 2.4 to 4.2 times for J2 (root system)-1) in plants that had been transplanted compared with plants that had been direct seeded.
For both varieties, the number of eggs in the roots (eggs per g roots and eggs per root system) were significantly (P < 0.05) higher in plants that had been direct seeded and had delayed irrigation compared with plants that had been direct seeded but had early irrigation (2.9 and 5.9 times more eggs (g root)-1 and eggs (root system)-1, respectively, in variety Thihtatyin; 2.4 and 2.8 times more eggs (g root)- and eggs (root system)-1, respectively, in variety Yatanartoe). For both varieties, the number of eggs in the roots, with the exception of eggs per root system of plants of variety Thihtatyin that had been transplanted and delayed irrigated, was significantly (P < 0.05) higher in plants that had been transplanted compared with plants that had been direct seeded (3.4 and 2.8 times more eggs (g root)-1 in plants of variety Thihtatyin that had early and delayed irrigation, respectively, and 7.7 times more eggs (root system)-1 in plants of the same variety that had early irrigation; 17.6 and 6.7 times more eggs (g root)-1 in plants of variety Yatarnatoe that had early and delayed irrigation, respectively, and 5 and 5.7 times more eggs (root system)-1 in plants of the same variety that had early and delayed irrigation, respectively).
For both varieties, the Mf-eggs of plants that had been transplanted were higher, irrespective of irrigation practice, compared with plants that had been direct seeded. Also for both varieties, the Mf-eggs of plants that had delayed irrigation was higher, irrespective of planting practice, compared with plants that had early irrigation. For all treatments, the Mf-eggs of plants of variety Thihtatyin were always higher compared with the Mf-eggs of plants of variety Yatanartoe.
Severity of root galling. For both varieties, the root galling index of plants that had been direct seeded and had delayed irrigation was significantly (P < 0.05) higher compared with plants that had been direct seed but had early irrigation (5.8 vs 2.7 for variety Thihtatyin; 2.4 vs 1.8 for variety Yatarnatoe, respectively). For both varieties, no significant differences in root galling index of plants that had been transplanted and had either early or delayed irrigation were observed. For both varieties, plants that had been transplanted and had early irrigation had a significantly (P < 0.05) higher root galling index compared with plants that had been direct
Table 2. Effect of two irrigation practices and two planting practices on the soil and root population densities of Meloidogyne graminicola, severity of
root galling (RGI), and nematode multiplication factor (Mfe??s) of two lowland rice varieties.
Irrigation Thihtatyin (VI) Yatanartoe (V2) Difference (V1-V2)
practice Direct seeding Transplanting Direct seeding Transplanting Direct seeding Transplanting
J2 (100 ml soil)"1
Early irrigation 2,825±1,291> a A 4,410±3,260 a A 64±70 a A 39±42 a A 2,761* 4,371*
Delayed irrigation 2,407±2,057 a2 A3 4,285±2,387 a A 97±30 a A 584±438 bA 2,310* 3,674*
J2 (g root)-1
Early irrigation 2,948±648 a A 4,518±1,835 a A 485±634 a B 2,830±2,786 a A 2,463* 1,688 ns
Delayed irrigation 5,815±4,518 b A 8,973±2,619 b A 752±658b B 3,870±3,657b A 5,063* 5,103 ns
J2 (root system) 1
Early irrigation 48,103±33,064aB 153,358±71,307 a A 17,847±15,743 aB 42,215±27,467 a A 30,256 ns 111,143*
Delayed irrigation 197,949±105,388 b A 218,605±83,649 a A 33,841±31,032 b B 141,861±147,233 b A 164,108 ns 76,743 ns
Eggs (g root) 1
Early irrigation 4,884±2,411 aB 19,494±8,363 a A 550±534 a B 9,691±11,181 aA 4,333* 9,803 ns
Delayed irrigation 14,398±12,523 b B 39,947±8,176 a A 1,343±1,192 b B 9,049±8,211 a A 13,056* 30,899 ns
Eggs (root system) 1
Early irrigation 87,811±75,939 aB 677,614±364,862 a A 21,412±14,923 aB 127,783±114,737 a A 66,400 ns 549,830*
Delayed irrigation 521,125±77,762 b A 994,712±421,164 a A 60,870±55,526 b B 345,170±362,809 a A 460,255 ns 649,542 ns
Root galling index4 (RGI)
Early irrigation 2.7±1.1 aB 6.4±1.0 a A 1.8±0.5aB 4.3±1.5 a A 0.9 ns 2.1*
Delayed irrigation 5.8±2.2 b A 7.4±1.3 a A 2.4±0.9 b A 3.3±2.5 a A 3.4* 4.1*
MTggs
Early irrigation 30.2 73.2 6.3 14.3 23.9 58.9
Delayed irrigation 78.0 94.3 11.8 50.2 66.3 44.1
1 Data represent means ± SD (n = 6).
2 Root galling index according to 0 = no swellings or galls, 1 = 10% galls, 2 = 20% galls, 3 = 30% galls, 4 = 40% galls, 5 = 50% galls, 6 = 60% galls, 7 = 70% galls, 8 = 80% galls, 9 = 90% galls and 10 = all roots of the root system galled.
3 Means in the same column followed by the same lowercase letter, between the two irrigation practices, are not significantly different according to the t-test (P < 0.05).
4 Root galling index according to 0 = no swellings or galls, 1 = 10% galls, 2 = 20% galls, 3 = 30% galls, 4 = 40% galls, 5 = 50% galls, 6 = 60% galls, 7 = 70% galls, 8 = 80%) galls, 9 = 90%) galls and 10 = all roots of the root system galled.
* indicates that the change is significantly different according to the t-test (P < 0.05).
seeded and had early irrigation (6.4 vs 2.7 for variety Thihtatyin; 4.3 vs 1.8 for variety Yatanar-toe).
Effect of nematode infection on plant growth and yield-contributing traits. The effect of M. graminicola infection on the plant growth and yield-contributing traits of the rice varieties Thihtatyin and Yatanartoe is presented in Table 3.
No significant effect of irrigation practice on plant height, fresh root weight and dry shoot weight of non-inoculated and inoculated plants of both varieties was observed, with the exception of the dry shoot weight of inoculated plants of variety Thihtatyin that had been transplanted and had delayed irrigation: the dry shoot weight of these plants was significantly (P < 0.05) lower compared with plants that had been transplanted and had early irrigation (7.3 vs 10.1 g, respectively).
A significant (P < 0.05) reduction (12.9%) in plant height between non-inoculated and inoculated plants of variety Thihtatyin that had been transplanted and had delayed irrigation was observed. A significant (P < 0.05) reduction in fresh root weight between non-inoculated and inoculated plants was observed in plants of variety Thihtatyin that had been direct seeded and that had early or delayed irrigation (50.6 and 48.8%, respectively), and in plants of both varieties that had been transplanted and had delayed irrigation (41.2% for plants of variety Thihtatyin; 40.7% for plants of variety Yatanartoe). In variety Thihtatyin, nematode infection significantly (P < 0.05) reduced the dry shoot weight of direct seeded plants that had been early or delayed irrigated by 32.5 and 52%, respectively, and of transplanted plants that had delayed irrigation by 31.8%. In variety Yatanartoe, nematode infection significantly (P < 0.05) reduced the dry shoot weight of plants that had been transplanted and had delayed irrigation by 37.1%.
For non-inoculated and inoculated plants that had been direct seeded and for both varieties, no significant effect of irrigation practice was observed on the yield-contributing traits measured with three exceptions. Non-inoculated plants of variety Yatanartoe that had been direct seeded and had early irrigation had a significantly (P < 0.05) lower number of filled grains per panicle compared with direct seeded and delayed irrigation plants (47.8 vs 56.8). Inoculated plants of variety Yatanartoe that had been direct seeded and had delayed irrigation had a significantly (P < 0.05) lower percentage filled grains per plant compared with direct seeded and early irrigated plants (89 vs 69.7%). Non-inoculated plants of variety Thihtatyin that had been
direct seeded and had early irrigation had a significantly (P < 0.05) lower filled grain weight per plant compared with direct seeded and delayed irrigation plants (10.9 vs 13.2 g, respectively).
For direct seeded plants of variety Thihtatyin, nematode infection significantly (P < 0.05) reduced the number of tillers per plant and the number of panicles per plant of plants that had early irrigation with 55.1 and 40.7%, respectively, and the number of tillers per plant, number of panicles per plant and filled grain weight per plant of plants that had delayed irrigation with 42.2, 45.3 and 55.3%, respectively. For direct seeded plants of variety Yatanartoe, nematode infection significantly (P < 0.05) reduced the number of filled grains per panicle and percentage filled grains per plant of plants that had delayed irrigation with 23.2 and 17.9%, respectively.
For plants that been transplanted and for non-inoculated and inoculated plants of both varieties no significant effect of irrigation practice was observed on the number of filled grains per panicle and percentage filled grains per plant. Non-inoculated plants of variety Yatanartoe that had been transplanted and had early irrigation had a significantly (P < 0.05) lower number of tillers per plant and a significantly (P < 0.05) higher weight of 1,000 filled grains compared with transplanted and delayed irrigation plants (15.8 vs 21.8 and 25.5 vs 23.7 g, respectively), whilst inoculated transplanted and early irrigation plants of the same variety had a significantly (P < 0.05) higher number of tillers per plant compared with transplanted and delayed irrigation plants (22 vs 11.3, respectively). Inoculated plants of variety Yatarnatoe that had been transplanted and had delayed irrigation had a significantly (P < 0.05) lower number of panicles per plant compared with transplanted and early irrigation plants of the same variety (7.6 vs 14, respectively). Inoculated plants of variety Thihtatyin that had been transplanted and had delayed irrigation had a significantly (P < 0.05) lower weight of 1,000 filled grains compared with transplanted and early irrigated plants of the same variety (8.5 vs 10.4 g, respectively).
For plants of variety Thihtatyin that had been transplanted a significant (P < 0.05) effect (% reduction) of nematode infection was observed on the number of filled grains per panicle irrespective of irrigation practice (21.3 and 40.7% for early and delayed irrigation plants, respectively), filled grain weight per plant (27.8 and 43.3 g for early and delayed irrigation plants, respectively) and weight of 1,000 filled grains (11.3 and 12.5 g for early and delayed irrigation plants, respectively).
Table 3. Effect of Meloidogyne graminicola on plant growth and yield-contributing traits of two lowland rice varieties (THY: Thihtatyin, YTNT: Yatanartoe)
grown under two irrigation practices and two planting practices.
vo
Irrigation practice Direct seeding (DS) Transplanting (TP) Differences (DS-TP)
N1 I % change N1 I % change N1 I
Plant height (cm)
THY
Early irrigation 68.0+3.71 a2 69.4±1.8 a +2.1 62.0+1.8 a 61.4+5.6 a -1.0 6.0* 8.0 ns
Delayed irrigation 70.5±3.3 a 63.0±7.7 a -10.6 64.3+1.5 a 56.0+5.3 a -12.9* 6.2* 7.0 ns
YTNT
Early irrigation 84.0±1.6 a 85.0±2.0 a +1.2 84.8+4.3 a 80.7+2.1 a -4.8 -0.8 ns 4.3*
Delayed irrigation 85.3±1.0 a 86.3±2.1 a +1.2 80.8+5.0 a 84.6+2.2 a +4.7 4.5 ns 1.7 ns
Fresh root weight (g)
THY
Early irrigation 36.2±5.8 a 17.9±7.9 a -50.6* 48.1+12.3 a 34.4+6.6 a -28.5 -11.9 ns -16.5*
Delayed irrigation 37.5±5.4 a 19.2±11.6 a -48.8* 45.6+9.3 a 26.8+5.3 a -41.2* -8.1 ns -7.6 ns
YTNT
Early irrigation 36.4±3.9 a 41.4±8.5 a +13.7 42.0+8.8 a 43.6+14.6 a +3.8 -5.6 ns -2.2 ns
Delayed irrigation 42.0±9.9 a 43.3±10.5 a +3.1 49.4+16.0 a 29.3+17.1 a -40.7* -7.4 ns 14.0 ns
Dry shoot weight (g)
THY
Early irrigation 8.3±1.8 a 5.6±1.5 a -32.5* 11.4+2.7 a 10.1+1.9 b -11.4 -3.1 ns -4.5 ns
Delayed irrigation 10.2±1.3 a 4.9±2.2 a -52.0* 10.7+2.8 a 7.3+1.0 a -31.8* -0.5 ns -2.4 ns
YTNT
Early irrigation 18.9±3.9 a 22.3±4.3 a +18.0 16.4+2.9 a 14.9+2.5 a -9.1 2.5 ns 7.4*
Delayed irrigation 17.0±2.0 a 19.5±8.2 a +14.7 18.6+3.4 a 11.7+4.9 a -37.1* -1.6 ns 7.8 ns
No. of tillers per plant
THY
Early irrigation 15.6±4.9 a 7.0±4.0 a -55.1* 21.0+5.6 a 20.0+1.2 a -4.8 -5.4 ns -13.0*
Delayed irrigation 17.3±2.1 a 10.0±4.4 a -42.2* 16.4+2.9 a 17.8+3.8 a +8.2 0.9 ns -7.8*
YTNT
Early irrigation 14.8±1.3 a 14.6±1.1 a -1.4 18.0+2.6 a 19.0+7.8 b +5.6 -3.2 ns -4.4*
Delayed irrigation 13.2±3.8 a 13.7±4.2 a +3.8 21.8+3.8 b 11.3+4.0 a -48.2* -8.6* 2.4 ns
No. of panicles per plant
THY
Early irrigation 10.8±1.8 a 6.4±3.2 a -40.7* 13.5+3.2 a 14.2+2.6 a +5.2 -2.7 ns -7.8*
Delayed irrigation 12.8±2.5 a 7.0±3.0 a -45.3* 12.3+3.5 a 11.8+1.0 a ^1.5 0.5 ns -4.8*
YTNT
Early irrigation 11.0±0.8 a 10.3±1.5 a -6.4 12.7+3.1 a 11.8+4.0 b -6.8 -1.7 ns -1.5 ns
Delayed irrigation 10.2±3.3 a 9.0±3.6 a -11.8 12.5+3.7 a 7.6+2.7 a -39.2* -2.3 ns 1.4 ns
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Table 3. (continued) Effect of Meloidogyne graminicola on plant growth and yield-contributing traits of two lowland rice varieties (THY: Thihtatyin,
YTNT: Yatanartoe) grown under two irrigation practices and two planting practices.
Irrigation practice Direct seeding (DS) Transplanting (TP) Differences (DS-TP)
N1 I % change N1 I % change N1 I
No. of filled grains per panicle
THY
Early irrigation 57.8±3.3 a 59.8±1.2 a +3.5 58.2+5.9 a 45.8+5.3 a -21.3* -0.4 ns 21.7*
Delayed irrigation 57.6±5.5 a 46.3±14.3 a -19.6 70.6+16.1 a 41.9+11.0 a ^10.7* -13.0 ns 4.4 ns
YTNT
Early irrigation 50.6±6.3 a 52.8±7.1 a +2.2 67.4+9.9 a 50.7+8.6 a -24.8* -16.8* 2.1 ns
Delayed irrigation 56.8±4.0 b 43.6±8.8 a -23.2* 63.7+10.2 a 60.3+6.0 a -5.3 -6.9 ns -16.7 ns
Filled grains per plant (%)
THY
Early irrigation 63.8±1.5 a 72.3±5.7 a +13.3 60.1+7.3 a 54.3+6.2 a -9.7 3.7 ns 18.0*
Delayed irrigation 64.6±4.2 a 66.6±5.6 a +3.1 65.5+1.1 a 60.3+8.7 a -7.9 -0.9 ns 6.3 ns
YTNT
Early irrigation 84.3±4.4 a 89.0±6.0 b +5.6 81.7+3.8 a 86.9+7.4 a +6.4 2.6 ns 2.1 ns
Delayed irrigation 84.9±2.4 a 69.7±9.0 a -17.9* 79.9+7.0 a 85.4+4.5 a +6.9 5.0 ns -15.7*
Filled grains weight per plant (g)
THY
Early irrigation 10.9±2.1 a 9.8±3.0 a -10.1 14.4+2.3 a 10.4+0.6 b -27.8* -3.5* -0.6 ns
Delayed irrigation 13.2±2.2 b 5.9±2.4 a -55.3* 15.0+2.4 a 8.5+1.5 a -43.3* -1.8 ns -2.6 ns
YTNT
Early irrigation 12.1±0.8 a 13.2±1.7 a +9.1 17.7+3.1 a 13.2+2.4 a -25.4* -5.6* 0
Delayed irrigation 12.9±2.2 a 12.6±5.5 a -2.3 18.1+3.0 a 11.2+3.7 a -38.1* -5.2* 1.4 ns
1,000 grains weight (g)
THY
Early irrigation 19.0±2.2 a 17.8±1.0 a -6.3 18.6+1.0 a 16.5+1.0 a -11.3* 0.4 ns 1.3 ns
Delayed irrigation 18.9+1.51 a2 17.2±1.0 a -9.0 17.6+1.5 a 15.4+0.4 a -12.5* 1.3 ns 1.8 ns
YTNT
Early irrigation 24.7±1.4 a 25.0±1.2 a +1.2 25.5+1.0 b 23.2+1.6 a -9.0 -0.8 ns 1.8 ns
Delayed irrigation 25.4±0.9 a 25.3±0.5 a -0.4 23.7+0.4 a 22.1+1.5 a -6.8 1.7 ns 3.2 ns
Yield (t ha"1)
THY
Early irrigation 3.3±0.2 a 2.9±1.0 a -12.1 4.6+0.6 a 2.7+0.4 a -41.3* -1.3* 0.2 ns
Delayed irrigation 4.0±0.6 b 2.4±1.0 a ^10.0* 4.6+0.9 a 2.9+0.6 a -37.0* -0.6 ns -0.5 ns
YTNT
Early irrigation 4.5±0.2 a 4.6±0.3 a +2.2 6.6+1.5 a 5.3+1.0 a -19.7 -2.1* -0.7 ns
Delayed irrigation 4.9±0.7 a 4.2±1.6 a -14.3 6.5+0.9 a 4.5+1.5 a -30.8* -1.6* -0.3 ns
1 Data represent means ± SD (n = 6).
2 Means in the same column under each rice variety, followed by the same lowercase letter, between irrigation practices, are not significantly different according to the t-test (P < 0.05).
(-): indicates a reduction in the inoculated plants compared to the non-inoculated plants. (+): indicates an increase in the inoculated plants compared to the non-inoculated plants. * indicates that the change is significantly different according to the t-test (P < 0.05).
For plants of variety Yatanartoe that had been transplanted a significant (P < 0.05) effect (% reduction) of nematode infection was observed on the number of tillers per plant and the number of panicles per plant of delayed irrigation plants (48.2 and 39.2%, respectively), number of filled grains per panicle of early irrigation plants (24.8%) and filled grain weight per plant (25.4 and 38.1 g for early and delayed irrigation plants, respectively).
Effect of nematode infection on yield. Figure 1 shows the effect of M. graminicola infection on the estimated yield of the two lowland rice varieties Thihtatyin and Yatanartoe under two irrigation practices and two planting practices. Irrigation practices had no significant effect on the yield of the non-inoculated and inoculated plants of both varieties with the exception of non-inoculated direct seeded plants of variety Thihtatyin. In these plants, the yield was significantly (P < 0.05) lower in plants
that had early irrigation compared with plants that had delayed irrigation (3.3 vs 4 t ha1). Transplanted non-inoculated plants had significantly (P < 0.05) higher yield than direct seeded non-inoculated plants in both varieties under both irrigation practices with the exception of variety Thihtatyin under delayed irrigation (3.3 vs 4.6 t ha1 under early irrigation in variety Thihtatyin; 4.5 vs 6.6 t ha-1 and 4.9 vs 6.5 t ha-1 under early irrigation and delayed irrigation, respectively in variety Yatanartoe). For variety Thihtatyin, nematode infection significantly (P < 0.05) reduced the yield of direct seeded and delayed irrigation plants with 40% (on average 1.6 t ha-1), of transplanted early irrigation plants with 41.3% (on average 1.9 t ha-1) and of transplanted delayed irrigation plants with 37% (on average 1.7 t ha-1). For variety Yatanartoe, nematode infection significantly (P < 0.05) reduced the yield of transplanted delayed irrigation plants with 30.8% (2 t ha-1).
0
Lowland varietv Yatanartoe
□ Unmodulated plants ■ Inoculated plants
7 -I a
6 - , □
5 - a a — H
i I I I
Early irrigation Delayed irrigation Early irrigation Delayed irrigation
Direct seeding Transplanting
Early irrigation ¡Delayed irrigation Transplanting
Planting practice and irrigation practice
Fig. 1. Effect of Meloidogyne graminicola infection on the estimated yield of two lowland Asian rice varieties a) Thihtatyin and b) Yatanartoe, grown under two irrigation practices and two planting practices. Bars with the same letter, within an irrigation practice, are not significantly different according to Tukey's HSD test (P < 0.05).
DISCUSSION
Based on Mresgs, both lowland rice varieties Thihtatyin and Yatanartoe were susceptible to M. graminicola under both irrigation and both planting practices used in our study but differences in susceptibility were observed between the two varieties and among the treatments.
Based on the number of J2 and eggs per root unit and per root system, and on M^ggs, variety Thihtatyin was more susceptible compared with variety Yatanartoe in all treatments. Also, significantly more J2 were extracted from the rhizosphere of variety Thihtatyin compared with variety Yatanartoe in all treatments. The severity of root galling was always higher on variety Thihtatyin compared with variety Yatanartoe in all treatments. This observation is in agreement with the results obtained in one of our previous studies (Win et al., 2014) that variety Thihtatyin is highly susceptible while variety Yatanartoe is less susceptible to M. graminicola infection based on Mreggs when the plants were harvested at the stem elongation growth stage.
The extremely high reproductive potential of M. graminicola on a highly susceptible lowland rice variety such as Thihtatyin grown under conditions that are favourable for the nematode can be illustrated by the observation that almost 1,000,000 eggs (root system)1 (a density of almost 40,000 eggs (g root)1) were counted at maturity of transplanted plants of this variety that had delayed irrigation.
For both varieties, the highest M^eggs were observed in transplanted plants that had delayed irrigation followed by direct seeded plants that had also delayed irrigation. For both varieties, the lowest Mreggs were observed in direct seeded plants that had early irrigation. This results shows that early irrigation limited the reproductive potential of M. graminicola, whilst delayed irrigation had the reverse effect (Netscher & Erlan, 1993; Prot & Matias, 1995; Soriano et al., 2000). These observations are in agreement with the observation that an about 7.5 times lower root population density of M. graminicola was observed in early irrigation rice plants of lowland varieties compared with delayed irrigation rice plants (46 vs 376 J2 (g root)1) during the survey carried out in the summer-irrigated lowland rice ecosystem in Myanmar in 2011 (Win et al., 2011). Also, a lower root population density of M. graminicola was observed in the early irrigation rainfed monsoon rice plants of variety Taungpyan in the rainy season compared
with delayed irrigation summer rice plants of variety Yatanartoe during a population dynamics study in a farmers' lowland rice field (Win et al., 2013).
In our study, high J2 and egg root population densities of M. graminicola were observed at maturity under all treatments which may be the result of the water regime (intermittent flooding) applied after the tillering plant growth stage and throughout the rest of the crop cycle in accordance with farmers' practice (i.e., sharing of irrigation water among the rice farmers during the dry summer season) in the summer-irrigated lowland rice ecosystem in Myanmar. Although intermittent flooding may limit the reproductive potential of M. graminicola compared with upland conditions, a high number of offspring can still be produced by egg-laying females of M. graminicola because this nematode species is exceptionally adapted to develop and reproduce inside flooded rice roots (Bridge & Page, 1982; Prot, 1992; Prot & Matias, 1995; Catling & Islam, 1999; Soriano et al., 2000; Bridge et al., 2005; De Waele & Elsen, 2007; Fernandez et al., 2014).
For both varieties, the highest number of J2 and eggs per root unit and per root system, and Mreggs were observed in transplanted plants compared with direct seeded plants irrespective of irrigation practice. This observation seems to contradict the observation in our previous study that on average a significantly higher root population density of M. graminicola was observed in direct seeded rice plants compared with transplanted plants (347 vs 46 J2 (g root)-1) at the tillering growth stage during the survey conducted in the summer-irrigated lowland rice ecosystem in Myanmar (Win et al., 2011). However, in about 90% of the surveyed fields, direct seeded rice plants had delayed irrigation and were cultivated in rice monoculture in a double rice cropping system, while transplanted rice plants had early irrigation and were cultivated in rice-blackgram-rice and rice-chickpea-rice cropping sequences. Also, the survey was carried out when the rice plants were at the tillering growth stage, while irrigation water (which was started during land preparation of the field) was being maintained in the transplanted rice fields. Soil and root population densities of M. graminicola were the highest in direct seeded summer-irrigated rice plants that had delayed irrigation, while very low soil and root population densities of M. graminicola were observed in transplanted rainfed monsoon rice that had early irrigation (Win et al., 2013). These rainfed monsoon transplanted rice plants were flooded (due to abundant rainfall) from soil preparation until a short time before maturity (harvest). In our study,
the high population density of transplanted rice plants may thus be influenced by the intermittent flooding applied after the tillering growth stage of the rice plants.
The higher root population densities of M. graminicola in transplanted rice compared with direct seeded rice observed in our study might also be due to the difference in root system between 21-day-old transplanted rice plants compared with 3-day-old pre-germinated seeds when the plants were inoculated. At the time of inoculation, the transplanted plants had a bigger root system (more and longer roots) compared with direct seeded plants. More and longer roots will allow more J2
(the infective developmental stage of Meloidogyne
species) to penetrate and establish feeding sites inside the roots (Elkins et al., 1979). Pokharel (2007) reported a higher reproduction of M. graminicola in rice seedlings inoculated when they were 2, 3 and 4 weeks old compared with inoculation of 1-week-old seedlings.
The severity of root galling was significantly higher on variety Thihtatyin compared with variety Yatanartoe in all treatments except direct seeded rice under early irrigation. For both rice varieties the severity of root galling was significantly higher on transplanted plants compared with direct seeded plants under early irrigation, and was for direct seeded plants of both varieties significantly higher on delayed irrigation plants compared with early irrigation plants. Only on transplanted plants of variety Yatanartoe was the root galling index higher (although not significantly) on early irrigation plants compared with delayed irrigation plants (4.3 vs 3.3, respectively). The higher root galling index observed on plants of variety Thithayin under early irrigation (which was comparable with plants of variety Yatanartoe under delayed irrigation) may be explained by the smaller size of galls observed under early irrigation. The root galls observed under early irrigation were smaller than under delayed irrigation rice. But as mentioned above, the severity of root galling is based on the percentage of roots galled, not on the size of the galls. It is thus also possible that in early irrigated plants J2 migration inside the roots was higher compared with delayed irrigation roots. But the difference in severity of root galling can also be explained by the higher J2 population density per root system and eggs per root system observed in delayed irrigation rice roots, which was about three times higher compared with early irrigation rice roots (141,861 vs 42,215 J2 and 345,170 vs 127,783 J2, respectively).
The highest root galling index (7.4) was observed on transplanted plants under delayed
irrigation of variety Thihtatyin, while the lowest root galling index (2.4) was observed on direct seeded plants of variety Yatanartoe under delayed irrigation. All these observations on the severity of root galling confirm the above described effects genotype, planting and irrigation practices can have on the M. graminicola population densities.
In the direct seeded plants of variety Yatanartoe, the delayed irrigation practice resulted in no significant effects on the plant growth variables measured although in these plants the Mf-esgs was relatively high (14.3) and 42,215 J2 (root system)-1 (a density of 2,830 J2 (g root)-1) were counted. This observation suggests that planting practice (direct seeding) and irrigation practice (delayed irrigation) made the plant growth of variety Yatanartoe less sensitive, even tolerant, to M. graminicola infection. By contrast, in transplanted plants of the same variety, the delayed irrigation practice resulted in a significant percentage reduction in fresh root weight and dry shoot weight. This observation supports the results obtained by Tandingan et al. (1996) that the sensitivity (including tolerance) of rice varieties to M. graminicola infection can vary under different water regimes.
For variety Thihtatyin, delayed irrigation reduced all plant growth variables measured, with the exception of plant height of direct seeded plants, thus confirming the high sensitivity to M. graminicola infection of this variety. This is in agreement with the results of one of our previous studies that showed a significant percentage reduction in fresh root weight and dry shoot weight of 15 lowland rice varieties (combined) (54.6 and 27.5%, respectively) in the screenhouse and (29.3 and 15.8%, respectively) in the field caused by infection of M. graminicola when the rice plants also had delayed irrigation and were harvested at maturity (Win et al., 2015b). By contrast, the percentage reduction in fresh root weight of variety Thihtatyin was only 9.9% when the rice plants were harvested at the stem elongation growth stage in one of our previous studies (Win et al., 2014). This can be explained by the different Mf-eggs observed on variety Thihtatyin between the plants harvested at the stem elongation growth stage and maturity (3.4 vs 47.5). In our study, Mf-^ was 78 and 94.3 in direct seeded and transplanted rice plants, respectively, that had delayed irrigation and this could explain the high sensitivity to M. graminicola infection of variety Thihtatyin. In our study, the percentage reduction in fresh root weight of direct seeded plants of variety Thihtatyin was about 50% under both early and delayed irrigation, whilst no significant reduction in fresh root weight of direct
seeded plants of variety Yatanartoe was observed under the same irrigation practices. However, when plants of both varieties were transplanted, a significant 40% reduction in fresh root weight was observed under delayed irrigation. In all our previous studies, lowland rice varieties were usually direct seeded and had delayed irrigation (Win et al., 2014; 2015b). The percentage reductions in fresh root weight caused by M. graminicola infection on direct seeded and delayed irrigation plants of variety Thihtatyin observed in our study (48.8%) are, in general, in agreement with the results of our previous studies carried out in the screenhouse and in the field (35.6 and 29%, respectively) when the rice plants were harvested at maturity (Win et al., 2014), although the reduction was greater in this experiment.
In our study, no significant percentage reduction in fresh root weight of plants of variety Yatanartoe was observed, which seems to contradict the results obtained in our previous study that the percentage reductions in fresh root weight caused by M. graminicola on variety Yatanartoe in the screenhouse and in the field were 22.7 and 27%, respectively, when the rice plants were harvested at maturity (Win et al., 2015b). This contradiction can be explained by the differences in root population densities of M. graminicola of these two rice varieties observed between the two studies. For variety Yatanartoe, a 9.6 times lower population density of J2 (g root)-1 (752 vs 7,188 J2) and a 3.5 times lower population density of J2 (root system)-1 (33,841 vs 118,681 J2, respectively) was observed in direct seeded plants that had delayed irrigation in our study compared with the previous study, also carried out under screenhouse conditions. By contrast, for variety Thihtatyin, only a 0.25 times lower population density of J2 (g root)-1 (5,815 vs 7,743 J2) and a 1.5 times higher population density of J2 (root system)-1 (197,949 vs 128,753 J2) were observed in direct seeded plants that had delayed irrigation in our study compared with the previous study, also carried out under screenhouse conditions. In the previous study, the rice plants of both varieties were inoculated 1 week after planting of 3-day-old pre-germinated seeds to simulate the farmer's field practices in the summer-irrigated lowland rice ecosystem in Myanmar. A possible explanation that in our study a significant percentage reduction in fresh root weight was only observed for variety Thihtatyin but not for variety Yatanartoe is the thinner root system of variety Yatanartoe compared with variety Thihtatyin when nematode inoculation of the direct seeded 3-day-old pre-germinated seeds took place. The number of
rootlets of variety Yatarnatoe was higher but the rootlets had a smaller diameter compared with variety Thihtatyin. The effect of root morphology of rice varieties on the penetration and establishment of M. graminicola J2 at the time of infection should be further investigated.
When all treatments are compared, of the plant growth variables measured, plant height was the least affected by M. graminicola infection while, in general, the highest percentage reductions in plant growth between non-inoculated and inoculated plants were observed when delayed irrigation has been applied. These observations are in line with the results of our previous studies (Win et al., 2014; 2015b).
It is difficult to conclude which of the yield-contributing traits measured had been the most affected by M. graminicola infection. The results of our study indicate that the damaging effects of M. graminicola on these traits were influenced by genotype, planting and irrigation practice at planting but general conclusions are difficult to make. For instance, no significant effect on the number of tillers per plant and number of panicles per plant between non-inoculated and inoculated transplanted plants of variety Thihtatyin was observed irrespective of irrigation practice, whilst early and delayed irrigation reduced the number of tillers per plant and number of panicles per plant of direct seeded M. graminicola-infected plants of this variety significantly with about 40 to 50%, respectively. By contrast, none of the other yield-contributing traits (number of filled grains per panicle, percentage filled grains per plant and weight of 1,000 filled grains) of plants of variety Thihtatyin grown under similar planting and irrigation practices (direct seeding, early and delayed irrigation) was observed.
In the majority of the treatments in which significant percentage reductions in yield-contributing traits were observed, these observations were made on transplanted plants (of both rice varieties) grown under delayed irrigation but there were several exceptions. For instance a significant percentage reduction in number of tillers per plant and number of panicles per plant (48.2 and 39.2%, respectively) of transplanted plants of variety Yatanartoe was observed under delayed irrigation. By contrast, no significant percentage reduction in number of filled grains per panicle of transplanted plants of this variety was observed under delayed irrigation.
Although, in general, the percentage reductions in yield-contributing traits were higher in plants (of both varieties) grown under delayed irrigation
compared with plants grown under early irrigation, we suggest that the effect of irrigation practice on these variables is lower compared with the effect of irrigation practice on fresh root weight and dry shoot weight (i.e., plant growth variables). For instance, direct seeded M. graminicola-infected plants of variety Thihtatyin grown under delayed irrigation had Mf-esgs of 78 or about 200,000 J2 (root system)-1 (a density of 5,815 J2 (g root)-1) but no significant percentage reduction in number of filled grains per panicle, percentage filled grains per plant and weight of 1,000 filled grains was observed. This suggests that planting practice (direct seeding) and irrigation practice (delayed irrigation) made the reproductive development of both varieties less sensitive, even tolerant, to M. graminicola infection. This less sensitivity of plant growth variables related to grain development in direct seeded M. graminicola-infected plants of variety Thihtatyin is probably caused by the high reduction of number of tillers per plant and number of panicles per plant (42.2 and 45.3%, respectively) under the same condition. We will later see that this observation is also supported by the yield loss data of both varieties.
Surprisingly, among the yield-contributing traits measured, the percentage reduction in number of tillers per plant, number of panicles per plant and the traits related to grain development of M. graminicola-infected plants in all treatments of both rice varieties except directed seeded plants of variety Yatanartoe under early irrigated were, in general, inversely affected by M. graminicola infection. When M. graminicola-infected plants produced a very low number of tillers or panicles per plant compared with non-inoculated plants, grain development traits were comparable with those of non-inoculated plants and vice versa. Only in direct seeded plants of variety Yatanartoe under early irrigation, was none of the grain development traits significantly reduced by nematode infection.
In our study, the number of tillers and panicles per plant were reduced by about 42.2 and 45.3%, respectively, in M. graminicola-infected direct seeded plants of variety Thihtatyin under delayed irrigation, while the other traits related to grain development were not significantly reduced. However, the filled grain weight per plant and yield were significantly reduced by 55 and 40%, respectively. By contrast, in the M. graminicola-infected transplanted plants of the same variety, the number of tillers and panicles per plant were not significantly reduced under delayed irrigation. However, the percentage filled grains per plant and weight of 1,000 filled grains were significantly
reduced by 40.7 and 12.5%, respectively, which, in turn, resulted in a significantly reduction in filled grains weight per plant and yield by 43.3 and 37%, respectively. Apparently, M. graminicola-infected rice plants can still 'mobilise' enough nutrients for grain filling even after they have produced only half the number of panicles per plant, a number that was comparable with the non-infected rice plants. Wei et al. (2011) reported that the percentage filled grains per panicle of direct seeded plants of the aerobic rice variety HD 297 was reduced, while the number of spikelets per unit area (which, in turn, is positively correlated with the number of panicles per unit area) was increased when two field experiments were conducted in a clay loam and a sandy loam soil with two nitrogen applications under intermittent irrigation. In their experiments, a higher number of tillers per plant leading to a reduction in percentage filled grains per plant were also observed. In our study, this less sensitive, even tolerant, ability of direct seeded plants of variety Thihtatyin and transplanted plants of variety Yatanartoe to M. graminicola infection in percentage reduction in number of filled grains per panicle, percentage filled grains per plant and weight of 1,000 filled grains observed under delayed irrigation is caused by the reduction of number of tillers and panicles per plant.
The results of our study confirm again the impact M. graminicola infection can have on the yield of both lowland rice varieties (Win et al., 2015b). When the rice plants are grown under planting and irrigation practices (especially transplanting and delayed irrigation) that are favourable for M. graminicola penetration, development and reproduction a yield loss of about one-third (or a yield loss of about 1.5 to 2 t ha-1) can be expected. The results of our study show that yield losses caused on rice by M. graminicola must be very high also in the farmer's fields. In one of our previous studies (Win et al., 2015b), M. graminicola caused on average a reduction in the yield of 15 lowland rice varieties by 31.1% under screenhouse conditions and 16.5% under summer-irrigated lowland field conditions.
The effect of planting practice on the percentage reduction in plant growth, yield-contributing traits and yield of both rice varieties caused by M. graminicola infection varied with irrigation practice. For instance, a significant percentage reduction in fresh shoot weight and dry shoot weight of direct seeded plants of variety Thihtatyin was observed (50.6 and 32.5%, respectively) under early irrigation and (48.8 and 52%, respectively) under delayed irrigation. By contrast, a significant percentage reduction in fresh shoot weight and dry shoot weight
of transplanted plants of variety Thihtatyin was observed (41.2 and 31.8%, respectively) only under delayed irrigation. No significant percentage reduction in filled grain weight per plant and yield of direct seeded plants of variety Thihtatyin was observed under early irrigation. By contrast, a significant percentage reduction in filled grain weight per plant and yield was observed under both early irrigation (27.8 and 41.3%, respectively) and (43.3 and 37%, respectively) delayed irrigation when this variety was transplanted. For instance, no significant percentage reduction in fresh shoot weight, dry shoot weight, and number of tillers per plant, number of panicles per plant and yield of direct seeded plants of variety Yatanartoe was observed under both irrigation practices. By contrast, significant reductions of these variables (40.7, 37.1, 48.2, 39.2 and 30.8%, respectively) were observed under delayed irrigation when this variety was transplanted. The number of filled grains per panicle of direct seeded plants of variety Yatanartoe was not significantly reduced under early irrigation but a significant percentage reduction in the number of filled grains per panicle (24.8%) was observed under early irrigation when plants of this variety had been transplanted.
The percentage reduction in yield of M. graminicola-infected direct seeded plants of both varieties Thihtatyin and Yatanartoe grown under early irrigation was -12.1 and +2.2%, respectively, which was not significantly different from non-infected plants of both varieties grown under the same conditions. These infected plants yielded 2.9 and 4.6 t ha-1, respectively. This observation indicates that under early irrigation (and in the case of variety Yatanartoe also under delayed irrigation) direct seeded plants of both varieties were tolerant to M. graminicola infection. This observation supports again the observation made by Tandingan et al. (1996) that the sensitivity (including tolerance) of rice varieties to M. graminicola infection can vary under different water regimes.
In one of our previous studies (Win et al., 2015b), we observed that the yield of variety Yatanartoe was reduced in both the screenhouse and the field experiment (with 27.7 and 14.2 %, respectively) when 3-day-old pre-germinated seeds were broadcasted in a naturally M. graminicola-infested field or 3-day-old pre-germinated seeds were inoculated 6 days after direct seeding in the screenhouse. In our study, M. graminicola infection of direct seeded plants of variety Yatanartoe did not reduce significantly any plant growth variable or yield-contributing trait measured under both delayed and early irrigation, except a reduction in number of
filled grains per panicle and percentage filled grains per plant (with 23.2 and 17.9%, respectively) under delayed irrigation. This observation shows that under certain conditions (lowland) rice varieties may become less sensitive and even tolerant to damage caused by M. graminicola infection but that these conditions are very specific. Further research is necessary to determine these conditions more precisely.
The results of our study confirm the important effect soil type, planting and irrigation practices at planting, and water regime during the crop cycle, can have on yield losses caused by M. graminicola infection on rice (Prot & Matias, 1995; Soriano et al. , 2000). A heavier soil, direct seeding, early irrigation at planting and permanent flooding may limit these yield losses. A lighter soil, transplanting, delayed irrigation at planting and (dry) upland water regime conditions may increase these yield losses.
ACKNOWLEDGEMENTS
This study was supported by a Flemish Interuniversity Council (VLIR-UOS) Ph.D. scholarship to P.P. Win. The authors express appreciation to the Plant Protection Division (Yangon) of the Ministry of Agriculture and Irrigation, Myanmar, for the facilities and assistance in conducting the experiments.
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Резюме. В тепличных экспериментах исследовали воздействие технологий посева и ирригации на репродуктивный потенциал и вредоносность для риса Meloidogyne graminicola. В условиях эксперимента провели оценку воздействия двух технологий ирригации (раннее и отложенное подтопление) и двух технологий посадки (внесение семян и пересадка) на урожайность и потери урожая от M. graminicola на двух широко культивируемых азиатских сортах (Thihtatyin и Yatanartoe). Оба сорта риса оказались менее чувствительными к поражению нематодами при условии раннего подтопления посевов, сделанных прямым внесением семян. Для обоих сортов наивысший показатель размножения нематод Mf-eggs (Multiplication factor) наблюдали у пересаженных растений при отложенном подтоплении. Также, для обоих сортов наибольшее количество личинок второй стадии и числа образовавшихся яиц нематод на одну корневую систему наблюдали в пересаженных саженцах по сравнению с прямым посевом семенами вне зависимости от способа ирригации. Наивысший индекс галлообразования (7.4) отмечен на пересаженных растениях сорта Thihtatyin, тогда как самый низкий индекс галлообразования (1.8) отмечен при прямом посеве семенами при раннем подтоплении на сорте Yatanartoe. Сокращение основного числа параметров роста растений и урожайности, вызванных M. graminicola, отмечено на пересаженных растениях обоих сортов риса при отложенном подтоплении. Напротив, устойчивость для обоих сортов была отмечена при прямом посеве семенами и раннем подтоплении чек. Сорт Yatanartoe отличается большей устойчивостью по сравнению с Thihtatyin. Для сорта Yatanartoe существенные потери урожая были отмечены только на пересаженном рисе с отложенным подтоплением (30.8%). Для сорта Thihtatyin, потери выхода зерна наблюдали как на рисе, высеянном семенами, так и на пересаженном рисе (40 и 37%, соответственно) при отложенном подтоплении и на пересаженном рисе при раннем подтоплении чек (41.3%).
