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Journal of Stress Physiology & Biochemistry, Vol. 20, No. 1, 2024, pp. 125-136 ISSN 1997-0838 Original Text Copyright © 2024 by Sissoko, Camara, Diawara, Diarra, Dembele, Dolo, Katile and
Toure
ORIGINAL ARTICLE
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Effects of two Ralstonia Solanacearum strains on Yield of Tomato (Lycopersicon esculentum Mills 1768) Genotypes Planted at Three Different Planting Seasons at the Sotuba Station, Sudano-Sahelian Area of Mali
Sory Sissoko12*, Abdoulaye Youssouf Camara2, Mamadou Oumar Diawara1, Allihoun Diarra2, Ibrehima Dembele2, Aminata Dolo2, Seriba Ousmane Katile3, Kadiatou Toure2
1 Department of Education and Research of Biology, University of Sciences, Techniques and Technologies of Bamako, Bamako, Mali
2 Fruit and Vegetable Programme, Institute of Rural Economy (IER), Bamako, Mali
3 Pearl Millet Programme, Institute of Rural Economy (IER), Niono, Mali
*E-Mail: s.s382(mesrs.ml [email protected]
Received November 29, 2023
The tomato, a genus of Lycopersicon, is cultivated worldwide for fruit. It is an important gastronomic ingredient and an important source of income for small farmers in Mali. However, its production faces a number of phytosanitary constraints that are still poorly understood. Among phytosanitary problems, bacterial wilt, caused by R. solanacearum, is proving to be the most dangerous and devastating of all tomato crops in Mali. The aim of this study is to contribute to the improvement of tomato production in Mali by identifying tomato varieties tolerant to R. solanacearum and adapted to the growing periods. To achieve this objective, trials under artificial and natural infestation conditions with 12 tomato genotypes during three periods were conducted at the Sotuba research station from December 2018 to December 2019. Analysis of variance revealed no significant difference between the two strains tested. Both strains were more severe, especially in period 2 of the experiment. The Caraibo variety was the most susceptible to both strains, with an average severity rate of 2.8 (=3) and the most stress-susceptibility index (0.69). The varieties Carioca and SF-83-61 were the most productive in both experimental conditions. With average yields > 3.5 t/ha under artificial infestation conditions and > 6 t/ha under natural conditions. In contrast, the Roma and Rossol varieties recorded the lowest yields. To minimize R. Solanacearum damage and improve field yields at all cropping periods, the SF-83-61 and Carioca varieties may be recommended for release in areas where R. Solanacearum, responsible for bacterial wilt, causes significant damage in tomato crops after confirmation tests.
Key words: Tomato, R solanacearum, yield, period of growing, Mali
The tomato (Lycopersicon esculentum Mills 1768) is one of the most widely cultivated field and garden vegetable crops in the world (Salunkhe and Kadam, 1998). It is an important cash crop for smallholders and commercial farmers with modest landholdings; it is also an important source of vitamins and essential minerals in the food supply (Naika et al., 2005).
The tomato is in great demand throughout the world in all seasons (Freeman et al., 2011), and is an essential condiment in the gastronomy of many countries, particularly Mali. Used in cooking as a vegetable, tomatoes can be eaten raw, in salads or mixed with other ingredients (Dembele, 1994).
Vegetable crops are attacked by a wide range of pests, often resulting in substantial crop losses each year (IPM-CRSP 2006). Pest pressure often reduces the range of crops that can be grown.
The tomato, which is the subject of our study, is the most widely cultivated fruit in Africa, yet its production is subject to a number of phytosanitary constraints during its development (Camara et al., 2022). Tomato crops are subject to multiple parasitic pressures, caused by pests and diseases as well. Among diseases, bacterial wilt, caused by R. solanacearum, is the most dangerous and devastating for tomato crops in Mali (Thera et al., 2010). Currently, there are no preventive or curative treatments for this disease, so the only solution is to respect crop rotations and use resistant or tolerant varieties (Courchinoux, 2008).
The aim of this study is to contribute to the improvement of tomato production in Mali by identifying tomato varieties tolerant to R. solanacearum and adapted to the different cultivation periods.
MATERIALS AND METHODS
Experimental site
The Station de Recherche Agronomique de Sotuba is located in the district of Bamako on the left bank of the Niger River, about 7 km from Bamako city center, and covers an area of about of around 265 ha. Sotuba is located between altitude 12°39 North and longitude 07°56 West at an altitude of 320m. The climate is
Sudano-Sahelian, with rainfall varying from 800 to over 1,000 mm. The soil is silty clay or sandy clay.
Plant material
The plant material consists of 12 tomato varieties, all introduced. The details of the varieties are given in Table 1. Bacterial material (bacterial strains)
Two bacterial strains, K5 (Kati) and Sd (Samanko), both were obtained from the AVRDC-Mali collection (WorldVegetable) where they are being cryopreserved. Were used to prepare the inoculum. The Kati strain (k5) is irregular in shape, small in size, russet in color with numerous isolated colonies. The Samanko strain (sd) is irregularly shaped, round and grouped, dark red in color with rapid colony growth. The two strains both host tomatoes belonging to Phylotype I.
Preparation of bacterial strains
R. solanacearum strains collected at the AVRDC (World Vegetable Center) were stored in glycerol at 20-70°C before use. Then, the strains were put into pure growth by repeated streaking of a single colony on agar medium containing triphenyltetrazolium chloride (TTC) (Kelman, 1954). To prepare the inoculum, bacterial cells were grown in nutrient bouillon (Difco, USA) under agitation at 28°C during two days. They were then in suspension in distilled water and an optical density of 0.1 at 600 nm wavelength (using the spectrophotometer) of around 108 cfu/ml/ml (D0600 = 0.8) and adjusted. The inoculum was then ready for use.
Inoculation of tomato plants
The 12 tomato genotypes germinated in containers, 4 weeks old (at the stage of 3 to 4 true leaves, about 15 cm high) were infected using 5 mL of inoculum applied to the substrate after scarification of the roots with a scalpel. The plants were watered regularly to prevent the substrate from draining. A treatment without infestation with the pathogen constituted the control trial, which also included the 12 genotypes placed 2 m from the infested trial to evaluate them under the same practical and environmental conditions.
Experimental design.
To assess the severity of the two bacterial strains on the 12 genotypes, a split plot design was used with the
two strains as the main factor and the 12 genotypes as the secondary factor. The main factor consisted of two strips subdivided into four blocks (replications) each. Each genotype was sown in 10-litre containers 3/4 filled with compost. Each container constituted a treatment and the 12 containers constituted a replicate, i.e. 48 containers for a block. A total of 96 containers made up the two blocks. To limit cross-contamination, the two plots were placed two metres apart and 20 cm was between the containers in which the different genotypes were sown. A non-infested control plot was set up not far (2 m) from the infested plot in a simple Fischer block design with four (4) replications. The trial was conducted during all three periods using the same design.
Data collection
- Observation of tolerance parameters
Observations of tolerance parameters began 15 days after inoculation: the severity of symptoms was observed 15 days after inoculation. Plants were observed weekly according to the development of symptoms of wilting caused by R. solanacearum on a scale of 0 to 5, a descriptive key for measuring the severity of the disease based on a scale developed by Winstead and Kelman, (1952).
0 = 0% Healthy plant with no visible symptoms
1 = < 5% On single leaf "fully or partially" wilted
2 = 5 - 25% 2 to 3 wilted leaves
3 = 25 - 50% All leaves wilted except 2-3
4 = 50 - 75% Total wilting of the plant
5 = 75 - 100% Dead plant.
- Observation of agronomic parameters
For the trial under artificial infestation, the observations concerned yield and some yield components, such as the mean height of the tomato plants, the mean number of fruits per plant and the mean weight of a fruit, in order to identify correlations between yield and these parameters under artificial infestation. In the control plot, grown under natural conditions, only the yield per hectare was observed in order to estimate stress indices.
- Method of calculating stress indices
Six stress indices and the percentage reduction in
yield were calculated on the basis of yield under infested and non-infested conditions. These parameters were calculated using the following formulae:
Tolerance index (TOL) (Hossain et al., 1990), TOL = (Yp -Ys)
Mean productivity (MP) (Hossain et al., 1990),
(Yp+ Ys )
MP = —--
2
Geometric mean productivity (GMP) (Fernandez, 1992),
GMP = V YpxYs
Stress susceptibility index (SSI) (Fischer and Maurer, 1978),
SSI
[ 1-(Ys )/(Yp )]
SI
where SI is the stress intensity and is calculated as follows:
SI = [ 1-(Ys)l(Yp)]
Stress tolerance index (STI) (Fernandez, 1992) as: (Yp)x(Ys)
STI = ■
(Yp )2
Reduction percentage (Red%) (Choukan et al., 2006).
, , N (Yp-Ys)
Red (%) = -'-
Yp
Where Ys and Yp are the yields under infested and non-infested conditions and Ys and Yp are the mean yields of all genotypes under infested and non-infested conditions.
Statistical analysis of data
Data was input into Excel. Analysis of variance (ANOVA) was performed using R i386 3.1.2. software. The effects of replicates and blocks were considered to be random, whereas those of genotypes were considered to be fixed. Means were separated by the least significant difference (LSD).
RESULTS
Experimental conditions
During the first experimental period, the minimum and maximum temperatures were almost constant during the first two months (December 2018 and
January 2019) of the experiment; we noted a slight increase in these minimum and maximum temperatures during the last two months of the experiment (February 2019 and March 2019). In contrast to temperatures, there was a decrease in minimum and maximum humidity in the last two months of the experiment.
The average monthly temperature and humidity during the first period are shown in Figure 1a. During the second experimental period, the minimum and maximum temperatures are high for the first two months (April 2019 and May 2019), then decrease for the last two months of the experiment. There was an almost exponential increase in minimum and maximum humidity towards the last two months of the experiment. The average monthly temperature and humidity during the second period as shown in Figure 1b. During the third experimental period, the minimum temperature decreased slightly in the last two months of the experiment (June 2019 and July 2019), while the maximum temperature increased slightly in the same months. There was also an almost exponential drop in minimum and maximum humidity levels towards the last two months of the experiment. The average monthly temperature and humidity during the third period is illustrated in Figure 1c.
Trials under artificial infestation conditions during the three experimental periods at the Sotuba station.
Analysis of variance (ANOVA) of the squares of the means of the various traits studied in the trials infested with two bacterial strains of R. solanacearum during the three periods.
The results of the analysis of variance of the means squared for the different traits studied are presented in Table 2. The analysis reveals a highly significant difference (p<0.01) between the periods for all the traits studied. A significant difference (p<0.05) was observed between strains only for yield. Except for plant height and fruit weight, a highly significant difference was observed between varieties for all the other traits studied. The effects of interactions between periods and strains were highly significant for the number of fruits per plant and yield. Interactions between periods and varieties were significant at p<0.05 and <0.01 for strain
severity, height, number of fruits per plant, fruit weight and yield. Interactions between strains and varieties were significant (p<0.05) for fruit weight and yield.
Analysis of the results of the means of the various traits studied by strain and by experimental period under artificial infestation conditions
In the interpretation of the results, Strain 1 represents the Kati strain (K5) and Strain 2 represents the Samanko strain (Sd).
Results of strain severity on genotypes over the three periods
The results of the mean analyses in Table 3 show that the two strains were generally moderately severe during the three experimental periods, with an overall mean of 1.7 (=2). A minimum value of 0.6 (=1) in period 1 and a maximum of 2.4 (=2) during the second period (Table 3). On the other hand, the analysis reveals that several genotypes were susceptible to both bacterial strains and especially during the second trial period, with a degree of severity between 3.5 and 4.3. These genotypes are: Caraibo, Estrella, Floradade, Roma, Rossol and UC-82. It should also be noted that strain 1 was slightly more severe than strain 2 during the 2nd trial period, with a mean of 2.6 versus 2.3.
Results of the analysis of variance of some yield components during the three experimental periods under conditions of artificial infestation.
The overall mean plant height was 55.5 cm. The results of the analysis show that the greatest plant height was recorded during the 3rd experimental period, with an average of 84.9 cm. The plants were shortest during the 2nd period (32.8 cm), when the bacterial strains were most severe. The study did not reveal any significant difference in height between the strains.
Period 3 of the experiment still produced more fruit per plant than the other two periods. The results show that the genotypes behaved differently for the same strain and the same period. During the 1st period of experimentation, the varieties SF-83-61, Estrella and Carioca were more tolerant to strain 1 than the varieties UC-82, Floradade and C-20-5. And the Estrella variety was more tolerant to strain 2 than all the other varieties during the same period. During period 2, varieties SF-
83-61, Carioca and Estrella were the most tolerant to both bacterial strains. During period 3, the varieties least sensitive to strain 1 were C-20-5, Estrella and Carioca. The varieties least sensitive to strain 2 were Estrella, SF-83-61, C-20-5, Carioca and Formosa. The varieties most tolerant to strains for fruiting were: Estrella followed by SF-83-61. The results of the statistical analysis of the mean weight of a fruit show us that the fruits harvested during the 1st period of the experiment were much heavier, followed by the 3rd period. The varieties with the highest mean of a fruit weight were Floradade and UC-82; the Estrella variety had the lowest fruit weight.
Results of the analysis of average yield per hectare over the three experimental periods under artificial infestation conditions
The results in Table 4 show that yields were highest in period 3 (4,201 kg/ha), followed by period 1 (3,287 kg/ha) and period 2 (305 kg/ha). The genotypes were much more sensitive to strain 2 during periods 1 and 2 of the experiment.
The best yields were obtained with the SF-83-61, Carioca and Buffalo varieties infested with strain 1 during period 1, with 9,626 kg/ha, 6,688 kg/ha and 6,424 kg/ha respectively; the Floradade and Buffalo varieties were better with strain 2 during the same period. Only the variety SF-83-61 had an above-average yield (3,085 kg/ha) with both strains during the 2nd period. During the 3rd period, the best yields obtained with strain 1 infestation were the varieties Mongal F1 (7,728 kg/ha), C-20-5 (6,764 kg/ha) and Carioca (6,531 kg/ha). With strain 2, the best varieties were Carioca (7,408 kg/ha), Formosa (7,279 kg/ha) and C-20-5 (6,133 kg/ha).
The best performance over the three periods and with both strains was obtained with the Carioca variety (3,938 kg/ha), while the lowest performance was observed with the Roma variety (1,292 kg/ha). Several genotypes died before the production period, which may be due to the severity of the strains linked to the production period.
Correlation analysis between the different traits studied under artificial infestation conditions during the three periods.
The analysis in Table 5 shows a negative and highly significant correlation (p<0.01) between the degree of severity and all other variables. This means that as the value of the degree of severity increases, the value of all the other traits (yield, height, number of fruits, etc.) decreases, and vice versa.
Trials under natural infestation conditions (controls) during the three periods
Control (non-infested) trials were conducted at the same time as trials infested with two bacterial strains of R. solanacearum.
Results of the analysis of the mean yield (kg/ha) of the genotypes during the three experimental periods under non-infested conditions.
The results show that the best production of the genotypes was achieved during the 3rd period of the experiment (Table 6). The most productive varieties were, on average, SF-83-61, Carioca and Estrella, with yields per hectare: 7,459 kg/ha, 6,084 kg/ha and 5,888 kg/ha respectively. Analysis of yields and stress indices correlated with the tolerance of genotypes to R. solanacearum
Analysis of yields (kg/ha) and stress indices
Various indices were calculated on the basis of yields under infested and non-infested conditions. The results are shown in Table 7. The varieties Roma, Rossol, Caraibo and UC-82 obtained the highest values for the stress susceptibility index (SSI) and percentage yield reduction (Red %). With yield reductions of over 60% (Table 7) due to the incidence of stress induced in the above varieties. The results showed that these four varieties also had the lowest stress tolerance indexes (STI), which explains their higher susceptibility to bacterial strains. The Carioca variety recorded the lowest Stress Susceptibility Index (SSI) value and percentage yield reduction, and was the most tolerant variety with the highest Stress Tolerance Index (STI) value. Variety SF-83-61 showed the best values for geometric mean productivity (GMP), mean productivity (MP) and tolerance index (TOL). This explains the
tolerance of SF-83-61 to bacterial strains.
Correlation analysis between yields and stress indices
The results in Figure 4 show a highly significant and positive correlation at p<0.01 and equal to 1 between the stress susceptibility index (SSI) and percentage yield reduction (Red%). These two variables had a significant negative correlation at p<0.05 or p<0.01 with all the other variables except the tolerance index (TOL). This means that as the values of SSI and Red% increase, the
values of the other variables will decrease and vice versa, except for TOL. Significant positive correlations at p<0.05 or p<0.01 were recorded between the variables: yield in non-infested conditions (Yp), yield in infested conditions (Ys), geometric mean productivity (GMP), mean productivity (MP) and stress tolerance index (STI). The tolerance index showed no significant correlation with the other variables, except for the stress tolerance index (STI), with which there was a significant negative correlation at p<0.05. (Figure 2).
Table 1: List and some characteristics of the different tomato varieties used in the study.
Ord N° Variety Origin Type genetics Growth type Cycle planting maturity 1st Harvest Color of fruit at maturity
1 SF 83-61 Taiwan Pure line Determinate 90 DAP Red
2 Formosa Cap Vert Pure line Determinate 90 DAP Red
3 Carioca Martinique Pure line Determinate 70 to 75 DAP Red
4 Mongal F1 France Hybrid Determinate 65 DAP Bright red
5 Caraïbo Martinique Pure line Determinate 75 DAP Red
6 Rossol France Pure line Determinate 75 a 80 DAP Red
7 C-20-5 France Pure line Determinate 90 DAP Red
8 Roma Italie Pure line Determinate 70 a 80 DAP Red
9 Floradade France Pure line Determinate 75 a 80 DAP Red
10 Estrella Cap Vert Pure line Determinate 90 DAP Red
11 Buffalo France Pure line Semi-Determinate 70 to 75 DAP Red-orange-gold
12 UC-82 USA Pure line Determinate 70 to 80 DAP Red
DAP = Day After Planting. (Source : Catalogue régional. 2017 ; Aïssa et al., 2014)
Table 2: Results of analysis of variance of agronomic parameters observed in trials infested by two bacterial strains during the three periods at the Sotuba station.
Source of variation d.f Severity PLH (cm) NF/PL WF (g) Yield (kg/ha)
Period 2 93,764** 68457.8** 1573,07** 3045,2** 3,99E+08**
Strain 1 0,222 913.8 18,00 2,4 2,69E+07*
Variety 11 7,836** 1343.8 171,79** 167,6 1,76E+07**
Period*Strain 2 1,514 285.1 271,95** 3,4 6,21E+07**
Period*Variety 22 4,711* 1387.0* 49,01** 211,3** 1,22E+07**
Strain*Variety 11 3,093 840.8 25,77 198,8* 1,33E+07*
Residual 216 2,868 838.9 19,42 104,6 6,00E+06
CV % 100,8 52,2 85,4 131,9 94,3
SE 1,7 28,96 4,4 10,2 2449,2
d.f. = degree of freedom, PLH (cm) = Plant height, NF/PL = Number of fruits per plant, WF (g) = Weight of a fruit, CV % = coefficient de variation, SE = Standard error.
Table 3: Results of strain severity analysis on the different genotypes over the three periods under artificial infestation conditions.
Period 1 Period 2 Period 3 Overall
Strain 1 Strain 2 Strain 1 Strain 2 Strain 1 Strain 2 mean
Buffalo 0,3 0,5 3,0 2,5 2,8 2,5 1,9
C-20-5 0,0 0,3 2,5 1,8 0,8 1,3 1,1
Caraïbo 1,5 0,0 3,8 4,3 3,8 3,3 2,8
Carioca 0,0 1,0 0,0 0,8 0,3 1,0 0,5
Estrella 0,8 0,0 4,5 2,5 2,5 0,8 1,8
Floradade 0,5 0,0 3,5 1,0 2,3 1,8 1,5
Formosa 0,0 2,8 3,8 2,0 2,0 1,5 2,0
Mongal F1 1,0 1,3 2,5 2,3 0,8 1,3 1,5
Roma 0,0 0,5 1,3 1,3 3,5 3,5 1,7
Rossol 0,3 0,0 3,8 3,8 2,5 2,5 2,1
SF-83-61 0,0 1,3 1,3 2,0 1,8 1,5 1,3
UC-82 1,3 0,5 1,3 3,5 2,3 3,3 2,0
Mean strain 0,5 0,7 2,6 2,3 2,1 2,0 1,7
Mean period 0,6 2,4 2,0
LSD 1.7 1.9 3.3 3.1 2.0 2.2 2,4
Table 4: Results of the analysis of the mean yield (kg/ha) of the genotypes during the three experimental periods under artificial infestation.
Variety Period 1 Period 2 Period 3 Overall
Strain 1 Strain 2 Strain 1 Souche 2 Strain 1 Strain 2 mean
Buffalo 6424 4080 0 0 2802 2891 2700
C-20-5 3358 1561 264 0 6764 6133 3013
Caraïbo 3806 3440 0 0 0 3985 1872
Carioca 6688 1090 1099 814 6531 7408 3938
Estrella 5930 1774 409 44 3611 5795 2927
Floradade 2828 5366 0 674 2461 5511 2807
Formosa 4831 809 0 228 4904 7279 3008
Mongal F1 4435 528 89 426 7728 3561 2794
Roma 2369 1026 0 0 1886 2470 1292
Rossol 984 1716 0 0 2159 3891 1458
SF-83-61 9626 2335 3085 198 3178 3910 3722
UC-82 2450 1436 0 0 3821 2149 1643
Mean strain 4477 2097 412 199 3820 4582 2598
Mean period 3287 305 4201
LSD 3536 3246 2399 491 4226 5208 3414
Table 5: Correlation matrix between the different traits studied under artificial infestation conditions during the three
periods.
Severity NF/PL WF PLH Yield
Severity 1
NF/PL -0,389** 1
WF -0,382** 0,158** 1
PLH -0,488** 0,470** 0,292* 1
Yield -0,411** 0,748** 0,545* 0,516** 1
**. Correlation is significant at 0.01, NF/PL = Number of fruits per plant, WF (g) = Weight of a fruit, PLH = Plant height.
Table 6: Results of the analysis of the mean yield (kg/ha) of tomato genotypes during the three experimental periods under non-infested conditions.
Variety Period 1 Period 2 Period 3 Overall mean
Buffalo 6626 2227 7525 5459
C-20-5 5138 2540 8186 5288
Caraibo 7400 1731 5364 4832
Carioca 6954 2705 8591 6084
Estrella 7203 1914 8548 5888
Floradade 5645 1744 9109 5499
Formosa 5077 2276 9541 5631
Mongal F1 4912 1809 9672 5464
Roma 3771 2329 6711 4270
Rossol 3568 1877 8359 4601
SF-83-61 10560 3095 8721 7459
UC-82 4302 1635 6424 4120
Mean variety 5930 2157 8063 5383
LSD 3208 2864 4321 3440
Table 7: Results of yields (kg/ha) and stress indices
Genotypes Yp (kg/ha) Ys (kg/ha) TOL MP GMP SSI STI Red (%)
Buffalo 5044 2700 2344 3872 3690 0,91 0,54 46
C-20-5 5288 3013 2275 4151 3992 0,84 0,57 43
Caraïbo 4832 1872 2960 3352 3008 1,20 0,39 61
Carioca 6084 3938 2146 5011 4895 0,69 0,65 35
Estrella 5888 2927 2961 4408 4151 0,99 0,50 50
Floradade 5332 2807 2525 4070 3869 0,93 0,53 47
Formosa 5431 3008 2423 4220 4042 0,88 0,55 45
Mongal F1 5233 2794 2439 4014 3824 0,91 0,53 47
Roma 4270 1292 2978 2781 2349 1,37 0,30 70
Rossol 4601 1458 3143 3030 2590 1,34 0,32 68
SF-83-61 7459 3722 3737 5591 5269 0,98 0,50 50
UC-82 4120 1643 2477 2882 2602 1,18 0,40 60
Mean 5299 2598 2701 3948 3690 1,0 0,5 52
Yp (kg/ha) = yield under non-infested conditions, Ys (kg/ha) = yield under infested (stress) conditions, TOL = tolerance index, MP = Mean productivity, GMP = Geometric mean productivity, SSI = stress susceptibility index, STI = stress tolerance index, Red (%) = percentage yield reduction
c. Third experimental period Figure 1. Monthly mean temperature and humidity (max and min). (Source: Sotuba weather station 2018 - 2019).
Variables (axes F1 et F2 : 99.70 %)
0,75
0.5
-r- 0,25
S 0
N <N
"--0.25
-0.5 -0.75 -1
TOL
/ \ • Yp (kg/ha)
Red (9 >) sst J № Ys(kg/ha)
STI
\
\ \ y.
\ s.
-1 -0.75 -0.5
-0.25 0 0.25 F 1(77.64%)
0.5 0.75
Figure 2. Correlation between yields and stress indices
Yp (kg/ha) = yield under non-infested conditions, Ys (kg/ha) = yield under infested (stress) conditions, TOL = tolerance index, MP = Mean productivity, GMP = Geometric mean productivity, SSI = stress susceptibility index, STI = stress tolerance index, Red (%) = percentage yield reduction
DISCUSSION
The results of the experiment during the three periods revealed that the yield and some yield components of all the genotypes improved under non-infestation conditions compared with artificial infestation conditions. This indicates the effect of the bacterial wilt agent (R. Solanacearum) on genotypes under artificial infestation conditions. According to Elphinstone (2005), bacterial wilt caused by R. solanacearum is one of the most destructive diseases of food and cash crops in the world, and one of the threats to the sustainability of agriculture. The 2nd period of the experiment (April to July 2019) has the lowest value for yield and other traits studied in the two experimental conditions. These results may be due to the effect of temperatures and high humidity during this period (hot and humid), not only on bacterial strains but also on tomato genotypes. The development of fungal diseases is strongly linked to high humidity accompanied by high temperatures (Laumonier, 1979). Tilma and Fontaine (2015) maintain that conditions favourable to the development of R. solanacearum are high temperatures. Other authors, such as Lallmahomed & Rakotobe (1988), Rakotondramanana & Rabehevitra (1984), support the same idea, i.e. that bacterial wilt usually occurs on crops in hot, humid periods. The best values for average yields and components under both growing conditions were obtained during the 3rd period of the experiment (August to December 2019). This is a period when temperatures generally decrease, which would be favourable for yield and its components. According to Maiga (2004), tomato cultivation gives better results in the cool, dry season (December to July). Yields for the 1st period (December 2018 to March 2019) under both conditions are also acceptable, with the genotypes probably benefits from the cool periods from December to February and the drop-in humidity from January to March. Bacterial wilt causes losses depending on the variety and the growing period (Ahmed and Sing, 1975; Sing and Rao, 1977).
The analysis did not reveal any significant difference between the two strains (K5 and Sd) for the parameters studied, except for yield per hectare and the interaction
of the strains with the periods. The average yield values of genotypes inoculated with strain 1 (K5) were higher than those inoculated with strain 2 (Sd) during experimental periods 1 and 2, and low during period 3. According to Buddenhagen and Kelman (1964), the speed of appearance and intensity of the disease depend on the host (species, cultivar), the physiological stage, the nature of the strain and the quantity of inoculum, but especially on interactions with the environment.
A highly significant difference was observed between varieties and interactions between varieties and period, for the degree of severity of bacterial strains. These results were confirmed by the correlation analysis, which revealed a highly significant negative correlation between all the traits and the degree of severity of the bacterial strains.
The results of the analysis of the mean yield under non-infested conditions show that the best production of the genotypes was achieved during the 3rd period of the experiment.
Stress indices were calculated on the basis of yield in infested (Ys) and non-infested (Yp) conditions. This method of analysis is a means of identifying genotypes with better stress tolerance. A higher value of SSI, Red % and TOL indicates a relatively higher sensitivity to stress (Golabadi et al., 2006). The results of the analysis identified the varieties SF-83-61 and Carioca as genotypes with better performance at different stress levels. On the other hand, Roma, UC-82 and Rossol were identified as the genotypes with the lowest yields. A number of authors both confirm and refute the results obtained in this study. Sikirou et al., (2007) found in their study that the varieties Carioca and Roma were all susceptible to bacterial wilt. According to Huat and Prier (2005), the Mongal F1 and Carioca varieties were both relatively resistant to R. solanacearum and had the best yields in their study.
CONCLUSION
The tomato is one of Mali's most important vegetables and is adapted to a wide variety of growing conditions. Bacterial wilt is one of the main causes of
tomato yield losses in the field worldwide, and in Mali in particular.
Our study confirmed that bacterial wilt, caused by R. solanacearum, is one of the main bacterial diseases causing damage to tomato crops in Mali at all growing seasons. But its damage is greatest during the growing season from April to July. To minimize the damage caused by R. Solanacearum and improve field yields at all growing seasons, the SF-83-61 and Carioca varieties can be offered for release in areas where R. Solanacearum, which causes bacterial wilt, causes significant damage to tomatoes after confirmation tests.
CONFLICTS OF INTEREST
The authors declare that they have no potential conflicts of interest.
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