Bacterial wilt caused by the soilborne bacteria Pseudomonas solanacearum is found in most of the tropical, sub-tropical and temperate countries. Among the wide range of hosts of this bacteria, tomato is susceptible to suffer severe losses. Breeding resistant varieties is the most successful way to control this disease. The objective of this study was to find genotypes resistant to bacterial wilt in the French West Indies, a lowland tropical region characterized by both hot seasons with cool nights and wet seasons with warm nights.
Twenty-four tomato varieties know to be resistant to P. solanacearum in the countries where they were selected (French West Indies, Taiwan, Japan, South Africa and Hawaii) and two control varieties, one susceptible: Floradel, and one tolerant: Caraibo, were sown in sterilized mould.
Three isolates of P. solanacearum isolated in the main Solanaceae growing areas of Martinique (our collection: PS6, PS8 and PS11) were used. Prior to each inoculation, the isolates were isolated from wilted plants previously inoculated, in order to keep them a high level of agressivity. They were then grown on Kelman medium for 72 hours and suspended in sterilized water (10^7 bacteria/ml). The inoculum was obtained by mixing the three bacterial suspension in equal proportions. The roots of three-weeks old tomato seedlings were contaminated by cutting them with a scalpel along one side of the plant to a depth of 10 cm, and 2 ml of the inoculum was poured over the injured roots. Plants were transplanted in field where air temperatures ranged between 30-32 deg.C day and 20-24 deg. C night during the crop season.
Bacterial wilt damage, expressed as percentage of wilted plants, was evaluated 3 weeks after inoculation. Results are presented in Table 1. Three groups of varieties are constituted according to their resistance against P. solanacearum: R (resistant), no wilting; MR (moderately resistant), 1-10% of wilting; S (susceptible), 11-100% of wilting. Among the 24 varieties tested, 15 are resistant to bacterial wilt (group R). However, 6 varieties of this group produce fruits with a mean weight lower than 45 grams. The striking result is that some of the varieties resistant in the countries where they were selected (CL5915 136D4 1 0 and CL5915 206D4 2 2, in Taiwan; Rodade in South Africa; 8SCR 15 14 42 and 8SCR 15 6 40 in Burkina Faso) are susceptible (group S) in Martinique. This may result from the effect of the environmental conditions on the severity of bacterial wilt or from the variability of the pathogenicity of P. solanacearum. The last hypothesis is supported by next observations that West Indies isolates are more aggressive than reference isolates (K60, K105 and K136 from the collection of the University of Wisconsin).
Following this preliminary selection of varieties resistant to bacterial wilt in West Indies, we intend to constitute a genetic pool of resistant varieties from diverse origins and in preference with a mean fruit weight higher than 45 grams. According to these criteria, the following varieties have been selected: CL9 0 0 1 3, CL1219 0 8 0, F1 King Kong, F1 Dynamo, IRAT L3, Caraibo and Hawaii 7996.
Table 1: Severity of bacterial wilt (caused by Pseudomonas solanacearum) in 25 wilt-resistant and one susceptible varieties, 3 weeks after inoculation with bacterial isolates from Martinique (French West Indies)
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Plants wilted^y (%) Bacterial
Variety Origin Plant^w Fruit^x 1985 1986 wilt
habit weight trial1 trial2 rating^z
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L 366 AVRDC-Taiwan D 50 0 NT NT R
L285 AVRDC-Taiwan D 15 0 NT NT R
CL8d 0 7 1 AVRDC-Taiwan D 50 0 NT NT R
CL9 0 0 1 3 AVRDC-Taiwan D 50 0 0 0 R
CL143 0 10 3 0 1 10 AVRDC-Taiwan D 35 0 NT NT R
CL1131 0 0 43 8 1 AVRDC-Taiwan D 30 0 NT 0 R
CL1219 0 8 0 AVRDC-Taiwan D 45 0 0 0 R
CL1351 1 9 10 0 AVRDC-Taiwan D 30 0 NT NT R
CL5915 93D4 1 0 AVRDC-Taiwan D 55 0 NT 0 R
F1Vanguard K.Y.S.-Taiwan I 115 NT 0 0 R
F1King kong K.Y.S.-Taiwan SD 150 NT 0 0 R
F1Dynamo Sakata-Japan I 85 NT 0 0 R
CRA66 INRA-F.W.I. D 30 NT 0 NT R
IRAT L3 IRAT-F.W.I. I 45 NT 0 NT R
Hawaii 7996 U.Florida-USA D 30 NT 0 NT R
CL949 0 8 0 AVRDC-Taiwan D 45 8 NT NT MR
CL1131 0 0 7 2 0 9 AVRDC-Taiwan D 35 5 10 5 MR
CL5915 206D4 2 5 AVRDC-Taiwan I 95 5 NT NT MR
Hawaii 1997 U.Florida-USA I 50 NT 5 0 MR
CL5915 136D4 1 0 AVRDC-Taiwan D 30 20 60 NT S
CL5915 206D4 2 2 AVRDC-Taiwan I 95 20 15 0 S
Rodade IRH-South Africa D 110 NT 25 10 S
8SCR 15 14 42 IRAT-Burkina Faso D 55 90 NT NT S
8SCR 16 6 40 IRAT-Burkina Faso D 60 85 NT NT S
Caraibo
(resistant Control) INRA-F.W.I. D 115 0 0 0 R
Floradel
(susceptible control) Petoseed-USA D 120 100 100 100 S
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w: D, determinate; I, indeterminate; SD, semi-determinatex: the mean fruit weight was determined on 30 fruits
y: for each variety, the mean percentage of bacterial wilt losses was calculated on 3 repeats of 10 plants (30 plants); NT: not tested
z: S: susceptible; MR: moderately resistant; R : resistant
K.Y.S., Know You Seed; F.W.I., French West Indies