Evaluation of resistance of tomato plants to fusarium wilt (Race 2): Three approaches

Developing resistant cultivars requires reliable screening methodologies. In this study, we compared the reliability of three tests for resistance of tomatoes to Fusarium oxysporum f. sp. lycopersici race 2. These tests were carried out using seedlings, 14 d old after sowing, 35 d old transplants or field tests with infested soil of one susceptible and nine resistant lines, either heterozygous [I2/+] or homozygous [I2/I2] (validated by using PCR marker). The root-dip inoculation method was used for the first two tests. In the susceptible line, 93-100% of the inoculated seedlings showed typical disease symptoms when 14 and 35 d old plants were used. Among the resistant lines, 3.3- 31.6% and 0-7% of the plants were diseased, respectively. In two field tests, none of the resistant genotypes showed severe symptoms, and seldom slight discoloration without wilting, occurred. Cuttings of asymptomatic (classified resistant) and symptomatic (classified susceptible) seedlings of a heterozygous line from the seedling test, were rooted, and seeds were extracted from ripened fruits. Comparisons using the seedlings evaluation test, revealed that up to 29.2% and 24.8% of the offspring originated from respective plants, developed disease symptoms, compared to the 25% expected disease incidence. It is concluded that disease symptoms in resistant plants, using the 14 d old seedlings inoculation test, do not necessarily indicate susceptibility, that this test is rather severe and that the 35 d old transplants test reflects better the response in infested fields. Seedling tests can be routinely used, but transplant tests should be used for final validation of resistance, when necessary.