41 Evaluation of Resistance of Tomato Plants to Fusarium Wilt (Race 2): Three Approaches Y. Rekah a , H.D. Rabinowitch and J. Katan Faculty of Agricultural, Food and Environmental Quality Sciences The Hebrew University of Jerusalem Rehovot 76100 Israel Abstract 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 [I 2 /+] or homozygous [I 2 /I 2 ] (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 sympto- matic (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. INTRODUCTION The I2 gene codes for resistance to the soilborne pathogen Fusarium oxysporum f. sp. lycopersici (Sacc.) Snyd. & Hans. race 2 (FOL2) in tomato, Lycopersicon esculentum Mill.. This gene has been introgressed to the domesticated plant from the wild species L. pimpinellifolium (Sarfati et al., 1989; Stall and Walter, 1965). Retig et al. (1967) and Alon et al. (1964) attributed the higher disease incidence than expected, of heterozygous tomato plants, to incomplete penetrance of the gene I which confers resistance against race 1 of Fusarium. This response of the heterozygous plants might be affected by factors such as inoculum concentration, temperature or seedlings age at time of inoculation. The common method for evaluating resistance is by dipping 10-14 d old seedlings in an inoculum suspension of high concentration of conidia (Alon et al., 1973; Laterrot, 1976). Vascular discoloration is a main diagnostic criterion for evaluation of resistance in seed- lings, but it may occur also to certain extent in dominant genotypes, thus may lead to false evaluation (Rodrigues-Molina et al., 1995). The International Union for the Protection of New Varieties of Plants (UPOV) recognizes the diagnostic limitations and allows up to 10% disease incidence in plants of resistant cultivars but the percentage of susceptible plants might exceed 10%. The objectives of this study are to examine whether the common evaluation procedure of resistance by the seedling test reflects regular response under field conditions and to assess an additional method to improve the evaluation of resistance of tomato cultivars. a rekah@agri.huji.ac.il Proc. II nd Intl. Symposium on Tomato Diseases Eds.: H. Saygili et al. Acta Hort. 808, ISHS 2009