Evaluation and identification of basil germ plasm for resistance to Fusarium oxysporum f. sp. basilicum

Growth chamber evaluation of several cultivars of basil and related herbs examined in the United States revealed that identical cultivars from different sources did not differ in their reactions to artificial inoculation with Fusarium oxysporum f. sp. basilicum. Cultivars differed in susceptibility to the pathogen: "Spicy globe" miniature was the most susceptible, and lemon basil (Ocimum basilicum var. citriodorum), Origanum majorana, and Thymus vulgaris were rated as not susceptible. Twenty isolates of F. oxysporum, originating from stems of diseased basil plants in Israel, were pathogenic on basil in growth chamber and greenhouse tests. Under artificial inoculation, 2 isolates of F. oxysporum f. sp. basilicum from stems were pathogenic to basil but not to 9 species representing Lamiaceae, Cucurbitaceae, Solanaceae, and Compositae, indicating the specificity of the pathogen to basil. These isolates were used for additional resistance tests. Ocimum basilicum var. purpurascens (Exotic) and var. citriodorum were rated as not susceptible to the pathogen under artificial inoculation. Resistant germ plasm was identified in several basil plants of a local variety originally introduced from the United States and reselected at Newe Ya'ar. Seeds were planted in the greenhouse in naturally highly infested soil. Symptomless plants that survived in naturally infested soil were the source for F1 seeds of resistant germ plasm, which was confirmed by artificial inoculations with both isolates of the pathogen. Further selection tests to improve resistance were conducted up to the F4 generation in infested soil in the greenhouse. All individuals of the present genetic line remained symptomless, while all individual plants of the original susceptible cultivar defoliated 3 weeks after planting into infested soil, suggesting that the resistance may be a single, dominant gene. The causal organism was reisolated only from the susceptible plants and not from the symptomless resistant plants through all the experiments.