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Factors Affecting Penetrance of Resistance to Fusarium oxysporum f. sp. lycopersici in Tomatoes. H. Alon, Department of Plant Pathology and Microbiology, The Hebrew University of Jerusalem, Rehovot, Israel; J. Katan(2), and N. Kedar(3). (2)(3)Department of Field and Vegetable Crops, The Hebrew University of Jerusalem, Rehovot, Israel. Phytopathology 64:455-461. Accepted for publication 23 September 1973. DOI: 10.1094/Phyto-64-455.

Resistance of tomatoes to Fusarium oxysporum f. sp. lycopersici is based on the action of a single dominant gene (I). Penetrance of resistance is incomplete, however, since various proportions of diseased seedlings are found in resistant lines. Penetrance is affected by the interaction between host genotype and inoculum concn, seedling age, or soil temp. As a general trend, an increase in inoculum concn causes a decrease in penetrance, i.e., an increase in disease incidence, but the susceptible cultivar 'Marmande' (ii) was affected at much lower concns than the homozygous (II) 'Roma VF' or 'Homestead 24'. The heterozygous (Ii) hybrids showed a lower penetrance than their respective homozygous resistant parents, and resistance in Roma VF was of lower penetrance than that in Homestead 24. Penetrance in the resistant cultivars was the lowest when seedlings were at ages of 2 or 5 days postemergence, whereas Marmande showed a complete susceptibility at all ages tested. At soil temp ranging 17-20 C, disease incidence among resistant cultivars and hybrids was higher than that at 27-30 C; the opposite was true for Marmande. Diseased seedlings of heterozygous resistant plants were less affected than diseased plants of Marmande in terms of disease progress, water conductivity in the vessels, population level, and distribution of the pathogen in various plant parts.