The nature of tolerance to Fusarium oxysporum f. sp. lycopersici in polygenically field-resistant marglobe tomato plants

Abstract

Plant-to-plant variation in the level of resistance to Fusarium oxysporum f. sp. lycopersici (Fol) is great within the polygenically resistant tomato cultivar Marglobe, a variation which precludes the use of seeded plants to obtain statistically valid data with respect to resistance mechanisms. The number of individual plants can be increased by vegetative propagation, however, to permit statistical analysis of experimental results. Polygenically tolerant or field resistant Marglobe tomato plants were therefore propagated vegetatively and inoculated with race 1, of Fol. Consistent levels of disease expression were found within these vegetative clones. Statistically significant differences in symptom expression were found between clonal families, indicating that these differences were genetically based. Three clones with consistently low, and three with consistently high levels of resistance to race 1 were then selected for further study. Disease development in these clones paralleled the extent of fungal colonization of vascular tissues in stems and petioles. Vascular colonization was markedly higher in susceptible than in resistant plants (P=0·001). Foliar symptom expression was ultimately related to petiolar infection and petioles of symptomatic leaves were always infected. Symptomless leaves were either entirely free of fungal infection or only the lateral xylem bundles were infected, but the central xylem bundle was free of infection. A colonization quotient (Qco1) was calculated for each clone/Fol race 1 interaction to obtain a quantitative expression of the rate at which the vascular system of plant stems had been colonized. Variable levels of resistance to Fol race 1 in plants of the cultivar Marglobe apparently resulted from different levels of success in restricting colonization within vascular tissues. It is concluded, therefore, that the apparent tolerance to vascular infection of seeded plants of the polygenically resistant cultivar Marglobe results from the segregation and recombination of the resistance genes within that population. On average, such a population is moderately resistant and produces a reasonable crop in a wilt-prone field. That diversity, based on segregation and recombination of the polygenes, does, however, serve to protect the polygenic plant population from cataclysmic inroads by any one of the many selected races derived from a genetically malleable population of the pathogen. The results suggest, also, that this genetic diversity results in different levels of recognition and response to the pathogen and, thus, results in differences in the degree of localization of Fusarium infections within the polygenic population.