The reactions of susceptible and resistant tomato cultivars to strains of Verticillium albo‐atrum

Abstract

SUMMARYThree strains of Verticillium albo‐atrum causing severe wilt of tomato (T), progressive (Hp) and fluctuating (HF) wilt of hop, were inoculated through the roots of four tomato cultivars at different inoculum concentrations. Symptoms were assessed visually 42 days after inoculation, and quantitatively on the change in total leaf area compared with controls. Distribution of mycelium and tyloses was determined by sections at 2 cm intervals of root, stem and petiole.Cultivars Loran Blood and Moscow showed resistance to disease expression at all levels of inoculum concentration with the T strain. Bonny Best and Potentate were both susceptible to this strain, but whereas in Potentate, disease severity increased from mild to severe with increase in inoculum concentration, Bonny Best was severely diseased at the lowest level of inoculum. All cultivars showed some susceptibility to the HP and HF strains; the ‘resistance’ of Loran Blood and Moscow was no longer apparent and Bonny Best was most severely affected. The relative susceptibilities to the strains wereHF Bonney Best > Loran Blood > Potentate > Moscow,HP Bonny Best > Loran Blood, Moscow > Potentate,T Bonny Best > Potentate > Loran Blood, Moscow.In general, vascular colonization was less in the cultivars Loran Blood and Moscow with all three fungal strains at io5propagules/ml level of inoculum, but this was not always correlated with an increase in disease severity. With the exception of the host‐pathogen combinations Bonny Best/T, Bonny Best/HF, Potentate/T and Moscow/T, increasing the inoculum concentration to 107propagules/ml increased disease severity but had little or no effect of increasing vascular colonization. In Bonny Best/T, Bonny Best/HF and Potentate/T vascular colonization was reduced with the higher level of inoculum. Moscow showed complete resistance to symptom expression and little vascular colonization with the T strain at 105prop./ml. At 107prop./ml resistance was maintained but there was very extensive growth of mycelium in the vessels.Tylosis resulted from an interaction of host, fungal strain and the level of inoculum and was not always correlated with the degree of vascular colonization. Contrary to previous reports the resistant varieties Loran Blood and Moscow developed acute disease symptoms after inoculation with HP and HF and these were associated with a high level of tylosis rather than mycelial growth. Tylosis and disease severity but not mycelial growth increased with higher levels of inoculum.The results suggested that susceptibility to Verticillium wilt was a complex response depending on host cultivar, fungal strain and the initial inoculum concentration. In some cultivar‐pathogen combinations susceptibility was directly proportional to the amount of mycelium present in the vessels, while in others a physiological resistance mechanism independent of the degree of colonization appeared to operate. In a third category, increased disease development rather than resistance was associated with high levels of tylosis.