Colonization of tomatoes by Verticillium dahliae: Determinative phase II

To investigate the relationship between stem colonizing ability of Verticillium and wilt disease development, the colonization capacities of Verticillium dahliae, race 1 or race 2, were compared in near-isolines of the cultivar Craigella, with (Vet) and without (Ve(-)) Ve-gene resistance, and in the cultivar IRAT-L3, which also lacks the Ve-gene. Only the Ve(+)/V.d.1 interaction was resistant to disease. Lateral and vertical invasiveness of the pathogen were assessed cytologically and fungal biomass was monitored by PCR-based diagnostics. The pathogen was most aggressive in the Ve(-)/V.d.1 interaction; colonization capacity was reduced in the other five combinations, but most severely, and equivalently, in Ve(+)/V.d.1, IRAT/V.d.1, and IRAT/V.d.2. Further study of the development and role of the vascular coating response in IRAT-L3 indicated a pattern of expression similar to that observed in other susceptible plants, and it was concluded that the reduced colonization of this cultivar by V. dahliae resulted from another unknown defensive mechanism. Stem colonizing ability that is compatible with resistance in one cultivar and race combination may promote symptom expression and even death in another.