THE ROLE OF THE COATING RESPONSE IN CRAIGELLA TOMATOES INFECTED WITH VERTICILLIUM-DAHLIAE, RACE-1 AND RACE-2

In tomato, near-isolines are available that differ in the presence (Ve(+)) or absence (Ve(-)) of the Ve-gene conferring resistance to wilt. Two races of Verticillium dahliae Klebahn have been identified, V.d.1 and V.d.2. Craigella Ve(-) v. V.d.1 or V.d.2 and Craigella Ve(+) v. V.d.2 are susceptible interactions, but Craigella Ve(+) is resistant to V.d.1. Among factors determining susceptibility or resistance to verticillium wilt are those governing colonization. One aspect of colonization is the ability of the fungus to escape laterally from trapping site vessels. These four host-parasite combinations were examined after infusion of conidia into the petiolar xylem vessels to compare early host reactions which are elicited by V.d.1 and V.d.2 and affect lateral penetration capacity. Samples were quantified by light microscopy al times from 4 to 24 h post-infusion for suberin coating responses and lateral escape by hyphae. The chemical nature, pattern of development and role of vascular coating in plant defence (i.e. blocks lateral penetration) appear to be similar in tomato infected with V. dahliae or V. albo-atrum. However, the effectiveness of this defence response evidently is compromised in the V. dahliae pathosystems by the aggressiveness of the pathogen. This study along with previous analyses of Craigella infected with V. albo-atrum, suggests that the following factors correlate with susceptibility: a high capacity for horizontal spread of Verticillium and a high capacity for Verticillium to actively suppress at least one host defense response. (C) 1995 Academic Press Limited