Studies on the Colonization of Plant Roots by Verticillium dahliae Klebahn With Use of Immunofluorescent Staining

Abstract

The colonization of plant roots by Verticillium dahliae was studied with use of fluorescent antibodies to stain the fungus in soil and on plant roots. Microsclerotia were observed germinating in rhizosphere soil and in soil amended with nutrients. Germination occurred much earlier in soil amended with either a sugariamino acid solution or a solution of wheat root extract than in soil amended with distilled water. Usually only a single germ tube was observed, but some microsclerotia produced two, three or more initials. These grew into hyphal strands up to 100 μm long, but most began to lyse when the length exceeded 30 μm. Hyphae of V. dahliae were observed on the roots of both immune (wheat) and susceptible (common thorn-apple, cotton and Noogoora burr) plants growing in field soils infested with microsclerotia. These hyphae were sparse, up to 1500 μm in length and appeared to enter the cortex of the young roots directly, without the aid of specialized penetrating structures such as appressoria. Sporulating hyphae were observed on the rhizoplane of each of the four species examined. Conidial germination, however, was not observed on the rhizoplane. By cultural methods it was established that V . Dahliae colonized plant roots 2-3 cm back from the root tip, suggesting that microsclerotia are stimulated to germinate by the advancing root. Examination of the rhizosphere soil around the root cap and the zone prior to vascular differentiation failed to produce evidence of conidia or other structures that may have functioned as an intermediate stage in the colonization of plant roots by V. dahliae.