Infection of Subterranean Clover (Trifolium subterraneum) by Kabatiella caulivora

Abstract

Kabatiella caulivora is a serious pathogen of clover (Trifolium) spp. Subterranean clover (T. subterraneum) cv. Woogenellup was inoculated with K. caulivora, to study the attachment and germination of conidia, germ‐tube penetration of the plant surface, and histochemistry and ultrastructure of changes in the host associated with lesion development. The foliar architecture caused the conidia to concentrate at the base of leaflets and on the petiolules (between the leaflets and petioles). Epidermal cells immediately beneath conidia and, occasionally, also adjacent cells developed a yellow‐brown discoloration 1 day post‐inoculation. Penetration appeared to be directly through the cuticle, characterized by constricted hyphae at the point of entry. No appressoria were observed. In leaves, invasion was restricted to the area proximal to the petiolule and leaf mid‐rib. In petioles and petiolules, the hyphae initially remained between the epidermal cells and first layer of mesophyll cells before moving intercellularly through the mesophyll tissue towards phloem tissues. The cuticle was occasionally degraded in petiole and petiolule infections, the loss of epidermal and mesophyll cell wall components was detected, and chloroplasts and starch grains were disrupted. Plants developed macroscopic symptoms 10–11 days post‐inoculation with necrotic lesions occurring on leaves, petioles and petiolules. Sporulation occurred approximately 15–18 days post‐inoculation when affected plants collapsed. This information may be useful for breeding programmes aimed at selecting varieties with improved resistance to the clover scorch disease.