Formation of conidial anastomosis tubes and melanization of appressoria are antagonistic processes in Colletotrichum spp. from apple

Abstract

Colletotrichum spp. cause two important apple diseases, i.e., bitter rot (ABR) and Glomerella leaf spot (GLS). The present study aimed to compare the development of conidial anastomosis tubes (CATs) of strains of Colletotrichum spp. originating from ABR and GLS. For that, conidia were distributed on polystyrene slides and detached apple leaves surfaces, and the development of CATs and pre-infection structures, was microscopically examined. CATs connections were quantified and categorized into three types: conidium-conidium (c-c), conidium-germ tube (c-t) and germ tube-germ tube (t-t). CATs were observed in half of the strains, and Colletotrichum nymphaeae markedly produced more c-c connections at a higher speed. The t-t connections were less often observed in all strains. CATs were also frequently found in strains of Colletotrichum fructicola from fruits, but not in those from leaves, or in Colletotrichum theobromicola. Conidial germlings that produced CATs on polystyrene also did on leaves. Nuclear transference was observed in CATs. Appressoria melanization and CATs development were found to be antagonistic processes. The possible contribution of CATs and consequences for increasing variability of Colletotrichum on apple are discussed.