Variability of Ascochyta fabae in South Australia

Abstract

Fifty-two isolates of Ascochyta fabae were established from 23 collections made in 3 States of Australia and were purified through 2 cycles of single-spore isolation. The isolates were evaluated for spore size, spore production, colony diameter, aerial mycelium, and pycnidia production. Variation in all of these traits among related single-spore cultures was comparable to that among unrelated ones and only colony diameter varied significantly among isolates. Spore size was 3–6 by 10–26 µm.

Eight of these 52 isolates were chosen for further investigations of pathogenicity characteristics using 8 populations of faba bean. Plants were scored daily for rate of appearance of symptoms and then 15 and 21 days after inoculation for lesion size and number, production of pycnidia on the lesions and overall disease score. Leaves and stems reacted differently to the disease, with one isolate producing many leaf lesions but few stem lesions on one bean accession but many stem lesions on another. Lesion size was not strongly correlated with the other measures of disease. Resistant accessions had longer incubation periods, fewer total lesions and fewer pycnidia-producing lesions than susceptible accessions. The 8 isolates on the 8 bean accessions showed 7 distinct patterns of resistance. The results showed that in southern Australia, A. fabae exhibited great variability which was incompatible with classification into biologically meaningful pathotypes.