Colonisation of resistant and susceptible bean tissue by Phaeoisariopsis griseola (Sacc.) Ferr

Abstract

Fungal development and the associated cellular reactions in three bean (Phaseolus vulgaris L.) cultivars/lines inoculated with Phaeoisariopsis griseola isolate PG18 were studied. Differences in the extent of fungal development and host cell integrity were used to compare the reactions of the three cultivars/lines (resistant, intermediate resistant and susceptible). Conidia of P. griseola germinated 4 hours after inoculation by producing germ tubes at either one, both tips or at the sides. However, germination in the resistant bean line M26 was slightly inhibited and most of the conidia had not germinated 4 hours after inoculation. Penetration occurred 24 hours after inoculation and was either direct or through the stomata. On M26, minute brown flecks were observed on the inoculated area 4 days after inoculation, whereas in cv Rosecoco-GLP-2 (susceptible) and line M29 (intermediate resistant), development of conspicously septed hyphae was observed. A prolonged biotrophic phase was noted in cv Rosecoco-GLP-2 and line M29 and lesions appeared on the 6th and 10th days, respectively, characterised by conspicously septed hyphae. Such septations were not observed in hyphae of the same isolate in slide culture. A transverse section of the infected tissue in cv Rosecoco-GLP-2 and line M29 taken 9 days after inoculation showed cell disintegration whereas the cells in line M26 were intact. The fungus only colonised the lower epidermal and spongy mesophyll layer. Sporuration in cv Rosecoco-GLP-2 and line M29 occurred 8 and 12 days after inoculation, respectively, but did not occur in line M26. The number of synnema per lesion and number of conidiophores per synnemata in cv Rosecoco-GLP-2 was significantly (P=0.05) higher than in line M29. Thus, resistance in bean line M26 to angular leafspot is probably due to inhibited germination, colonisation and sporulation whereas the intermediate response in line M29 could be due to delayed and limited sporulation of the fungus