STUDIES ON CERCOSPORA AND ALLIED GENERA

Abstract

Five species of pathogens were identified and confirmed on the 3 host plants studied. They were Cercospora balsaminiana and Cercoseptoria balsaminicola on leaves of Impatiens balsamina; Cercospora canescens and Pseudocercospora cruenta on leaves of Vigna unguiculata and Cercospora gloriosicola on leaves of Gloriosa superba. This is the first report on the occurence of C. canescens on V. unguiculata in Singapore. Isolation of these fungi into pure cultures was successful by the single-spore isolation technique. Such cultures sporulated on v-8 and TLDA. The hyphal-tip method resulted in non-sporulating cultures. Each of the 5 species in cultures showed characteristic colony morphology, which for each did not differ much on either v-8 or TLDA.

The growth rate of the 5 species differed but in general, growth was extremely slow. The increase in colony diameter was about 0.7 to 3.5 mm per day depending on the species. Variations in temperature and light produced marked effect on growth and sporulation o of these fungi in culture. Room temperature of 27-300C, tended to favour vegetative growth compared to the lower minimum temperature o range of 21-300C. Sporulation of most species was enhanced under continuous blacklight, 12 hours alternating blacklight and darkness or continuous darkness, but poor or totally inhibited under diurnal light condition or continuous fluorescent light. Conidia produced in cultures were longer than those from naturally infected leaves; and conidia produced in cultures kept in continuous darkness were relatively shorter than those produced under light though the width was not affected to any significant degree. In pathogenicity tests, leaves of balsam developed lesions about 1-2 weeks after inoculation by spore suspensions of C. balsaminiana. Older and wounded leaves were more susceptible than younger and unwounded ones. Leaves of Gloriosa were not susceptible to infection by C. balsaminiana.