Some factors affecting in vitro production, germination and viability of conidia of Corynespora cassiicola from Hevea brasiliensis

Abstract

The fungus Corynespora cassiicola causes the very destructive Corynespora leaf fall disease of Hevea brasiliensis. The present population of C. cassiicola from Sri Lanka includes highly, moderately and poorly sporulating isolates. The fungus sporulated freely on potato dextrose agar at 10 - 35 °C with a peak at 30 °C. Cultures showed maximum conidia production after an incubation period of 12 days. Conidia produced well-branched germ tubes from both apices. The percentage germination reduced significantly at 1.5 × 106 conidia / mL. Germination of conidia occurred at temperatures between 5 - 40 °C with the optimum between 15 - 35 °C. Free water was not essential for germination but promoted germination. Conidia remained viable when the relative humidity was above 90 % and the ultraviolet radiation (254 nm) inactivated conidia. The most favourable temperature conditions for germination of C. cassiicola conidia described above has been observed to be prevalent in the main rubber growing areas in Sri Lanka, particularly during the period of refoliation, making it favourable for the establishment of the disease.