Effect of exogenous pH on development and growth of Colletotrichum musae and development of anthracnose in different banana cultivars in Sri Lanka

In the present study, the relationship between the observed inter-cultivar variation of anthracnose development in banana by Colletotrichum musae and fruit peel pH was investigated. The results of a survey showed that anthracnose does not develop on Mondan (ABB), a cooking-type cultivar. In contrast, the highest susceptibility to anthracnose was reported by the dessert cultivars Anamalu and Embon (AAA). Variations of the pH of fruit peel exudates (FPE) of different banana cultivars were monitored during the ripening stages. The effects of FPE, collected from different banana cultivars at two maturity stages, on germ tube formation and elongation and appressoria development of C. musae were also determined. Furthermore, the effects of culture medium pH on the growth of C. musae, anthracnose development and the expression of a virulence-related gene, pelB were determined. With ripening the pH of FPE and fruit pulp decreased. The highest pH of FPE was shown by cultivars Seenikehel, Alukehel and Mondan with pH values greater than 6.0 at mature-unripe stage. At the ripe stage, pulp of all cultivars had pH values less than 4.5 with the lowest being 3.9 in Embul. When grown in culture media at a pH range from 4.0 - 8.0, the highest mycelial growth of C. musae was observed at 4.5, indicating that an exogenous pH of 4.5 can favour colonization by C. musae. The expression of pelB gene was detected by RT-PCR only in C. musae grown in a culture medium having a pH of 4.5. In addition, soluble solid content of the fruit peel was quantified as a possible factor responsible for the variation in anthracnose development among different banana cultivars, and its effects on germ tube formation, elongation and appressoria formation are discussed.