The infection of the fruit of ‘Cavendish’ banana by Pseudocercospora fijiensis, cause of black leaf streak (black Sigatoka)

Abstract

Black leaf streak (black Sigatoka) caused by Pseudocercospora fijiensis (teleomorph Mycosphaerella fijiensis) is the most destructive leaf disease of banana world-wide. It is capable of destroying plantations unless controlled and successful commercial production is dependent on frequent applications of fungicides. A study was made to determine whether the pathogen is capable of infecting the fruit of ‘Cavendish’ banana and completing its life cycle on banana skin as the sole substrate. Emerging and recently emerged bunches of ‘Cavendish’ banana were inoculated in the field with carbendazim-resistant strains of P. fijiensis, incubated in situ in a humid environment for 6 days and then allowed to develop and mature naturally. The fruit were harvested approximately 16 weeks after inoculation, ripened to green tip stage, and skins incubated either as whole skins in humid containers or as thin strips of skin removed from the fruit and incubated on water agar modified with penicillin, streptomycin and carbendazim in Petri dishes. Supradermal fascicles of P. fijiensis were found on the incubated skin strips and whole skins within 5 days. From a total of approximately 300 field inoculated fruit, 18 colonies of P. fijiensis were detected. Small pieces of skin, each supporting a single fascicle, were excised, incubated individually on modified water agar and monitored for the production of conidia. It was shown that the fungus colonised, and was able to sporulate profusely on the excised skin pieces. This finding has implications for the transfer of strains of P. fijiensis between countries via commercial banana shipments and represents a potential pathway of entry to countries currently free of the disease.