Fruit-spot (‘speckle’) of Jamaican bananas caused by Deightoniella torulosa (Syd.) Ellis : I. Symptoms of disease and studies on pathogenicity

Abstract

The external symptoms of a serious fruit-spot (‘speckle’) of Jamaican ‘Lacatan’ bananas are described. Spots are round, up to 4 mm. in diam., with a reddish brown centre and a characteristic dark green halo. Deightoniella torulosa was isolated with a high frequency from spotted tissue; no other fungus (or bacterium) was isolated with significant frequency. In pathogenicity tests, D. torulosa caused typical spots to develop on the fruit peel after 48 hr. After 72 hr. some spots had diameters of approximately 3 mm. Conidia of D. torulosa germinate after 1 hr. in a film of water on the surface of banana fruit peel. One or two appressoria are produced by each conidium. The appressorium develops a fine infection hypha which penetrates the cuticle and epidermal wall by mechanical means. This stage in infection was observed after 12–20 hr. Further tissue invasion occurs by hyphal penetration of cell walls. Shortly after infection, cell walls and contents develop a reddish brown colour and may contain large numbers of spherical inclusion-bodies. Similar stages in this infection process were observed on naturally infected fruit. These results are discussed in relation to ‘Johnston fruit-spot’, which occurs in Central and South America.