Fruit-spot (‘speckle’) of Jamaican bananas caused by Deightoniella torulosa (Syd.) Ellis : II. Factors affecting spore germination and infection

Abstract

Some factors affecting germination of conidia of Deightoniella torulosa have been studied. In tap water on glass or on the surface of banana fruits, approximately 50% germination occurs after 1hr. incubation at 28–30 ° C. After 4–6 hr., nearly 90% of conidia have germinated. Germination does not occur below 95% R.H. : percentage germination in a water film is markedly higher than that at 100% R.H. Using a series of citrate-phosphate buffers, the optimum pH for germination and germ-tube elongation is in the range 4–6. Substances present in water extracts of banana leaves and fruit peel stimulate germination, as do drops of distilled water that have remained on the surface of young banana fruits for 24 hr. Solutions of several compounds present in synthetic nutrient media provide better germination than does distilled water; some carbohydrates tested are particularly favourable. The longevity of D. torulosa conidia is dependent on external humidity. Viability is lost within 3–4 days at R.H. < 95%. At 98% R.H. , a small number of conidia may be viable after 10 days storage. The severity of fruit-spot caused by D. torulosa increases progressively with advancing maturity of fruit. Spotting tends to be most intense near the tips and on the inner sides of individual fingers. Presence of dew or rain water appears to be essential for successful infection by D. torulosa . Experimental inoculations have shown that resistance to infection increases to some extent as the fruit matures. The severity of fruit-spot undergoes seasonal variation, being highest after the rainy periods during April–May and September–October. D. torulosa sporulates on diseased, dead or dying leaves, withering flower parts and on transition leaves and bracts. The relative importance of these inoculum sources in relation to fruit-spot epidemiology is discussed. Removal of dead or dying ‘trash’ leaves from the plantation at frequent intervals is recommended. Isolates of D. torulosa from fruit-spots are able to cause a spotting of banana leaves (‘black-spot’) and a rotting of the tips of banana fruits (‘black-tip’). Conversely, isolates from ‘black-spot’ and ‘black-tip’ lesions may cause fruit-spot. A black scab-like fruit-spot, also caused by D. torulosa , occurs in a few widely separated plantations.