Fruit-spot (‘speckle’) of Jamaican bananas caused by Deightoniella torulosa (syd.) ellis : III. Spore formation, liberation and dispersal

Abstract

The air in two Jamaican banana plantations was sampled during the period 19 July–5 October 1960 at 3 m. above ground with a Hirst automatic volumetric spore trap. Each day's slide was scanned and conidia of Deightoniella torulosa were counted at positions representing 2-hourly intervals. In both plantations there was a clearly defined diurnal periodicity, the peak concentration of spores/m. 3 of air normally occurring near 08.00 hr. and then decreasing to a low level between 12.00 and 24.00 hr. each day. The period of most rapid liberation (06.00–10.00 hr.) corresponds with that of rapidly increasing temperature and decreasing R.H. This may be due to a previously undescribed active discharge mechanism operating under conditions of decreasing vapour pressure. In plantation A, which was surface irrigated, daily mean concentration varied between o and 58 spores/m. 3 when the previous 2–12 days were rainless. After 0·55 and 2·55 in. rain, concentrations of 377 and 536 spores/m. 3 , respectively, were recorded : the highest 2-hourly estimate recorded was 5280 spores/m. 3 . In plantation B, where under-tree irrigation was practised, successive irrigation periods (each of 48 hr.) were associated with considerable increases in spore concentration; rainfall resulted in similar large increases. The highest daily mean concentration recorded was 7115 spores/m. 3 , while the highest 2-hourly estimate was 72,335 spores/m. 3 at 08.00 hr. on 8 September—approximately 12 hr. after cessation of irrigation. As the plantation dried out after irrigation or rainfall, spore concentration decreased rapidly within 2–3 days. Both rainfall and irrigation spray resulted in heavy sporulation of D. torulosa on banana-leaf ‘trash’; this helps to explain the increased atmospheric spore concentrations referred to above. There was a marked increase in incidence of fruit-spot during under-tree irrigation periods and when rainfall caused the plantation r.h. to remain high for more than 12 hr. during the day. Incidence of fruit-spot in plantation B was higher than in A; this was related partly to the greater numbers of dead banana leaves in the former, and partly to the favourable effects of under-tree irrigation on sporulation, spore germination and infection.