Dispersal of conidia of Dothidella ulei from Hevea brasiliensis

South American leaf blight caused by Dothidella ulei occurs only in tropical America, on both indigenous and cultivated Hevea spp. The conidium (Fusicladium macrosporum) is a 1-septate, dry, air-borne spore about 40 × 7 μ, occurring on the abaxial surface of dry leaves in dense, powdery, olive-green masses, and with one or both cells collapsed. The conidia adhere to the surface of water droplets, becoming turgid, and are disseminated in splash droplets. A Hirst volumetric trap, placed within a prepared source in north-west Trinidad, showed a diurnal periodicity of conidial production, with a maximum at 10.00 h and minima at night or in the early morning. On rainless days there was also a minor peak at 20.00 h. Transient increases occurred after rain, most of which fell around noon. On wet days almost equal numbers of conidia were dispersed between 10.00 and 12.00 h. Large increases occurred in 87% of all rain showers between 09.00 and 13.00 h. After 13.00 h fewer rain showers caused such increases; the lowest (36%) was between 21.00 and 01.00 h. Twice as many were trapped on sunny days (> 9 h sun) at 09.00 h when there was full sunshine, compared with overcast days (< 5 h sun). A more clearly defined morning maximum occurred on relatively windy days, compared with calmer ones. Conidial sporulation became very low, or ceased, where rain fell below a mean of 3–4 mm per day for at least 20 days. Abundant sporulation occurred with a daily rainfall about twice this amount. The results support the belief that if Dothidella ulei appeared in Malaysia its spread would be rapid and its effects damaging.