PRODUCTION OF CONIDIA BY PERONOSCLEROSPORA-SORGHI ON SORGHUM CROPS INZIMBABWE

Factors affecting the production of conidia of Peronosclerospora sorgh i, causing sorghum downy mildew (SDM), were investigated during 1993 a nd 1994 in Zimbabwe. In the field conidia were detected on nights when the minimum temperature was in the range 10-19 degrees C. On 73% of n ights when conidia were detected rain had fallen within the previous 7 2 h and on 64% of nights wind speed was <2.0 m s(-l). The time period over which conidia were detected was 2-9 h. Using incubated leaf mater ial, conidia were produced in the temperature range 10-26 degrees C. L ocal lesions and systemically infected leaf material produced 2.4-5.7 x 10(3) conidia per cm(2). Under controlled conditions conidia were re leased from conidiophores for 2.5 h after maturation and were shown to be well adapted to wind dispersal, having a settling velocity of 1.5 x 10(-4) m s(-l). Conditions that are suitable for conidia production occur in Zimbabwe and other semi-arid regions of southern Africa durin g the cropping season.