Effect of dew point temperature and conidium age on germination, germ tubegrowth and infection of maize and sorghum by Peronosclerospora sorghi

The effect of the environment on the germination, survival and infection of sorghum by conidia of Peronosclerospora sorghi is unknown in Africa. Dew p oint temperature, and the effect of conidium age was characterized for an i solate of P. sorghi from Zimbabwe. Germination and germ tube growth took pl ace in the range 10-34 degrees C (optimal at 10-34 and 20-33 degrees, respe ctively). Infection was optimal at 14-30 degrees. Incidence of infection at different temperatures was correlated with germ tube growth (r = 0.8, P < 0.001). Germination and germ tube growth occurred from 5 h after commencing incubation of infected leaf material, although immature conidia harvested at 3 h caused a low incidence of infection. Plant age also affected the inc idence of infection. Container grown sorghum plants older than 20 d, and ma ize plants older than 15 d were resistant to systemic infection by conidia. The results indicate that germination, germ tube growth and infection take place over a wide temperature range, and that some limited biotypic variat ion may exist when these data are compared to other reports of the environm ental requirements of P. sorghi from the U.S.A. and India.