On the relation between weather variables and sorghum ergot infection

Sorghum ergot (Claviceps africana) has had a significant impact on seed pro duction and breeders' nurseries in Australia since it was first found in 19 96. In this paper, 3 distinct key development stages of sorghum that are re lated to ergot infection were identified: flag leaf stage, pollen starch ac cumulation stage, and flowering period. Relationships between weather varia bles during these 3 stages and ergot severity as well as pollen viability w ere analysed using observed data from 2 field trials, a serial planting tri al and a genotype trial, conducted at Gatton, Queensland. The duration of t he flag leaf stage and of the flowering period was estimated from thermal t ime. An infection factor was introduced and calculated based on hourly temp erature during the flowering period. This infection factor and the mean rel ative humidity at 0900 hours during the flowering period were the main fact ors influencing ergot infection. Mean daily minimum temperature during flag leaf stage also had a significant effect on ergot severity, although no si gnificant relation was found between this mean daily minimum temperature an d pollen viability. A linear regression model using the above 3 factors acc ounted for 94% of the environmentally caused variation in ergot severity ob served in the genotype trial.