Sorghum ergot (Claviceps africana) has had a significant
impact on seed production and breeders’ nurseries in Australia since it
was first found in 1996. In this paper, 3 distinct key development stages of
sorghum that are related to ergot infection were identified: flag leaf stage,
pollen starch accumulation stage, and flowering period. Relationships between
weather variables during these 3 stages and ergot severity as well as pollen
viability were analysed using observed data from 2 field trials, a serial
planting trial and a genotype trial, conducted at Gatton, Queensland. The
duration of the flag leaf stage and of the flowering period was estimated from
thermal time. An infection factor was introduced and calculated based on
hourly temperature during the flowering period. This infection factor and the
mean relative humidity at 0900 hours during the flowering period were the main
factors influencing ergot infection. Mean daily minimum temperature during
flag leaf stage also had a significant effect on ergot severity, although no
significant relation was found between this mean daily minimum temperature and
pollen viability. A linear regression model using the above 3 factors
accounted for 94% of the environmentally caused variation in ergot
severity observed in the genotype trial.