Patterns of release of the secondary conidia of Claviceps africana, the sorghum ergot pathogen in Australia

Abstract

Trials were conducted in southern Queensland, Australia between March and May 2003, 2004 and 2005 to study patterns of hourly and daily release of the secondary conidia of Claviceps africana and their relationships with weather parameters. Conidia were trapped for at least one hour on most (> 90%) days in 2003 and 2004, but only on 55% of days in 2005. Both the highest daily concentration of conidia, and the highest number of hours per day when conidia were trapped, were recorded 1–3 days after rainfall events. Although the pattern of conidial release was different every day, the highest hourly conidial concentrations occurred between 10·00 hours and 17·00 hours on 73% of all days in the three trials. Hours when conidia were trapped were characterized by higher median values of temperature, windspeed and vapour pressure deficit, lower relative humidity, and leaf wetness values of 0%, than hours when no conidia were recorded. The results indicate that fungicides need to be applied to the highly ergot‐susceptible male sterile (A‐) lines of sorghum in hybrid seed production blocks and breeders’ nurseries as soon as possible after rainfall events to minimize ergot severity.

Sorghum ergot can develop without local Claviceps africana inoculum from nearby infected plants

“…Although there was no attenuation within 200 m, the abundance of aerial inoculum did decline when the closest likely inoculum source was more than 500 m away. Using spore trapping Ryley & Chakraborty (2008) failed to detect diurnal periodicity in the patterns of secondary conidia release in C. africana from nearby infected sorghum crops, although a previous study in Zimbabwe had reported a late afternoon peak (Frederickson et al ., 1993). One likely explanation is that not all secondary conidia trapped were from the local source and trapped conidia were released into the air from local and/or distant sources over a period of time.…”

“…Assessment of infected spikelets within 7–9 days of exposure further ensured that only infections from field inoculum trapped during an exposure were considered. However, secondary conidia of C. africana are often dispersed in clumps (Ryley & Chakraborty, 2008) and the assumption of a single infection resulting from a single conidium may have underestimated their abundance.…”

“…Although moisture related weather variables significantly influenced spikelet infection in this study, these were not associated with rainfall events and very little rain fell during the 2003 and 2004 exposure periods. A recent spore trapping study at two field sites over 3 years have shown that high levels of conidia were trapped during periods of higher median temperatures, wind speed and vapour pressure deficit (Ryley & Chakraborty, 2008). Spore release events were not directly related to rainfall.…”

Ergot: from witchcraft to biotechnology

“…Nevertheless, it seems to be beyond any doubt that climatic conditions, such as low temperatures and humid weather before and during flowering, contribute to ergot outbreaks and the rapid distribution of the disease (see, for example, Bandyopadhyay et al. , 1998; Ryley and Chakraborty, 2008; Workneh and Rush, 2006).…”

“…However, the means by which C. africana spread to Brazil in 1995 and to Australia in 1996 still remains to be elucidated (Bandyopadhyay et al, 1998;Chakraborty and Ryley, 2008;Pažoutová and Frederickson, 2005). Nevertheless, it seems to be beyond any doubt that climatic conditions, such as low temperatures and humid weather before and during flowering, contribute to ergot outbreaks and the rapid distribution of the disease (see, for example, Bandyopadhyay et al, 1998;Ryley and Chakraborty, 2008;.…”