Infection occurs during flowering when spores of the fungus land on the feathery stigmas of flowers in sorghum heads.
If the flower has been fertilised by pollen it resists infection and normal seed will develop. If the flower has not been fertilised, the spore germinates and grows into the unfertilised seed (ovary) and the ovary is rapidly replaced by a fungal mass.
About seven days after infection, sticky honeydew oozes out of the flowers and drips onto leaves and the ground.
When the weather is wet or humid, the honeydew turns white due to the production of the infective spores just above the surface of the honeydew. Ultimately (near grain maturation), the fungal mass develops into a hard fungal body - the sclerote.
Occasionally the developing sclerote can be overgrown by the black fruiting body of another fungus called cerebella. The fungus can survive year-round in honeydew on other sorghum hosts, such as Johnson grass (halepense) and columbus grass (almum).
The ergot which affects sorghum does not infect paspalum or winter cereals such as wheat and barley. Although ergot spores can survive in honeydew on sorghum seed, the fungicide thiram that is used routinely on sorghum planting seed will kill the spores.
Sorghum ergot does not survive from season to season in sorghum stubble or as sclerotes in the soil.
[Source: Sorghum disease management bulletin, Qld Department of Agriculture and Fisheries.]
There is no known resistance to ergot in Australian grain sorghum hybrids and fungicidal control is not practicable due to the high cost of effective fungicides.
Therefore management relies on good agronomic practices. If ergot develops in a crop the honeydew on the seed heads makes harvesting difficult. Honeydew is modified plant sap, so it will be produced from infected heads for as long as the plant is alive.
If planting outside the recommended window, growers need to aim for an evenly flowering crop to ensure there are large quantities of viable pollen during flowering, because flowers fertilised by pollen are resistant to infection by the ergot spores.
Pre-harvest spraying of grain sorghum crops with a herbicide registered for that practice (eg. glyphosate) is commonly used to aid in rapid plant dry down and harvesting. In ergot-affected crops it will also assist in stopping the production of honeydew.
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