A Queensland Department of Primary Industries (DPI) plant pathologist has assured graingrowers that sorghum ergot disease does not carry over from season to season in soil or stubble.

DPI principal plant pathologist Malcolm Ryley was responding to concerns of Darling Downs graingrowers about the possibility of carry-over sorghum ergot from last season's crops infecting 2003-04 season plantings.

Dr Ryley said sorghum ergot did not survive from season to season in sorghum stubble or as sclerotes in the soil, which meant zero-till operations did not exacerbate ergot problems.

He said sorghum ergot could survive year-round in honeydew on the heads of volunteer sorghum, Johnson grass, and columbus grass in frost-free areas such as the tropics.

The major source of reinfection of crop and weed hosts in southern Queensland during summer probably originated from these hosts because sorghum ergot spores could travel long distances on the wind.

Ergot could also survive on dead infected sorghum heads over winter in southern Queensland but the importance of this disease source was unknown.

"We're unsure at this stage if controlling sorghum weed species on a grower's property will reduce the buildup of ergot during the season.

"All sorghum weeds in southern Queensland would probably have to be killed to have a major effect."

Dr Ryley said a Grains Research and Development Corporation-supported project in collaboration with CSIRO was investigating the roles of different ergot sources in the spread of the disease.

He said sorghum ergot infected only grain, forage and weed sorghums, not paspalum or winter cereals like wheat and barley.

Dr Ryley said adverse weather during flowering was the major risk factor for the development of sorghum ergot.

Crops at risk from ergot infection were those that were poorly pollinated, or tillering grain sorghum crops, forage sorghums and ratoon crops.

"All commercial grain sorghums varieties are susceptible to ergot infection during cool, wet weather conditions such as those which we experienced at the end of the 2002-03 season.

"This means it's not yet possible to use varietal resistance to minimise the problem."

He said some breeding lines from Africa had good resistance to ergot and DPI plant breeders were transferring this resistance to high-yielding grain sorghum varieties.

Sorghum ergot made harvesting difficult and could reduce yields. The presence of ergot sclerotes in stock feed had toxicity and nutritional implications for some animals.

He said detailed information on ergot was available in a DPI leaflet called "Biology, toxicity and management of ergot in sorghum", which was distributed to every graingrower in the northern grain belt by the GRDC.