HOW WET will it be next spring? That's the pick-a-box riddle that points to whether fungicide spraying is likely to be required or effective, according to the results of trials run with the Mingenew-Irwin group, 100 km south-east of Geraldton, WA.
"A wetter spring generally increases the yield potential of a crop and the crop's disease level. This combination is more cost-effective to spray," says AGWEST scientist J atinderpal Bhathal, who has overseen the trials.
Dr Bhathal and the Mingenew-Irwin group started the trials when growers' wheat crops were affected by yellow spot and powdery mildew late in the growing season. A later epidemic of leaf rust in WA led them to expand the trials to test options for treating leaf rust.
When to spray
According to Dr Bhathal, the trials demonstrated that a single spray treatment at flag emergence can be effective in controlling leaf rust, increasing yields from 2.7 t/ha to 3.5 t/ha for Ajana wheat (in 1999). "But it is crucial to achieve control before there is significant disease on the top two leaves and this means spraying when the crop is producing flag leaves and before booting stage," Dr Bhathal said.
However, the trials demonstrated that fungicides are economic only in higher-yielding crops, with expected yields higher than 3 t/ha in the case of wheat.
Going after yellow spot
The same single spray treatment was also effective in controlling yellow spot. As a rule of thumb, it is most economic to spray for yellow spot when more than 100 mm of rainfall is expected in the two months after flag leaf emergence.
With severe early infections of yellow spot unlikely to be economically controlled by fungicides, Dr Bhathal points out that it is very important to reduce early disease risk through crop rotations.
Trials indicated that even a one-year wheat-lupin rotation reduces the risks from early severe disease in stubble retention systems, by avoiding the build-up and transfer of pathogens from one crop to the next.
Mingenew-Irwin grower Clancy Michael of 'Geraldine' says the trial results and advice from Dr Bhathal's research at the time led him to spray 500 ha of his wheat crop for powdery mildew. "We achieved good control and a grain yield of 3.2 t/ha, but I would consider spraying some varieties earlier than we did in that season," says Mr Michael.
Spraying in conjunction with a herbicide spray is also a strategy that Mr Michael would consider in future crops to improve cost-efficiency, particularly if he was unable to avoid planting some of the less disease-resistant varieties.
Look after those top leaves
According to Dr Bhathal, the aim of any fungicide spraying program is to keep the top two or three leaves on wheat green as long as possible for good grain fill.
"To this end, a two-spray program may occasionally be required if a crop is infected early by leaf rust, or in cooler areas where the growing season is longer," Dr Bhathal said.
Planting disease-resistant varieties of wheat saw a predictable decline in the value of spraying, with a single spray for leaf rust at flag emergence increasing the yield of Brookton and Carnamah by only less than 5 per cent in 1999.
Program 3 Contact: Dr Jetinderpal Shethal 08 9956 8555
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