WHILE many farmers are rejoicing about the rain, the threat of disease is pushing some to spray their crops unnecessarily.
Department of Agriculture and Food (DAFWA) development officer Greg Shea said he was extremely concerned about crop decisions that were being made in response to fear and emotion, rather than science.
"It is more important than ever that farmers keep a cool head and not overreact to the presence of leaf diseases," Mr Shea said.
"If the crop has flowered then you really shouldn't use a fungicide spray, as trials have shown it to be completely uneconomic.
"The only exception is stem rust where spraying with fungicide after flowering can be worthwhile."
According to Mr Shea, there were crops in the eastern Wheatbelt that now have great potential after late rain was received.
But with budgets still tight and some doubts as to where wheat prices will end up, he said farmers needed to be careful and not jump the gun when they spotted an anomaly in their crop.
DAFWA has confirmed that regular rain events through late July and August have led to the development of fungal infections in some Wheatbelt crops.
Necrotrophic leaf blotching diseases such as yellow spot and septoria nodorum have been detected in many crops, especially those sown onto wheat stubbles.
And reports of both leaf and stem rust have also been received.
DAFWA plant pathologist Geoff Thomas said there was a clear frenzy among WA growers who were taking every available preventative option to ensure nothing happened to their crop.
"A number of crops have varied degrees of yellowing, which growers are mistaking for disease," Mr Thomas said.
"But the fact of the matter is that it's not and they're wasting their money."
He said the real exception of the rule was stem rust, in which case he said a later fungicide may be effective.
"But first they need to confirm the disease is definitely in the crop," Mr Thomas said.
As the season progresses and crops reached maturity, he said decisions on whether to apply a fungicide to infected crops became more difficult.
"This season, the wheat variety Mace which dominates the area planted has regularly suffered from 'non-specific' yellowing and blotching on the leaves," Mr Thomas said.
DAFWA reported that while initially it was most obvious in crops through the Lower Great Southern, the frequency of the symptom had increased over recent weeks.
"While yellow blotching is present in both conditions, the difference between this 'non-specific' blotching and yellow spot/septoria nodorum is the absence of necrotic (or dead) spots within the yellow blotched area," Mr Thomas said.
"Where yellowing and blotching on leaves is not caused by disease but by this 'non-specific' blotching, then responses to fungicide are unlikely to be economic."
Given that yellow spot has been known to reduce grain quality and cause yield losses of up to 30 per cent throughout the WA Wheatbelt, it is little wonder growers are acting impulsively.
An analysis of 30 years of DAFWA field trials on fungicide management of the yellow spot found that application of one fungicide spray can recover some of this loss to provide economic returns in high and medium rainfall zones, but is unlikely to be economic in the low rainfall zone.
But Mr Thomas said as the season progressed, the likelihood of an economic response to fungicide diminished.
"Spraying decisions should bear in mind the level of disease in the crop, the time of the season and growth stage of the crop and the chance of ongoing rainfall after spraying being favourable for disease development," he said.
"This applies to yellow spot, septoria nosorum particularly, but is a general rule for all crop diseases."
