Fungus causes spot of trouble

Fungus causes spot of trouble

Grains
Agriculture Victoria plant pathologist Grant Hollaway. Photo by Brad Collis/GRDC.

Agriculture Victoria plant pathologist Grant Hollaway. Photo by Brad Collis/GRDC.

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The disease leads to significant yield loss and reduced tonnages.

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NEW research has found that in wet winters, yellow leaf spot, a stubble-borne fungal leaf disease which occurs commonly throughout the Western Australia Wheatbelt, leads to significant yield loss and reduced tonnages.

While the disease is most prolific in winter, it's not necessary to wait for it to rain to know whether a crop might be hit by yellow leaf spot, with a PREDICTA B test before the first paddock is sown helping growers to plan more tactically and reduce the threat of the disease.

Agriculture Victoria plant pathologist Grant Hollaway said there has been some conjecture in the grains industry as to whether yellow leaf spot is simply a cosmetic disease that has little impact on yield and the contrary view that yield loss can be significant.

"In the absence of good information to inform those decisions, we set about running several field trials over a number of years to clearly establish the amount of yield loss caused by yellow leaf spot," Dr Hollaway said.

"When we grow our highly susceptible varieties, the yield losses in season really conducive to disease was in the range of 20 to 25 per cent and there were significant yield losses in those seasons where we had continual wet conditions.

"In those same trials, where we had more resistant varieties, the yield losses were negligible which shows us the importance of avoiding those highly susceptible varieties in risky situations."

 Yellow spot on wheat plants. Photo by Julie Monroe/GRDC.

Yellow spot on wheat plants. Photo by Julie Monroe/GRDC.

A lot of rain on a frequent basis over winter and spring will cause greater yield loss from yellow leaf spot due to leaf wetness and rain splashing spores up the plant.

In general, winter conditions are more conducive for spores to germinate and infect leaves.

Furthermore, there is also a higher risk of yield loss because of the disease when planting highly susceptible varieties into paddocks that have wheat stubble present.

Dr Holloway said there are a range of fungicides available, with their experience being that fungicides will provide partial control, but the level of control is variable.

"Sometimes we get near complete control, others it might only do half the job we want, so it does vary from one season to the next," he said.

"When we get a dry season, we still see the disease there, particularly in paddocks with a lot of wheat stubble, but in the drier season the disease doesn't progress up the plant and the yield losses will be much less than what we see in wetter seasons.

"The extent of yield loss in the susceptible varieties does depend very much on the seasonal conditions."

Conducting a PREDICTA B test allows paddocks to be ranked for their relative levels of yellow leaf spot inoculum.

Dr Holloway said that means growers can test the paddocks they're going to plant to wheat and identify the paddocks which have the higher level of inoculum and therefore put their most resistant varieties into those paddocks.

"It's really important that stubble is present in the sample," he said.

"That can be done through either physically adding pieces of stubble to the test, or sampling from the rows that stubble will be getting collected in the sampling process and end up in the bag.

"If we don't have stubble in that bag, then the test can't detect the disease and will underestimate the risk that may be posed."

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