FUNGUL disease is threatening to trash WA's forecast better-than-average wheat yield this season.
Waterlogged Wheatbelt growers have been advised to monitor crops carefully for early signs of rust, powdery mildew and other fungal diseases caused by unusual seasonal conditions.
Wet and cool conditions, coupled with higher humidity in dense crop canopies, has seen the return of stripe rust - not evident in WA in recent seasons.
It has been found in crops of susceptible wheat varieties Calingiri and Bonnie Rock in the north-east Wheatbelt from Cadoux to Bencubbin.
The Department of Agriculture and Food (DAFWA) PestFax newsletter also lists an unconfirmed report of stripe rust in adjoining Mace crops north of Coorow.
This has caused some concern because it may indicate a new pathotype that could be virulent in Mace, previously considered less susceptible to stripe rust.
According to DAFWA, stripe rust can cause up to 60 per cent yield loss and reduce grain quality when susceptible wheat varieties are infected early.
Stem rust, which can cause yield losses of up to 50pc and significant grain quality issues, has already been reported, mainly in volunteer regrowth wheat, across the north and central Wheatbelt.
It usually appears later in the season when temperatures are warmer, up to 30 degrees.
Lower levels of leaf rust have also been found across the Wheatbelt and powdery mildew is thriving in northern and central areas, with a hot spot near Northam.
Other damp-conditions diseases, including barley leaf rust and sclerotinia in canola, have also been reported on PestFax.
But rust is causing most concern because of its potential substantial yield and quality impact on WA's recently-estimated 13 million tonne grain harvest and the ease with which spores are spread.
A problem for farmers trying to combat the diseases with fungicide is that many paddocks are still too wet to get larger spray rigs onto.
Some, aware of the risk of rust and mildew after the entire Wheatbelt received a drenching at the start of the month, have already come to grief trying to get onto soft paddocks.
The WA Wheatbelt Rainfall Reports Facebook page has featured posts of pictures of tractors, spray rigs and other implements bogged to the wheel hubs.
Gnowangerup farmer and 4Farmers agricultural chemical company owner Phil Patterson alerted Farm Weekly to the stripe rust problem last week.
"We have to get the message out there to farmers that they have to check for rust and do something about it as soon as they can get onto paddocks," Mr Patterson said.
"I know people will just think I'm saying that to sell more fungicide, but the problem is worse than I first thought."
He said he wanted to warn farmers after talking at a social function to a Bencubbin grower who had discovered the tell-tale yellow leaf of stripe rust in his wheat crop.
"He's a big grower and he's got it in his Calingiri crop.
"We need to get the message out because it's an unusual season - whoever heard of the Wheatbelt being waterlogged in the middle of August?
"There's some big crops out there this season and they could be at risk.
"Stripe rust forms when there are colder and wetter conditions, and it stops again when the temperature gets back to 22 degrees."
He said he realised the ground was too soft to get heavy spray equipment onto paddocks.
"If it's too wet to get the boom spray onto the paddocks then use the mister," Mr Patterson advised.
Mister spray units on utilities were lighter and could get onto damp paddocks long before larger tractors and spray rigs could, he said.
"We know the mister works, I used it to save a crop in 2003," he said.
He said the problem of spraying fungicide on wheat crops in soft paddocks was compounded by aerial crop-spraying contractors already being fully booked.
"I'm told the planes are all booked out," Mr Patterson said.
He said there was some urgency to act before the first signs of rust appeared.
"Growers have to keep in mind that if they treat for it before it's there, that will get them six weeks protection out of their fungicide.
"If they treat after it shows up then they will only get three to four weeks.
"They also have to remember, it takes five to seven days to stop the rust."
Mr Patterson's warning to check carefully for rust was echoed by DAFWA plant pathologist Geoff Thomas who pointed out managing rust at low levels in a crop was far more effective than when it was well established.
Mr Thomas, who leads a crop disease project conducted by DAFWA and supported by the Grains Research and Development Corporation (GRDC), said seasonal conditions favoured wheat rust, powdery mildew, barley leaf rust and sclerotinia developing.
"Stripe rust, which has been found on susceptible varieties Calingiri and Bonnie Rock in the eastern grainbelt this year, has been very rare in WA over the past few seasons," he said.
"Stem rust has been reported at a couple of locations in wheat regrowth in central, eastern and northern areas and leaf rust has been detected at low levels in the popular wheat variety Mace."
Mr Thomas warned growers not to be complacent and to closely monitor all wheat crops irrespective of variety.
A new leaf rust pathotype was detected in WA in 2013.
If the pathotype was present in infected crops, Mace would be moderately susceptible, he said, and other varieties with increased susceptibility included Corack, Emu Rock and Wyalkatchem.
"Remember to look into the bottom of the crop canopy and stems, to check for the presence of stem rust," he advised.
"Fungicides that may have already been applied for powdery mildew are also effective on leaf rust."
He said the rust problem was not just confined to paddocks that had surface water in them.
Information on susceptibility to rust is available from the 2015 Wheat Variety Guide for WA on the DAFWA website www.agric.wa.gov.au and information on developing a rust management strategy was on www.rustbust.com.au, Mr Thomas said.
If rust is detected, it was important samples were sent to the University of Sydney Plant Breeding Institute for pathotype analysis, he said.
The Australian Cereal Rust Control Program based at the university is one of GRDC's core investments.
Centre for Crop and Disease Management (CCDM) researcher and fungicide resistance group leader Fran Lopez-Ruiz advised growers spraying for powdery mildew to avoid repeated doses of the same type of fungicide.
"Always spray fungicides at the first sight of symptoms and control the disease as early as practical," Dr Lopez-Ruiz said.
"Always opt for a fungicide mixture where possible, such as a DMI (DeMethylation Inhibitor) and QoI (strobilurin) fungicide mixture, to help prevent the development of fungicide resistance," he said.
"If this is not available, then mixtures containing different DMI fungicides are always better than just using a solo product.
"So far, we have not seen any signs of resistance development in wheat powdery mildew.
"However, as we saw in barley powdery mildew in 2009, and given that wheat and barley powdery mildews are very close genetically, it is highly possible that mutations endowing fungicide resistance could quickly occur in wheat powdery mildew under the right pressures.
"Growers therefore need to be mindful of how they use fungicides, and stick to integrated disease management strategies that do not depend solely on fungicides."
He said the Wheatbelt was experiencing "perfect seasonal conditions for disease outbreaks".
Dr Lopez-Ruiz encouraged growers who suspected fungicide resistance issues, to contact CCDM's fungicide resistance group for testing and advice at email@example.com.