RESEARCHERS are pleading with grain growers to be careful with their fungicide rotations after confirming a sample of a common barley disease had resistance to two different fungicide modes of action.
Speaking at the Crop Protection Forum earlier in the month at Moama, NSW, Fran Lopez-Ruiz, of the Centre for Crop and Disease Management (CCDM) said there had been samples of net form of net blotch (NFNB) coming in from South Australia's Yorke Peninsula with dual resistance.
The resistance was against two of the most common modes of action in fungicide, the succinate dehydrogenase inhibitor (SDHI) based products as well as some Demethylation Inhibitor (DMI) fungicides.
Testing found the strain of NFNB had mutated for resistance.
Growers in the area are being warned not to use SDHI based products next year, but the researchers are hopeful they will be able to bring them back into their fungicide management strategies at a later date.
There is also concern around the closely related spot form of net blotch (SFNB) which is showing some resistance to DMIs in WA.
Dr Lopez-Ruiz said resistance was a natural phenomenon but the issue was with selecting for resistance via agronomic practices.
"The resistance may be there even before the fungicide goes on, it is naturally occurring, but the big problem is when we select for it via over-reliance on one particular product and that population becomes the dominant strain of disease, which we do when we target the same mechanism over and over again."
Dr Lopez-Ruiz said the first cases of fungicide resistance in Australia occurred in Tasmania, which was to be expected as the cool, moist conditions in many parts of that state were more conducive to disease.
From there it was reported in Western Australia, also in the cooler areas near the Southern Ocean.
"At first we noticed it in SFNB it was just reduced sensitivity to the fungicide, there was no impact in production at first then we saw full resistance."
"The trick for other producers was to learn from others' mistakes, with very little crop rotation and no significant stubble management but unfortunately that's not what we've seen."
Dr Lopez-Ruiz said with relatively limited numbers of fungicide modes of action farmers had to use non-chemical controls such as rotation and stubble management to reduce the risk of resistance.
"Good farm hygiene and the removal of stubble maybe through burning can be very important in keeping the disease burden down, as will common sense measures like not planting barley on barley.
He said farmers should not spray prophylactically for disease, as was briefly the fashion, but only when the disease would have an economic impact if left uncontrolled.
However, he said it was a narrow window to decide whether or not to apply a fungicide as it needed to go out before there was widespread infection.
In terms of fungicide choices he said group 3 fungicides should not be used twice in a row, while Group 7 and Group 1 products should not be used more than once per season in any crop rotation.
The offending disease samples were identified by SARDI plant pathologists Tara Garrard and Hugh Wallwork and confirmed as resistant via tests conducted by CCDM's fungicide resistance team led by Dr Lopez-Ruiz.
"The samples, collected from two paddocks a short distance apart, clearly showed high NFNB disease levels when the SARDI team was first alerted to the problem by agronomist Sam Holmes," said Dr Wallwork.
He said the samples were found on the Spartacus variety of barley.
Additional samples collected at the site also showed resistance to the Group 3 DMI fungicide tebuconazole, and although the resistance mechanism is still being investigated, Dr Lopez-Ruiz said it was known to be different to the DMI resistance mechanisms already reported in barley net blotches in Western Australia.
The story New fungicide resistant strain of disease found on YP first appeared on Farm Online.