THE welcome arrival of a better season across most of the West Australian grain belt has brought with it the challenge of a return to more intense disease pressure.
Growers in the Esperance Port Zone in particular have taken a ‘hard hit’ from powdery mildew in barley, with the 2011 infection beginning in early tillering.
Yield losses from uncontrolled early infection can be as high as 50 per cent due to tiller abortion.
In the other three Port Zones, mildew developed slightly later as the crops started to elongate. Yield losses from post tillering infection are usually between 10 and 25pc.
The 2011 epidemic has partly been due to the widespread planting of varieties rated as very susceptible such as Baudin, or susceptible varieties including Hamelin, Stirling and Vlamingh.
Further fuelling the problem is the fact that Baudin and Vlamingh are widely grown in high mildew risk areas and that some of the older fungicide chemistries are no longer as effective as they once were.
To combat resistance in barley, Bayer CropScience has developed Jockey Stayer to fight against mildew infections.
Powdery mildew is a well understood disease that can be controlled by both genetic resistance and fungicides. Ideally both strategies should be used, but it appears that both have been neglected in the past few years.
DAFWA Plant Pathologist, Dr Kithsiri Jayasena said, “It is vital that integrated disease management practices are implemented by growers to reduce the dependence on fungicides for mildew management.”
He says the key message for next season is that growers will need to refine their disease strategy and combine as many different control techniques as possible to stay in front.
“These measures include removing the barley green bridge, use of resistant varieties and timely use of effective fungicides, both seed and foliar.”
“For example, growers should remember that Jockey Stayer should be used with an effective smuticide to control smuts and bunts.”
Bayer CropScience advise Raxil is ideal for this purpose.
Genetic resistance to powdery mildew needs to cope with the fact that like the rusts, the pathogen can be divided into numerous races, each of which can overcome a different resistance gene.
According to Dr Jayasena, new resistance genes have a history of failing in a few years, so the possibility of breakdown in barley varieties which possess major gene resistance is very real.
Equally concerning is the break down in effectiveness of some of the traditional fungicide staples used to control powdery mildew. Powdery mildew is at such high risk of resistance because it is wind borne, has mixed sexual stages and sporulates abundantly.
Fungicides from only two modes of action are currently registered for barley powdery mildew control in Australia.
One group is the DMI (sterol demethylation inhibitors) or triazole group and the second group is the QoI (quinone-outside inhibitors) or strobilurin group. All of the products registered to control powdery mildew contain a triazole. In Europe, many triazoles have lost efficacy and this has been linked to the occurrence of mutations in the target gene.
The Australian Centre for Necrotrophic Fungal Pathogens (ACNFP) detected one of these mutations in 2009 and from 2010 the mutation was found in 100% of the isolates tested.
This affects some triazoles but not others. There is direct evidence that this mutation has rendered the triazole fungicide tebuconazole less effective at current label rates. Mutants have been found that are about 10 fold less sensitive than susceptible populations.
Use of an up-front fungicide at seeding can effectively delay the onset of mildew infection in crops, therefore delaying the timing of a first foliar application.
Research from the 2011 season clearly demonstrates that the seed dressing fluquinconazole (the active ingredient in Jockey Stayer Systemic Seed Treatment), is not one of the fungicides that has had its effectiveness compromised.
In 2011 at Esperance Downs Research Station, Jockey Stayer was still effective against mildew infections for up to 11 weeks after sowing. One of the benefits of dressing seed with a fungicide is that it reduces losses from early disease infection, especially since early infection decreases tiller number and therefore grain yield.
The same trial clearly showed that flutriafol in-furrow was no longer effective for the extent of fluquinconazole against mildew at that site. Other work has shown that seed dressings containing triadimenol no longer provide commercially acceptable levels of mildew control in high mildew risk areas.
Further evidence suggests that some other older generation foliar triazoles in addition to tebuconazole have compromised activity. This group is thought to include propiconazole, flutriafol, triadimefon and triadimenol.
However, there is some good news for growers in that the newer generation DMI fungicides including cyproconazole (second generation), epoxiconazole (second generation), fluquinconazole (second generation) and an active ingredient in Prosaro i.e. prothioconazole (third generation), appear to be unaffected by the current mutation. No resistance to QoI fungicides has been found so far.
The results of the third generation fungicide Prosaro can be seen in figure 2, when in combination with Jockey seed dressing it delivered exceptional control. This trial was conducted on powdery mildew on Baudin barley at Gibson, 2011.