REPORTS of crop diseases are increasing around the State, with the discovery of spot form net blotch (SFNB) infection in Maximus CL barley of most concern given the variety is supposed to have better resistance to the disease than other widely grown options.
SFNB infection in Spartacus and Maximus barley was found at several locations across the Kwinana north zone, while net form net blotch (NFNB) was recently reported in RGT Planet in the Albany and Esperance port zones.
Of the seven most popular barley varieties, Maximus is the only one ranked as moderately susceptible to susceptible (MSS) to SFNB, the other six are all either susceptible (S) or very susceptible (VS).
Both SFNB and NFNB are stubble borne diseases, with the fungus carried from season to season on infested stubble.
The closer new crops are sown to stubble, the greater the risk of infection occurring at early growth stages.
"Barley varieties which are susceptible to SFNB (Spartacus CL) or NFNB (RGT Planet) and have not been treated with a fungicide seed dressing (Systiva) or in-furrow (Uniform) registered against net blotches and are sown into a high-risk situation (one to two year old barley stubble) are particularly vulnerable," said Department of Primary Industries and Regional Development (DPIRD) research scientist Kithsiri Jayasena.
Applying a fungicide spray is necessary in medium to high rainfall regions where disease threatens crops with high yield and quality expectations, particularly if the barley crop is a susceptible variety or has been sown into or adjacent to last year's barley stubble.
Fungicides in three groups - Group 3 (DMI-demthylisation inhibitors), Group 11 (Qol-quinone outside inhibitors) and Group 7 (SDHI-succinate dehydrogenase inhibitors) - are registered to manage both diseases in WA.
However, Dr Jayasena said due to the presence of NFNB and SFNB populations resistant to some fungicides it was important to follow fungicide resistance management guidelines.
"At present, populations of both NFNB and SFNB have developed resistance to the DMI group of fungicides in WA," he said.
"Resistance (SFNB) and reduced sensitivity (SFNB, NFNB) to SDHI fungicides has also been detected in WA over the past two years."
The choice of a single-spray or two-spray strategy depends on the presence of disease, the environment in which the crop is growing and the use of up-front fungicide at seeding.
In high rainfall environments it may be necessary to apply one or two sprays, the first at early stem elongation stage if disease is at damaging levels already, with a follow-up spray three to four weeks later (Z33-45).
Where up-front fungicides have been employed, such as seed dressing or in-furrow, it is expected they will reduce or remove the requirement for a stem extension spray.
DPIRD plant pathologist Geoff Thomas said in the medium rainfall regions, or rotation crops, growers should consider one well-timed spray between late stem elongation and early flag leaf emergence (Z33-39) to protect leaf two.
"Under high disease pressure best results may be obtained by using the maximum recommended rates," Mr Thomas said.
"Application of a registered foliar fungicide prior to stem extension - tillering, for example - can reduce disease levels after spraying but will not provide sufficient upper canopy protection and is likely to still require a follow-up fungicide later in the season.
"In low rainfall areas fungicide applications are most likely to result in a yield response to SFNB in earlier sown crops, when the disease pressure is high and there is reasonable spring rainfall or stored soil moisture."
To date, fungicide resistance for Group 3 or DMI actives such as prothioconazole, tebuconazole and epoxiconazole has been found in populations of NFNB at Dandaragan, South Stirling, Amelup, Tenterden, Kendenup and Scaddan.
Less sensitive NFNB pathogen populations were also present at several locations across the Wheatbelt.
DMI resistance and reduced sensitivity in SFNB populations is widely distributed across most barley production regions in WA.
Isolated occurrences of SDHI resistance in SFNB have been detected in the Central Wheatbelt and Albany regions and reduced sensitivity to SDHI in NFNB also exists in the Albany area.
"To prevent further development of fungicide resistance it is recommended growers only use fungicide if necessary, rotate fungicides with different modes of action, avoid using the same mode of action twice per season and use recommended label rates," Mr Thomas said.
"It is also recommended that tebuconazole is not used as a stand-alone product in barley for any other disease."
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