Two net blotches, net-type (net form) and spot-type (spot form), occur in Western Australia and can be quite variable in appearance depending on the form of the pathogen, the variety of barley and the stage of infection.
Net type net blotch
Lesions appear on leaves as thin brown streaks or blotches that may enlarge up to several centimetres in length. Darker longitudinal and horizontal lines sometimes develop in the lesions, creating a net like appearance. A narrow zone of yellowing usually surrounds the lesion (Figure 1). Futher details on symptoms of net-type net blotch can be found on the Diagnosing net-type net blotch MyCrop page.
Prevalence and yield loss
Net-type net blotch, caused by the fungus Pyrenophora teres f. teres, occurs throughout barley growing areas of Western Australia. Severe infection kills leaves prematurely and mainly causes reduced seed weight. It can also reduce number of ears and number of grains per ear. Yield reductions of 20-30% can occur and grain quality (sievings, protein, hectolitre weight) may also be affected.
Net-type net blotch can be both stubble- and seed-borne. Infection and spread of this disease is favoured by wet conditions and it is most evident following periods of rainfall. The most important source of inoculum is carried from season to season on infected barley stubbles. Spores produced on stubble or straw are spread by wind to initiate infections in new barley crops and so crops sown onto or adjacent to barley stubble are at greatest risk of infection and the the greatest yield loss is likely to occur in paddocks that are re-sown to barley without a break crop. Although seed infection levels up to 35% have been observed in WA, significant transmission of seed infection is rare and infected seed is considered to be of minor importance.
- Do not double crop susceptible barley, particularly in medium to high rainfall areas, as this can result in severe disease. In disease prone areas ensure barley is grown only once in three year rotations. A one-year break between barley crops in a paddock will markedly reduce the potential for serious disease. If highly effective stubble retention systems are in use, a two-year break may be required.
- Avoid sowing susceptible (S) and very susceptible (VS) varieties, particularly in high production situations and in disease prone environments - select more resistant varieties if available. See the current Barley variety guide for more information.
- Where the wind erosion risk is low, stubble management practices such as burning stubble will reduce carry over of the fungus.
- Crops grown in potassium deficient soil (<50ppm) are more prone to net blotch. Department of Primary Industries and Regional Development research reveals application of potassium to deficient soil could reduce the vulnerability to disease.
- If seed-borne net-type net blotch is suspected, seed can be sent to DPIRD diagnostic laboratory services (DDLS) for diagnosis prior to seeding. If seed is infected, treating it with a fungicide registered for suppression of seed-borne net-type net blotch can reduce seed-to-seedling transmission. However, this method of spread is not considered important in the overall level of infection by this disease in barley crops in WA. See the list of registered active ingredients on the Seed dressing and in-furrow fungicide page.
- The seed dressing Systiva® (fluxapyroxad, succinate dehydrogenase inhibitor (SDHI) group) and in-furrow product Uniform® (azoxystrobin and metalaxyl-M) have been registered for net-type net blotch control. Both products can act systemically and protect seedlings from stubble borne inoculum.
- Applying a fungicide spray is necessary in medium to high rainfall regions where disease threatens crops with high yield and quality expectations. A wide range of foliar fungicides are registered for net-type net blotch. The choice of a single-spray or double-spray strategy depends on the environment in which the crop is growing, the time onset of disease and use of seed dressing or in-furrow fungicides:
- In high rainfall environments it may be necessary to apply two sprays, such as at early stem elongation stage with a follow-up spray three to four weeks later.
- In medium rainfall regions, consider one well timed spray between late stem elongation and early flag leaf emergence (Z33 - 39) to protect leaf two (flag-1). Under high disease pressure, best results may be obtained by using the maximum recommended rates.
- Caution: Research at the Centre for Crop and Disease Management at Curtin University has identified net-type net blotch pathogen populations in WA showing low levels of resistance to DeMethylation Inhibitor (DMI) fungicide ingredients, particularly tebuconazole based products. Furthermore this pathogen population further shows resistance to metconazole, triticonazole, difenoconazole and prochloraz and lower levels of resistance to epoxiconazole, prothioconazole, propiconazole and triadimenol in laboratory experiments. Another detection of a separate resistance mutation in a sample from the Esperance region is being further investigated. To prevent further development of fungicide resistance, rotating fungicide modes of action, avoiding using the same mode of action twice per season, use of fungicide mixtures with more than one mode of action and using appropriate label rates are suggested. It is also recommended that tebuconazole is NOT used as a stand-alone product, in barley for any disease; its use in WA is now compromised for net type net blotch, spot type net blotch as well as powdery mildew.