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Net blotch is expected to be at damaging levels in Western Australian barley crops in 2014, particularly in high rainfall areas in the south coast, but growers can reduce the risks by adopting an integrated disease management approach.
According to DAFWA researcher Kith Jayasena, the high risk of damage to 2014 barley crops is mainly due to carry-over of the disease from last year, heavy stubble loads and warm and wet conditions following crop emergence.
There are two forms of net blotch, net-type net blotch is caused by the fungus Pyrenophora teres f. teres and spot type net blotch is caused by Pyrenophora teres f. maculata. Both diseases are common in barley growing areas of Western Australia (WA). The primary source of inoculum for both diseases is infected stubble. Net type net blotch can be seed borne however the importance of seed infection in initiating seasonal epidemics is not known and unlikely to be particularly important given the large areas of barley stubble every year. . Severe infection reduces green leaf area and kills leaves prematurely and mainly causes reduced seed weight and grain quality (sievings, protein, hectolitre weight). It may also reduce number of ears and number of grains per ear. Yield reductions varies from 30to 50% depending on the disease pressure.
Infection and spread of both diseases are favoured by wet conditions and it is most evident following repeated periods of rainfall. It will cause the greatest yield loss in paddocks that are re-sown to barley without a break-crop.
Both forms of the disease result in lesions forming on the leaves, or occasionally on the sheaths. Lesions appear on leaves as thin brown streaks or blotches that may enlarge up to several centimetres in length. Lesions may also be surrounded by chlorotic (yellow) tissue. Severe infecion can result in premature necrosis of large areas of leaf tissue.
The net type symptoms start as pin point brown lesions on the leaves which elongate and produce fine, dark brown streaks along and across the leaf blades to form a net appearance. As the disease continues to develop the lesions continue to elongate, following the veins, and are often surrounded by a yellow margin.
The spot type also starts as small solid brown oval lesions, surrounded by yellow edges. As infections age they may elongate and join together causing blotch symptoms.
It is important to remember that abiotic factors such as nutrient deficiencies or adverse weather conditions can also cause abnormalities in barley leaves. Some barley genotypes exhibit varying degrees of physiological spotting on leaves which can also be confused with disease symptoms.
An important starting point in disease management is correct identification of the symptoms as a disease as abiotic symptoms will not respond to disease management approaches.
If seed-borne net-type net blotch is suspected, seed can be sent to AGWEST Plant Laboratories 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.
Further assistance with disease identification can be obtained from AGWEST Plant Laboratories (phone 9368 3721).
Growers can reduce the risk of crop losses from net-type and spot-type net blotch by:
- Selection of paddocks (rotating crops and not seeding barley on barley stubble);
- Choosing varieties with good resistance profile (avoid sowing susceptible (S) and very susceptible (VS) varieties, particularly in disease prone environments and if sowing into barley stubble). Barley variety disease rankings are available in the WA Barley Variety Guide for 2013, available by searching this title on www.grdc.com.au;
- Burning or burying stubble can reduce disease carry-over, be aware of wind erosion risk with stubble management;
- Early sowing increases risk of damaging levels of disease developing in crops;
- Ensure crop has adequate nutrition, on K deficient soils (<50 ppm) applying potassium (K) fertiliser six to eight weeks after barley crop emergence can reduce the impact of net blotch and other foliar leaf diseases such as powdery mildew. Use registered seed dressing fungicides to eradicate possible seed infection and to protect the plants at early growth stage (Net type net blotch only);
- Applying a fungicide spray if required. A follow-up spray may be necessary in high rainfall areas depending on weather conditions and the disease pressure.
- Whenever possible, rotate fungicides with different actives, which will reduce the risk of development of fungicide resistant pathogen populations.
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