Yellow leaf spot
Description
YLS is often noticed in wheat seedlings, however in some areas where conditions are favourable, infection can occur right through to adult stages. YLS causes tan coloured oval lesions on leaves, which are often surrounded by a yellow halo. Lesions can merge and in severe infections the leaf tips turn yellow and die. Lesions which occur during adult growth stages are more defined in shape and tend to have less chlorosis. Wet conditions during spring can cause pink pigmentation to develop on the grain, resulting in quality downgrades. In Western Australia YSL can be confused with another wheat disease, septoria nodorum blotch, as symptoms are similar and can only be distinguished in a laboratory. In-crop management are similar for both, and therefore most growers don’t worry about correct identification.
YLS is mostly stubble borne, surviving from one season to the next as mycelium on infected stubbles, where it can remain for up to two years. During late summer/early autumn black fruiting bodies full of spores form on the stubble. Primary infection occurs during cool (15 °C), humid conditions when spores are released and land on nearby wheat plants. Infection will only occur if leaves remain wet for more than six hours. Secondary infection occurs during moist conditions when spores from diseased leaf tissue are spread by the wind, often causing YLS to develop higher up in the plant. Secondary infection often leads to high yield losses.
Control
Effective control of YLS is possible by taking an integrated approach to its management. Severe outbreaks are usually associated with successive wheat plantings, therefore rotating wheat with other crops will help reduce disease levels. Practices such as tillage, burning or grazing will help reduce stubbles and therefore disease load, however care must be taken where soils are at high risk of wind or water erosion. Where possible avoid planting susceptible varieties, particularly where wheat on wheat rotations exist, and replace them with resistant varieties which still meet agronomic and quality standards.It is generally not economical to apply fungicides for control of YLS unless it is a susceptible variety with a yield potential of more than 3.0 t/ha and has infection to five per cent of the flag (-2) and flag (-3). It is also important to ensure YLS is the primary cause of the symptoms rather than nutrient deficiency or environmental stresses. Crops should be monitored for symptoms particularly in wheat on wheat rotations where YLS was previously diagnosed and in this situation a fungicide application between tillering and stem elongation may be warranted.
Both Prosaro® and Aviator® Xpro® provide protective and curative control of YLS in wheat and should be applied before the disease has infected the top three leaves. Consult CropLife Fungicide Resistance Management Strategies as a guide to the responsible use of fungicides and always follow the product label for specific use instructions.
References
Department of Primary and Regional Industries (DPIRD) (2016), ‘Diagnosing yellow spot of wheat’, https://www.agric.wa.gov.au/mycrop/diagnosing-yellow-spot-wheatGRDC (2011), ‘Northern Region - Management to reduce the risk of yellow spot’, GRDC Yellow Leaf Spot fact sheet, https://grdc.com.au/__data/assets/pdf_file/0021/208713/yellow-spot-fact-sheet-northern-region.pdf.pdf
GRDC (2011), ‘Southern Region Management to reduce the risk of yellow spot’, GRDC Yellow Leaf Spot fact sheet, https://grdc.com.au/__data/assets/pdf_file/0030/209658/yellow-leaf-spot-fact-sheet-southern-regions.pdf.pdf
GRDC (2011), ‘Western Region - Management to reduce the risk of yellow spot’, GRDC Yellow Leaf Spot fact sheet, https://grdc.com.au/__data/assets/pdf_file/0021/224913/yellow-spot-fact-sheet-western-region.pdf.pdf
Holloway, G (2014), ‘Yellow leaf spot of wheat’, Agriculture Victoria, http://agriculture.vic.gov.au/agriculture/pests-diseases-and-weeds/plant-diseases/grains-pulses-and-cereals/yellow-leaf-spot-of-wheat