Yellow Leaf Spot of Wheat
Yellow Leaf Spot of Wheat
Note Number: AG1114
Published: June 2006
Updated: May 2014
Dr Grant Hollaway, Senior Plant Pathologist Cereals, DEPI Horsham
Yellow leaf spot, also known as tan spot, has become a widespread and important disease of wheat in Victoria. It has been supported by stubble retention, intense wheat production in the rotation and wide spread cultivation of susceptible wheat varieties like Yitpi.
What to look for
Yellow leaf spot is most often observed in seedlings, but when conditions are suitable it can progress up the plant where it causes significant yield loss.
The first symptoms appear on leaves as small tan oval spots or lesions surrounded by a yellow halo (Figure 1). Individual lesions may vary in shape and size, often expanding and joining together with other lesions. The tips of severely affected leaves soon yellow and die (Figure 2).
Figure 1. Early symptoms of yellow leaf spot infection
Accurate diseases identification is important as symptoms of yellow leaf spot can be confused with other disorders like aluminium toxicity or herbicide damage.
Figure 2. Severe infection, leaf tips dying
In isolated cases, in particular when susceptible wheat varieties are sown into wheat stubble, heavy infestations of yellow leaf spot can caused yield loss when the flag and upper leaves become infected. In most years, yellow leaf spot only infects the lower leaves and is generally regarded as causing limited yield loss. However, research at DEPI Horsham in 2012 and 2013 found infection of the lower leaves by the yellow leaf spot fungus reduced the yield of resistant varieties (MR and MRMS) by approximately 4 per cent and susceptible varieties (S and SVS) by approximately 15 percent.
Yellow leaf spot (tan spot) caused by the fungus Pyrenophora tritici-repentis, is predominantly a stubble-borne disease. This fungus survives between crops on cereal stubbles as mycelium in stubble and can survive on stubble for up to two years, if seasons are a very dry.
In the late summer/early autumn the fungus develops small black fruiting bodies on the stubble (Figure 3) The fruiting bodies, or pseudothecia, contain large numbers of sexual spores which are forcibly ejected during cool (15°C) humid conditions. Spores land on nearby wheat plants and will infect leaves if they remain wet for more than 6 hours.
Figure 3. Black fruiting bodies on stubble
Often the early infection of seedlings does not progress to adult plants. However, when conditions are wet during the season, asexual spores or conidia are produced in leaf lesions. This secondary spore is dispersed by the wind, and can result in rapid disease development higher up the plant; and in neighbouring wheat crops. It is this secondary spread that causes high yield loss.
- Figure 4. Cereal tiller showing leaf designations
Yellow leaf spot is most severe where successive wheat crops are grown on retained stubble. Rotating wheat with barley, oats or a non cereal crop will reduce the impact of this disease. Foliar fungicides are registered to control yellow leaf spot, but they may not be economical.
Management options include:
- Not sowing wheat into infected stubbles.
- Avoiding growing susceptible varieties. The pressure from yellow leaf spot will be greatly reduced if susceptible (S) and very susceptible varieties (VS) are replaced by varieties moderately susceptible (MS) or better to yellow leaf spot. Complete resistance is not needed to achieve sustainable control of this disease. See the Victorian Cereal Disease Guide for resistance ratings.
- Reducing the number of susceptible crops grown in a district will reduce inoculum load from season to season.
- Fungicides are most likely to give an economic return when yield potential is above 3.0 t/ha, a susceptible variety is being grown, and 5 per cent of the Flag (-2) leaf and Flag (-3) leaf are affected (see Figure 4). Under these conditions, a fungicide application should be made prior to or just after rain, between flag leaf emergence and late booting. This will prevent the disease from moving up onto the flag leaf.
- Seed and fertiliser treatments are not effective against this disease.
DEPI Information Note Series
Victorian Cereal Diseases Guide
Decimal Growth Scales of Wheat
DEPI Taking Care with Foliar Fungicides
Victorian Winter Crop Summary
Wallwork H (2000) Cereal Leaf and Stem Diseases. (Book) GRDC.
Contact/Services available from DEPI
DEPI Field Crop Pathology, Grains Innovation Park,110 Natimuk Rd, Horsham 3400. Tel (03) 5362 2111, or the DEPI Customer Service Centre 136 186.
This Information Note (AG1114) was prepared with assistance from Frank Henry DEPI Horsham. Financial support by the GRDC is gratefully acknowledged. Last Updated: 20th May 2014