YELLOW (LEAF) SPOT

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.

In most years, yellow leaf spot only infects the lower leaves and is generally regarded as causing limited yield loss. However, in isolated cases, in particular when susceptible wheat varieties are sown into wheat stubble, heavy infestations of yellow leaf spot can cause yield loss when the flag and upper leaves become infected.

Research at DEDJTR 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 per cent.

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 2.11). 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, (Figures 2.12 and 2.13).

Accurate disease identification is important as symptoms of yellow leaf spot can be confused with other disorders like aluminium toxicity or herbicide damage.

Figure 2.11 Early symptoms of yellow leaf spot infection

Figure 2.12 Yellow leaf spot infections on older leaves, leaf tips dying

Figure 2.13 Severe yellow leaf spot infections on mature plants

Figure 2.14 Black fruiting bodies of the yellow leaf spot fungus on stubble carry the disease from one year to the next

Disease Cycle

Yellow leaf spot, caused by the fungus Pyrenophora tritici-repentis, is predominantly a stubble-borne disease. The fungus survives from season to season on stubble in small black fruiting bodies, (Figure 2.14). Fruiting bodies contain large numbers of ascospores which are forcibly ejected during humid conditions. Spores land on nearby wheat plants and will infect leaves if they remain wet for more than 6 hours.

Often the early infection of seedlings does not progress to adult plants. However, when conditions are wet during the season and temperatures are between 10-25°C a second type of spore (conidia) is produced. This secondary spore is dispersed by the wind and can result in rapid disease development higher up the plant, as well as into other wheat crops. It is this secondary spread that causes high yield loss, (Figure 2.15).

 

Figure 2.15 Disease cycle of yellow leaf spot of wheat. Illustration by Kylie Fowler

Yellow (Leaf) Spot Management

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 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 (AG 1160) 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 2.16). Under these conditions, a fungicide should be applied prior to, or just after rain. This will prevent the disease from moving up onto the flag leaf
  • seed and fertiliser treatments are not effective against this disease.

Figure 2.16 Cereal tiller showing leaf designations. Illustration by Kylie Fowler

Further Information

More detailed information can be obtained from: The DEDJTR AgNotes Series www.vic.gov.au/graindiseases

Victorian Cereal Diseases Guide (AG 1160) www.vic.gov.au/cerealdiseaseguide

Victorian Winter Crop Summary www.vic.gov.au/victorianwintercropsummary

Decimal Growth Scale of Cereals (AG 0013) www.vic.gov.au

Wallwork H (2015) Cereal Seed Treatments (SARDI) www.pir.sa.gov.au, direct link: http://www.pir.sa.gov. au/ data/assets/pdf_file/0005/237920/cerealseedtreat2015_web.pdf

STEM RUST OF WHEAT

SEPTORIA LEAF BLOTCH OF WHEAT