There are two forms of net blotch present in Australia. The spot form of net blotch (SFNB), which is most common due to the cultivation of susceptible varieties, is caused by Pyrenophora teres f. maculata, (Figure 2.33). The net form of net blotch (NFNB), caused by the fungus Pyrenophora teres f. teres, is currently less common due to the majority of barley varieties being resistant. However, NFNB can be the more damaging of the two.

Figure 2.33 Pyrenophora teres a) stubble with pseudothecia b) ascospore c) conidia

What to Look For

Spot Form of Net Blotch

Symptoms are most commonly found on leaves, but occasionally also infect leaf sheaths. SFNB develops as small circular or elliptical dark brown spots surrounded by a chlorotic zone of varying width. These spots do not elongate to form net like pattern characteristic of the net form. The spots may grow in diameter to 3-6 mm. Older leaves will generally have a larger number of spots than younger leaves, (Figure 2.29).

Figure 2.29 Typical symptoms of spot form of net blotch.

Net Form of Net Blotch

The net form of net blotch starts as pinpoint brown lesions which elongate and produce fine, dark brown streaks along and across the leaf blades, creating a distinctive net-like pattern. Older lesions continue to elongate along leaf veins and often are surrounded by a yellow margin, (Figure 2.30).

Figure 2.30 Typical netting symptoms of net form of net blotch

Disease Cycle

Primary inoculum of both forms of net blotch comes from infected stubble. Net blotch can survive on infected barley stubble as long as the stubble is present on the soil surface. However, the inoculum levels are typically significantly reduced after 2 years.

Ascospores are produced by pseudothecia on the stubble residues, (Figures 2.31 and 2.32) that are spread by rain-splash or wind to infect neighbouring plants. Most of these ascospores only travel short distances within the crop. Infection requires moist conditions with temperatures below 25°C, but is most rapid at 25°C.

The disease cycles for the two forms of net blotch differ in that NFNB can be carried over on seed, while the SFNB is not seed-borne. Carryover of NFNB occurs when humid conditions are present at crop maturity.

Secondary infection is provided by conidia produced from lesions on leaves. These lesions usually start on the lower leaves which then infect the upper leaves during moist conditions. Unlike the ascospores, conidia are wind dispersed and therefore can travel considerable distances. The likelihood of infection decreases with distance from the source. As the barley plant begins to senesce, the fungus grows into the stem as a saprophyte. After harvest, it survives on the stubble and will begin producing ascospores when cool moist conditions are present. There is a positive relationship between the amount of ascospores produced and stubble load. Stubble breakdown and inoculum production may be prolonged during seasons with dry summer months.

Figure 2.31 Disease cycle of net form of net blotch of barley. Illustration by Kylie Fowler

Figure 2.32 Disease cycle of spot form of net blotch of barley. Illustration by Kylie Fowler

Economic Importance

When the net blotch diseases are severe they can cause significant reductions in grain yield and quality, leading to downgrading of grain from malting quality to feed. In general the flag and flag-1 leaves must be infected for yield loss to occur. Losses due to NFNB in susceptible varieties generally range between 10 and 20 percent with losses of more than 30 percent possible. Losses due to SFNB generally range between 1 and 10 percent, but when severe can exceed 20 percent. Severe infection of either net blotch can result in reduced grain plumpness which is generally observed as reduced retention and increased screenings.

Net Blotch Management

Varietal Selection

Avoiding growing susceptible (S) and very susceptible (VS) varieties and growing a variety with a rating of moderately susceptible (MS) or better significantly reduces the likelihood of grain yield and quality loss. Consult a current Victorian Cereal Disease Guide (AG 1160) when selecting varieties. The widespread use of varieties with resistance to NFNB has effectively managed this important disease in Victoria for 20 years.

Crop Rotation

Avoid growing susceptible barley varieties in successive years in the same paddock as net blotch inoculum will become established. Once net blotch is established, inoculum will persist for several seasons. At least two seasons of break crops are required to reduce stubble and inoculum loads sufficiently to reduce risk.

Time of Sowing

Early sowing favours the development of the net blotches and can increases the potential for loss. Have a proactive approach to disease management in early sown crops, by applying up-front fungicides if possible and monitor with a view to application of foliar fungicides if needed during the season.

Seed Dressings

Seed dressings registered for the net blotches provide suppression during the seedling stages of crop development. These are most effective when combined with an application of foliar fungicide with a different mode of action during the flag emergence growth stages of crop development.

Seed treatments containing the active ingredient thiram can reduce NFNB severity in seedlings, while treatments containing the active ingredients difenoconazole + metalaxyl can reduce the carry over of seed-borne NFNB. A new seed treatment Systiva (fluxapyroxad) is reported to give good control for net blotch in barley. These treatments will generally require additional applications of foliar fungicide to minimise risk of loss.

Foliar Fungicides

There are a number of foliar fungicide products registered for suppression of NFNB and/or SFNB. Monitor barley crops and apply a registered fungicide when required. Best suppression of the net blotches is provided by application of foliar fungicide during the beginning of stem elongation (GS31) to flag leaf emergence (GS39) growth stages. A single application of foliar fungicide may provide sufficient suppression during seasons with dry conditions but is unlikely to be sufficient to eliminate grain yield and quality loss in severe cases, where a second application is generally warranted. Application of foliar fungicide can be effective during the head emergence (GS49-59) growth stages, but will provide less benefit than if applied earlier.

Further Information

More detailed information can be obtained from the DEDJTR AgNotes Series

Victorian Cereal Diseases Guide (AG 1160)

Victorian Winter Crop Summary

Wallwork H (2015) Cereal Seed Treatments (SARDI), data/ assets/pdf_file/0005/237920/cerealseedtreat2015_web.pdf

Wallwork H (2000) Cereal Leaf and Stem Diseases (SARDI)

For rust identification, send rusted plant samples in a paper envelope (do not use plastic wrapping) to: Australian Cereal Rust Survey. Plant Breeding Institute. Private Bag 4011, Narellan NSW 2567