Net Blotches of Barley

Note Number: AG1165
Published: June2007
Updated: June 2009

 

Net blotch is a common disease of barley caused by the fungus Pyrenophora teres. It has two forms: the net form (NFNB) and the spot form (SFNB). The spot form is more common in Victoria. Both diseases can be effectively managed by using a combination of varietal selection, crop rotation and where necessary fungicides.

Disease symptoms

Spot form of net blotch

Symptoms are most commonly found on leaves, but can occasionally be found on leaf sheaths. They develop as small circular or elliptical dark brown spots becoming surrounded by a chlorotic zone of varying width. These spots do not elongate to the net-like pattern characteristic of the net form. The spots may grow to 3-6 mm in diameter. Older leaves will generally have a larger number of spots than younger leaves (Figure 1). 

 Figure 1. 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).

 

Figure 2. Typical symptoms of net form of net blotch 

 

Economic importance

The net form is less prevalent in Victoria, but can be potentially more damaging. Net blotch can cause a significant increase in screenings leading to downgrading from malting quality as well as reduced yields.
Yield losses from NFNB generally range between 10 and 20 per cent, but losses of more than 30 per cent have been reported. SFNB, although common early in the season, rarely develops sufficiently in spring to cause significant yield loss. Yield losses from SFNB generally range between 0 and 10 per cent, but in severe outbreaks can exceed 20 per cent.

Disease cycle

The main source of primary inoculum or initial crop infection comes from infected stubble. However, humid conditions while the crop is maturing may allow NFNB to infect seed, leading to infected crops the following year. Seed infection of SFNB is not regarded as an important source of primary inoculum.

The pathogen can survive for up to 2 years on infected crop residues. Initial crop infection occurs with up to 6 hours of moist conditions at temperatures between 10ºC and 25ºC. Primary inoculum is derived from airborne or splash dispersed spores, ejected up to 40 cm from stubble of the previous crop.

The formation and dispersal of secondary inoculum (conidia) takes place between 14 – 20 days after primary infection. The spores from these leaf infections are dispersed by rain splash or wind, and usually only travel short distances within the crop.
These secondary infections can occur repeatedly throughout the growing season whenever conditions are favourable. As the barley plant begins to senesce, the fungus grows into the stem and can survive on stubble for long periods, producing spores until the stubble is broken down. There is a positive relationship between the amount of stubble from the previous crop and the severity of net blotch in the current crop.

Management

Both SFNB and NFNB can be effectively controlled with an integrated strategy incorporating varietal selection, crop rotation, seed treatment (for NFNB only) and crop monitoring with a view to fungicide applications, if required.

Varietal selection

Avoid susceptible (S) and very susceptible (VS) varieties. Consult a current Cereal Disease Guide (AG1160) when selecting varieties.

Crop rotation

Avoid growing barley in successive years in the same paddock as most inoculum survives in stubble.  Spores are carried by the wind, so crops located close to infected stubble will receive more inoculum than those more distant. Also, disease levels will be higher in crops in districts where barley crops are grown in close rotation.

Seed dressings

Seed dressings are ineffective against the SFNB. However, treat seed with a registered seed dressing for NFNB if seed came from a crop infected by NFNB.

Time of sowing

Early sowing increases the risk of a net blotch infection, but the risk should be weighed up against other agronomic factors.

Foliar fungicides

Monitor the crop and apply a registered foliar fungicide if required. There are a number of products registered to control the net form and/or the spot form of net blotch. Check the product label to ensure the fungicide is effective on the form of net blotch you have, and also check withholding periods.

In Victoria, in trials with low to moderate levels of SFNB (up to 8 per cent leaf area affected), grain yield and quality of barley were not improved by the application of a foliar fungicide, regardless of the product or the time of application. In Western Australia, with more severe levels of SFNB a single application of propiconazole at flag leaf emergence increased grain yield by up to 32 per cent. 
Foliar fungicides are unlikely to improve yield in varieties that are moderately susceptible (MS) or better, and are only likely to be economical in susceptible (S) and very susceptible (VS) varieties when the disease is severe and potential yield exceeds 3 t/ha. The short grain filling period common in the Wimmera and Mallee, due to a lack of spring rain, can often limit crop yield more than SFNB. In such seasons, the best control is achieved by application at the beginning of stem elongation (GS 31). However, if a cool wet spring is predicted a later application closer to flag leaf emergence (GS 37-39) may be more beneficial.

Further References

More detailed information can be obtained from the DPI Information Note Series

Cereal Diseases Guide   (AG1160)
Victorian Winter Crop Summary
Wallwork H (2000) Cereal Leaf and Stem Diseases.

Contact/Services available from DPI

DPI Field Crops Pathology, Grains Innovation Park, 110 Natimuk Rd, Horsham 3400. Tel (03) 5362 2111, or the DPI Customer Service Centre 136 186.

Acknowledgements

This agnote was developed by Mark Mclean and Grant Hollaway in June 2006, and was reviewed by Frank Henry, Farm Services Victoria - BioSciences Research, June 2009. Financial support by the GRDC is gratefully acknowledged.