Barley net blotch

Barley net blotch is a fungal leaf disease caused by Pyrenophora teres. The pathogen occurs mainly as the net-form type, which creates brown, net-like lesions, and the spot-form type, which produces discrete brown spots with yellow halos. It survives in infected seed and barley residue, spreads by wind-driven spores, and is favored by humid weather and susceptible cultivars.

Barley net blotch is a foliar disease of Hordeum vulgare caused by the closely related net-form and spot-form types of Pyrenophora teres. The net form produces expanding brown lesions with dark longitudinal and transverse lines; the spot form produces more discrete brown, oval or elliptical lesions, often with chlorotic halos.

The disease is important wherever barley is grown, including the northern Great Plains and other U.S. barley-producing regions. Visual diagnosis is useful, but lesion appearance can vary with cultivar, pathogen population and environment. Laboratory confirmation may be needed when symptoms resemble barley stripe, tan spot or other leaf diseases.

• Net form: small brown flecks enlarge into elongated, brown lesions with a characteristic dark reticulated or net-like pattern, often surrounded by chlorosis.
• Spot form: dark-brown, round to elliptical spots develop on leaves and leaf sheaths, commonly with yellow or pale-green halos.
• Lesions may merge, causing large necrotic areas, premature leaf senescence and loss of photosynthetically active tissue.
• Severe disease can affect upper leaves, reduce kernel fill and contribute to yield or quality loss.

The pathogen is introduced or maintained through infected seed, infected barley residue and volunteer barley. Spores produced on infected residue or diseased leaves are dispersed by wind, rain splash and field operations. Repeated barley production, reduced rotation, susceptible cultivars, dense canopies and prolonged leaf wetness increase epidemic risk.

Both pathogen forms show substantial genetic and virulence variation. Consequently, a cultivar that performs well against one population or form may be less effective against another. Disease identification should distinguish net-form and spot-form symptoms where possible.

Pyrenophora teres survives between crops in infected barley seed and crop debris. Under moist conditions, the fungus produces airborne or splash-dispersed spores that infect young or mature barley leaves. Infection is favored by extended surface wetness and moderate temperatures.

New lesions produce additional spores, allowing repeated secondary infection during the growing season. The disease often intensifies from lower leaves upward, especially when wet weather, dense foliage and susceptible barley genetics maintain a humid canopy. Sexual reproduction and pathogen diversity contribute to changing virulence patterns, although the epidemiological importance varies among regions.

Begin with accurate diagnosis and an assessment of disease severity, crop growth stage, cultivar susceptibility and yield potential. In commercial barley, integrated management commonly combines resistant genetics, rotation, residue management, clean seed and, where justified, a properly timed foliar fungicide.

Fungicide decisions should follow current local extension recommendations and label directions. Protecting the upper canopy before disease becomes severe is generally more useful than attempting to recover extensively necrotic leaves. Avoid relying on repeated use of one fungicide mode of action, because P. teres populations can vary in sensitivity and resistance risk.

For home or small-scale plantings, remove heavily infected volunteer barley and dispose of diseased plant material rather than composting it in a cool backyard pile. Do not save seed from affected plants.

• Use certified clean seed and resistant or tolerant barley cultivars whenever available.
• Rotate away from barley and control volunteer barley to reduce the local inoculum reservoir.
• Remove or thoroughly incorporate infected residue where agronomically and environmentally appropriate.
• Maintain balanced fertility; avoid excessive nitrogen that produces dense, slow-drying foliage.
• Use regular scouting and sanitation as the primary low-input measures.

Biological products and organically permitted fungicides may have activity in some production systems, but performance depends on formulation, timing, disease pressure and local registration. They should be treated as components of an integrated program rather than guaranteed cures.

• Plant certified, high-quality seed from reputable sources and avoid planting visibly diseased seed lots.
• Select cultivars with documented resistance or improved field tolerance to the locally prevalent net-blotch form and pathogen population.
• Use crop rotation and avoid successive barley crops when practical, especially where residue-borne disease is severe.
• Manage volunteer barley and grassy hosts in and around fields.
• Reduce prolonged canopy wetness through appropriate seeding rates, fertility management and good field air movement.
• Scout from tillering through grain filling, concentrating on lower leaves first and checking whether lesions are net-like or spot-like.

• The species name Pyrenophora teres covers two epidemiologically important symptom types: net-form net blotch caused by P. teres f. teres and spot-form net blotch caused by P. teres f. maculata.
• The two forms are difficult to separate by ordinary spore shape alone; symptom phenotype, pathogenicity tests or molecular methods may be needed for confident identification.
• Pathogen populations are genetically and virulently diverse, which helps explain differences in cultivar performance between regions and years.
• Net blotch is a disease of barley rather than a general disorder of all cereal crops; similar lesions on wheat may indicate a different pathogen.