Net blotch symptoms and management in barley

Net blotch causes losses of yield and quality in barley crops across the UK. For management, it is important to understand the sources of infection and the variation in disease symptoms.

Net blotch is a very important disease of barley. It is prevalent across the UK, with no variation in the risk between regions. It can cause typical yield losses of 10–40% where it is not well controlled. It also reduces the quality of grain, increasing screenings and the risk of crop rejection for malting. The disease can be particularly damaging when symptoms continue to develop through the winter and into the early spring, producing an early epidemic as the crop develops.

The disease affects the yields of both winter and spring barley, but tends to be more important in winter barley. The most serious symptoms usually occur on upper leaves during warm humid weather in the summer on unprotected susceptible varieties.

Net blotch infects barley via infected seed or infected trash. Symptoms at the early growth stages can vary, depending on the source of the infection.

Seed infection causes brown stripes to spread from the base of leaves in seedlings and tillering plants, which can look similar to leaf stripe infection. Later on in the season, the symptoms of leaf stripe and net blotch become more distinct, with the symptoms of net blotch becoming more typical of that disease.

Infection from splash-borne spores, from infected trash or neighbouring plants, is the most common cause of infection. Symptoms start as individual spots that can easily be mistaken for ramularia leaf spot. These elongate and turn into brown stripes or blotches with a random netting of darker lines along and across the leaf. Symptoms can be extensive in winter but affected leaves die back and new leaves in spring can be symptom-free.

The ‘spot form’ of net blotch is less common. A chlorotic halo surrounds the oval lesions. Unlike the net form, these spots do not elongate, but grow to be 3-6 mm in diameter. This symptom can also be mistaken for ramularia leaf spot. However, spot form lesions are not rectangular and are not limited by the leaf veins. Net blotch also occurs throughout the season, whereas ramularia leaf spot symptoms typically appear after flowering.

Leaves frequently have yellowing associated with all of these types of lesion, particularly when the symptoms are severe. Late attacks can also affect the glumes and awns, producing dark brown flecking and striping.

Net blotch symptoms on barley leaves

• Left: The relatively common net form, which shows as dark-brown narrow-netted lesion
• Middle: The spot form, which shows as dark-brown circular-to-elliptical lesions
• Right: The stripe form, which shows as long, elongated stripes
• Note: Unlike ramularia, net blotch lesions are not rectangular or restricted by leaf veins

Pyrenophora teres f. teres (Drechslera teres)

Pyrenophora teres f. maculata (spot form)

The disease also affects a wide range of grasses but the forms on barley are specific to this crop.

The disease is seedborne and trashborne. Historically, infected seed was important in spreading the disease. However, the trashborne phase is now the main inoculum source. Infected stubble and debris, along with volunteer barley, allows the pathogen to overwinter.

Spores can be sexually or asexually produced on infected trash. Sexually produced ascospores that arise from pseudothecia are wind-blown and water splashed and can initiate infections. However, asexually produced conidia arising from mycelium initiate and spread most of the disease. Strong air currents release the spores from the leaf and then these conidia are splash dispersed over short distances in the canopy.

Straw residues into which a crop is drilled are the most important source of the disease, but straw in adjacent fields can also provide a source of wind-blown infection.

Infection occurs during periods of prolonged high humidity and temperatures of 10–25°C. Higher temperatures and dry weather inhibit infection. The disease cycle can complete in 14 days in optimal conditions.

• Infected barley trash and volunteers (e.g. second barley crops after minimum tillage)
• Susceptible variety
• High humidity and mild temperatures in the spring and summer
• Early drilling
• High level of seed infection
• Thick crop

Growing a more resistant barley variety reduces the risk of net blotch substantially. However, the pathogen has the ability to overcome this resistance. As a result, monitor all varieties, even those with a high net blotch rating, throughout the season.

Avoiding successive barley crops and minimising previous crop debris from infected crops will reduce the risk of net blotch infection. Controlling volunteers, avoiding early sowing, and managing nitrogen to avoid excessive concentrations in plants also reduce the risk of net blotch.

Use clean seed where possible and do not save seed from heavily infested crops. Test seeds to assess the need for seed treatment: use the agar plate method, which takes 7–10 days, or a molecular test, which takes 48 hours. This gives results as % infection of the seed, which should be treated if there is over 15% infection.

Fungicides

Fungicides used at the main spray timings can protect crops against net blotch.

• Grow a resistant variety
• Avoid second barley crops, minimise infected crop debris and control volunteers
• Manage nitrogen applications to avoid excessive concentrations in plants
• Avoid early sowing
• Avoid using highly infected seed
• Treat infected seed with an effective seed treatment
• Use fungicides to control net blotch at the T1/T2 timings
• Use mixtures to combat fungicide resistance