Spot blotch

Bipolaris sorokiniana (sexual stage: Cochliobolus sativus)

Biology

Spot blotch is a foliar disease of barley that can be found in some Alberta fields, although it tends to occur more frequently in Manitoba and Saskatchewan. The fungus that causes spot blotch on leaves can also cause seedling blight, is involved in the common root rot complex, and can cause kernel blight, black point or smudge.

The spot blotch fungus overwinters primarily in barley or wheat residue, but also as resting spores in the soil or as hyphae in infected seed. In the spring, spores produced by the spot blotch fungus on infested residue are transported via wind or rain to the growing plant where they can germinate and infect healthy leaves. Infection of the germinating seed and emerging seedling can occur from seed-borne or soil-borne inoculum resulting in poor emergence, discolouration, and seedling blight.

Temperatures above 20^oC and moist humid conditions within the crop canopy favour spot blotch development. Conducive weather conditions may favour successive production of new spores and lesions leading to rapid disease development during the growing season.

Damage Description

Initial infection by the spot blotch fungus results in small brownish spots on leaf blades and sheaths. As these spots or lesions enlarge they are typically oval in shape, brown to dark brown in colour with distinct margins. Chlorosis or yellowing of the leaf may occur around individual lesions. Individual lesions may be up to 1.0 cm in length. Small brown to dark brown spots and discolouration may be observed on head tissues and individual seeds. Although they are similar in colour, spot blotch lesions do not have the typical long, narrow appearance of net blotch lesions. However, spot blotch may be confused with the spot-form of net blotch, which is characterized by small circular to oval, dark brown spots.

Yield losses of 10-30% have been reported, especially when warm temperatures and moist conditions favour disease development during and after heading. Severe infections, especially of the flag leaf and second leaf, reduce the amount of leaf area available for photosynthesis resulting in reduced yield and grain filling. Head infection and subsequent seed discolouration is favoured by warm, moist conditions after heading. Seed discolouration will reduce the potential of obtaining malt status for malting barley varieties.

Diagnosis

Symptoms on seedlings can be observed shortly after emergence. The diagnosis section for barley scald can be used to obtain an estimate of potential yield loss. Differentiation of spot blotch and the spot-form of net blotch will require microscopic examination of spores that are produced from lesions. This can be done by a suitable diagnostic laboratory.

Management Strategy

• Use clean seed and seed treatment to reduce the potential for poor germination, seedling blight, and early season root rot. Consult the AAFRD Blue Book for a list of suitable seed treatments.

• Incorporation of residues into the soil may help to reduce disease risk.

• Use a crop rotation of at least 2 years between cereal crops or grasses. Rotation will help to reduce the amount of infested residue as well as the level of soil-borne inoculum.

• If weather conditions are favourable and spot blotch symptoms are readily present at flag leaf emergence, a fungicide application may be considered. Tilt (propiconazole) is registered for spot blotch of barley.

• Breeders and pathologists are starting to look more at the development of spot blotch resistant varieties, but their commercial availability is likely several years away.