Bacterial Blights

Bacterial blights first appear as small, water-soaked spots extending along leaf margins and midribs. Lesions appear translucent when held up to light. The lesions expand into spots, blotches, or streaks.

Bacterial leaf streak affects barley, wheat, triticale, rye, and, occasionally, oats. Lesions turn yellow, then eventually appear as brown, necrotic streaks along leaf veins. Droplets or a shiny film form on the streaks in damp conditions, leaving hardened yellowish granules or scales when leaves dry. Exudates turn milky and spread on the leaf surface when leaves are moist, ultimately forming a thin layer of transparent flakes on the leaf surface. Black chaff blackens glumes and is caused by the leaf streak pathogen when bacterial propagules land on the spike in wet weather during heading.

Bacterial leaf blight affects wheat. Lesions develop into light tan spots, blotches, or streaks. Halo blight affects oats. Light tan spots with a pale green margin (halo) may grow together to form blotches. Heavy infections kill leaves. Stripe blight affects oats. Light tan spots do not have a pale margin, and usually enlarge to form stripes. Heavy infections kill leaves.

Blights seldom cause significant damage in California because they develop only during wet weather. These bacteria survive in soil and on crop residue, volunteer grains and wild grasses, and grain seed. From the seed surface, bacteria spread to aboveground parts of the plant, and infection can occur before the first leaf emerges.

Cultural Control

Use certified seed because bacterial leaf streak is seed borne and seed is the most common source of primary inoculum. Practice crop rotation and exclude alternative hosts, including weeds. The pathogen is short lived in the soil, so rotation with a nonhost crop will reduce inoculum in the field. If blight becomes a problem, avoid overhead and sprinkler irrigation to reduce canopy wetness. Incorporate crop residue into the soil in infected fields to reduce inoculum in future seasons.