Figure 1. Irrigated winter wheat field with black chaff disease in Keith County. The disease typically develops in irrigated fields or under extremely wet conditions. (IANR Photos)	Figure 2. Necrotic spots and lesions on a wheat leaf, caused by black chaff.

Figure 3. Darkened glumes caused by black chaff.

Figure 4. Darkening caused by black chaff usually occurs on the upper portions of glume, but can occur on the entire head.

Figure 5. Wheat heads with black chaff disease.

July 3, 2007

Black chaff caused by the bacterium Xanthomonas campestris pathovar (pv.) transluscens has been confirmed in several wheat fields in Keith County. Disease incidence (percentage of heads showing black chaff) is higher in irrigated fields (Figure 1) than in rain-fed fields.

Yield losses of up to 40% due to black chaff have been reported.

Symptoms of black chaff include necrotic streaks and blotches on leaves (Figure 2) and darkened glumes (Figure 3). Usually, darkening occurs on the upper portions of glumes (Figure 3), but entire heads can be darkened (Figure 4).

During wet weather, slime or viscous droplets representing abundant bacterial growth may appear on diseased tissues. These exudates appear light-colored and scale-like when dry.

Black chaff is favored by wet weather. It is more common in irrigated than in rain-fed fields; however, if there is excessive rain during the growing season, the disease can also occur in rain-fed fields (Figure 5).

The black chaff bacterium survives on seed. It also can overwinter on winter cereals and on stubble of small grain cereals and some wild grasses. Infection of wheat occurs mainly through stomata and wounds.

Research has shown that under controlled conditions, transmission of the bacterium from infested seed to seedlings was higher in a susceptible wheat cultivar than in a resistant one. However, under field conditions, transmission from infested seed to seedlings was found to be very low even in the susceptible cultivar.

The bacterium is spread by splashing water, wind-blown stubble and water droplets, and plant-to-plant contact.

Recommendations

Black chaff can be managed by planting certified, pathogen-free seed. Farmer-saved or bin-run seed from fields with black chaff should not be planted.

Seed raised in fields without black chaff is safe to plant regardless of the location of those fields. Seed can be purchased from certified seed producers in southwestern Nebraska and elsewhere in the state.

Consider hauling all grain from fields with black chaff straight to the elevator rather than storing it in bins.

Stephen Wegulo
Extension Plant Pathologist
Robert Klein
Extension Cropping Systems Specialist