Symptoms and Signs
Halo blight symptoms first appear as small, angular, water-soaked spots (almost resembling little pin pricks) on the undersurfaces of leaves. As these spots grow and turn brown, a characteristic light green to yellow halo appears around the spots. This halo is due to the action of a toxin produced by the bacteria and is a diagnostic symptom of the disease. In severe infections, the leaves and upper parts of plants turn yellow (chlorotic). On pods, small water-soaked spots, about the size of pinpricks, develop that grow into sunken spots and turn reddish brown. Sometimes a creamy white ooze may be seen inside these spots. Pod symptoms of common and halo blight diseases are virtually indistinguishable (see photo comparison). Pod infections can transfer to the seeds inside, causing them to be shriveled, discolored, or smaller than normal size.
Comments on the Disease
Halo blight disease occurs worldwide. It can cause extensive losses under humid, moist conditions and moderate temperatures (60° to 73°F, 16° to 23°C). Fortunately, such conditions are unusual in California and thus halo blight is relatively uncommon. Halo blight bacteria can overwinter in infested debris or seed. Infested seed is the most important inoculum source. Use the table below to distinguish the symptoms of halo blight from common blight and bacterial brown spot.
|Common blight||Halo blight||Bacterial brown spot|
Control of halo blight is very similar to the control for common bacterial blight.
- Plant certified disease-free seed produced in arid regions unfavorable for development of bacterial diseases, such as California and Idaho.
- Use furrow irrigation.
- Avoid sprinkler irrigation since this can provide the needed moisture and humidity for halo blight development in California.
- In fields that have had halo blight problems, deep plow infested debris and practice a 2 to 3-year rotation with a non-legume crop.