Aerial blight, also known as Rhizoctonia foliar blight and web blight, is caused by the fungus Rhizoctonia solani. Aerial blight is mainly a problem in the southern U.S. where soybeans are grown in rotation with rice. First leaf symptoms appear as water-soaked, grayish green lesions that turn tan to brown at maturity. Symptoms normally develop on the lower portion of the plant after the canopy closes during late vegetative stages.
Foliar symptoms of aerial blight on leaves of soybeans. Image: T. Allen.
Leaves, pods and stems in the lower canopy may be infected. Reddish-brown lesions form on infected petioles, stems, pods and petiole scars. Mycelium spreads on the surface of plants during warm (77-90°F), wet weather and forms localized mats of “webbed” foliage. Small (1/16 to 3/16 inch diameter), dark brown sclerotia form on diseased tissue. The fungus overwinters as sclerotia in soil or plant residue.
White fungal mycelium of aerial blight formed on leaves of soybeans. Image: T. Allen.
Aerial blight of soybeans with white fungal mycelium visibile in the morning formed on leaves and petioles. Image: T. Allen.
Some varieties are less susceptible than others, however, there is little resistance in soybean. Fields with a history of sheath blight of rice have a higher risk of aerial blight. Crop rotation with poor or non-host crops for multiple years can reduce inoculum. Wide rows and lower populations may open the canopy and reduce aerial blight.