Watch Those Beans for Spots and Rots

June 9, 2000
Septoria leaf spot: Soybeans in Illinois are up and growing fast and furious. While you are out in the field scouting for insects, you may notice brown spots on the unifoliate soybean leaves, especially in areas that have been wet. These are most likely caused by a fungus called Septoria glycines, which is the causal organism of brown spot on soybeans. This disease occurs mainly on the leaves and typically does not cause economic damage (see the photo below). The symptoms appear as dark-brown spots that are irregular in shape on both the upper and lower surface of the unifoliate leaves. However, the pathogen can move upward into the plant canopy during periods of warm, moist weather, which favors sporulation of the fungus. Small, brown fruiting bodies of the fungus called pycnidia are visible in the dead tissue of old lesions. Under conditions of continuous cropping of soybeans, infection by this pathogen can be severe and can cause premature defoliation. However, crop-rotation practices reduce the severity of this organism, but brown spot may still be observed in the fields.

Septoria leaf spot or brown spot.

Fusarium root rot: A major problem with periods of cool wet weather, such as that occurring in some areas of Illinois, is that they create cool, wet soils that encourage the development of some root-rotting pathogens. Similar to damping-off and seedling blights (see the update article in issue no. 9 and the seed-treatment article in issue no. 2 of the Bulletin), root rots of soybeans are caused by a complex of soilborne fungi, such as Pythium, Rhizoctonia, and Fusarium spp. These pathogens are more severe under cool, wet conditions because the plants grow more slowly and remain in a susceptible stage for longer periods. This gives the pathogen more time to infect and damage the roots and lower stems. Typically, Pythium spp. causes more of a wet rot, whereas Rhizoctonia and Fusarium spp. cause more of a dry rot. Rhizoctonia spp. produces more of a dark reddish brown or rusty discoloration of lateral and taproots. Fusarium spp. causes more of a reddish or pinkish discoloration. Discoloration is variable because there are many different species, forms, and races of Fusarium. The form that causes sudden death syndrome (SDS), for example, produces blue spores on the taproot of mature beans under favorable conditions (more on this disease in July). Once invasion of the root system occurs by one soilborne pathogen, it becomes fair game for others. When seedlings and young plants that appear stunted and weak are dug up to expose the lower taproot, a pinkish red discoloration (as shown in the photo below) will indicate that the root has been invaded by some type of Fusarium organism. Replant decisions should be based on the time when infection occurs, the extent of infection per plant, and the potential for recovery as well as the amount of plants infected in the field. Seed treatments should be used when replanting soybeans into infected soil.--Loretta Ortiz-Ribbing

Root rot of soybean caused by Fusarium. (Photo courtesy of Mike Roegge.)

Author: Loretta Ortiz-Ribbing