Posted 6 October 2008. PMN Crop News.
Charcoal Rot in Beans
University of Illinois at Urbana-Champaign. aces.illinois.edu
Urbana-Champaign, Illinois (September 29, 2008)--Mason County Extension was recently able to examine a few soybean fields with unusual "dead spots" in them. Somewhat stumped by the problem, we contacted Dr. Suzanne Bissonnette with the Champaign Extension Center. Dr. Bissonnette asked us to shave away the epidermal tissue of the root which exposed black to gray streaking.
The sample was apparently plagued by the fungus Macrophomina phaseolina, better known as the organism responsible for the disease termed "charcoal rot".
Charcoal rot typically appears during flowering (mid to late season), when plaguing beans. Infected soybean plants often display a gray discoloration of the root and lower stem that often appears as if that portion of the plant were flecked with small charcoal-like bodies. A red-brown or black discoloration of the root may also be noted. Above ground symptoms may also occur. Leaflets may wilt, brown along the margins, and die, while remaining attached to the stem. Pod development may be significantly marred via charcoal rot in some cases. The disease can appear in hundreds of other plants including corn.
Overwintering fungal material forms a germ tube that penetrates the root of a host plant. Once infecting the host plant, the fungus proceeds to form microsclerotia (fruiting bodies) and mycelia (fungal hairs) in the root and lower stem that physically block xylem (water conducting) tissue. The disease is more likely to appear when soil temperatures are in 80s or low 90s for a couple weeks and when hot, dry conditions prevail. In some cases, the fungus can be transferred to/carried via infected seed which can significantly decrease germination rates. Aphid pressure may further stress the plant, promoting the appearance of this disease.
Charcoal rot management is especially difficult given the wide host range associated with M. Phaseolina, and while crop rotation is noted as a "text book" management tool for this root disease, its overall ability to deter this disease is disputed. Rotation may therefore benefit the producer if it encompasses a couple year shift to small grains and is always combined with additional tools such as the use of moderately resistant varieties (note – moderate resistance appears to be the only resistance option available), conventional or minimum tillage, and adequate but not excessive plant populations.