Charcoal rot in soybeans is a more recently discovered disease that has proven to be extremely impactful on yields. It is more prominent in the southern states but has been found to be inhabiting fields in the North Central regions of the U.S. and Ontario, Canada. It tends to occur in hot, dry conditions.
The fungus responsible for charcoal rot survives in dry soil or infected plant residue. Hosts include corn, soybean, grain sorghum, sunflowers, and a number of weeds. It prefers soil temperatures between 80 and 91 degrees F and infects plants earlier in the spring when moisture levels are higher. Symptoms appear during the hottest, driest parts of summer, often during reproductive stages.
If charcoal rot infects seeds or seedlings, plants may never emerge or seedlings may become discolored and die. In mature stands, usually plants in the dryest parts of the field begin to show symptoms first. The earliest symptoms are smaller-than-average leaves, reduced vigor, and premature yellowing of leaves at the tops of the plants. Plants also begin wilting during the hottest parts of the day. Upon closer inspection, the lower stem and root surfaces adopt a light-gray discoloration. If a cross-section of the stem is taken, a line of decay and discoloration can be observed. In order to not confuse the disease with symptoms of drought stress or SDS, it is helpful to pull plants and observe the stem cross-section. Most infected plants will eventually die prematurely.
The majority of serious infections during hot, dry periods result in moderate to extensive yield loss. Most plants are severely stunted in growth, while many die prematurely. There may be entire fields that show symptoms of charcoal rot. In the case that infection is moderate and weather is more favorable, root mass and drought tolerance will decrease, leaflets will be smaller, vigor is reduced, wilting occurs, and some plants may experience premature death. In each case, seed pods sometimes abort, yielding lighter and fewer seeds. Many growers incorrectly blame drought as the reason for the low-quality seed, rather than charcoal rot.
Tillage can be helpful in reducing the presence of infected crop residue, but it may also cause excess drying of the soil. No-till systems have been known to aid in moisture conservation and higher soil nutrition, leading to healther stands. Eradicating surrounding weeds is very important in order to avoid moisture competition as well as disease hosts. It can also be helpful to avoid plant stress through proper irrigation.
There is no known chemical treatment for charcoal rot. Please contact your local plant pathologist to learn what is most effective in your area and how to use it best. Always read and follow all label instructions.
Crop rotation has been somewhat effective, but it must be longer rotation for better overall effects. Reducing seeding rates of soybeans may reduce stress. Overall, the best option is to use varieties that have some level of resistance to charcoal rot.