Last week I wrote an article on scouting Rhizoctonia root rot in soybeans. Before the article was even in print, we saw a significant increase in this disease. The combined effects of fungi, weather, and herbicides have resulted in the increase of Rhizoctonia root rot in some parts of the state.

Rhizoctonia root rot of soybeans.

Rhizoctonia is most aggressive when soybeans are under stress. There are two types of stress involved in the prevalence of Rhizoctonia root rot this year. First is moisture stress. The dry weather has weakened some soybeans, thus they are not in ideal condition. These plants may have been lightly infected during seedling stages. Symptoms worsen when these plants receive excessive stress over a long period of dry weather, especially if they are planted on a hillside. Second, excessive stress from improper herbicide applications, mainly postemergence applications, can promote infections by this fungus. Typical symptoms resulting from this second type of stress are malformed, swollen stems with reddish brown Rhizoctonia lesions at soil lines. Rhizoctonia cannot cause malformation of stems. For a more detailed discussion of soybean root rots, see ICM #15 (pages 106–107).

Unfortunately, there are no effective methods to eliminate this disease if infection has already taken place. If the disease only causes light root rot and reduces plant vigor, using cultivation to mound soil around the base of the plant can promote root growth above the diseased portion. Soybeans can out-grow the light infections.

If you find severe Rhizoctonia root rot in one of your fields, make a note for your next soybean planting. You should consider using fungicides for seed treatment (fungicides containing PCNB) if the field is heavily infested with Rhizoctonia. Seed treatment not only prevents seedling blight but also promotes healthy seedlings tolerant to stress. Robust growth is very important in prevention of Rhizoctonia root rot beyond seedling stages.