Rhizoctonia root and stem rot on soybean
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Rhizoctonia root and stem rot is a common soybean disease that typically causes most damage to seedlings, but can also damage older plants. It can kill and stunt plants to result in significant yield losses, or the lesions can be superficial and have minimal effects on plant health. Rhizoctonia is a fungal pathogen that infects many different plants in the northern U.S., but only some types of this pathogen infect soybean.
Rusty-brown, dry sunken lesions on stems and roots near the soil line are a characteristic symptom of Rhizoctonia infection. Lateral roots may be decayed. Seedlings or older plants may develop these infections and become stunted, yellow, and may wilt. The infections can be superficial and cause no clear damage to plants, or they can girdle the stem and kill or stunt plants.
Conditions and timing that favor disease
Rhizoctonia root and stem rot occurs primarily in early to mid summer. Infected plants typically appear in patches in a row or field. Several different conditions can favor this disease including, high soil moisture, warm soil temperatures, soil types with high amounts of organic matter, and delayed emergence. Plant stress from herbicide or hail injury or the soybean cyst nematode (SCN) also may favor this disease.
Rhizoctonia solani (a soilborne fungus). This fungus has a wide host range that may include soybean, corn, alfalfa, and other crops; but only some types of this pathogen infect soybean. The most common strains of this pathogen (anastamosis groups also referred to as 'AG') that infect soybean are AG-2-2 and AG-4. Different AG groups can have different optimal conditions for growth and infection.
Encourage seedling health with good agronomic practices and the use of high quality seed. Avoid or reduce plant stress, for example from herbicide injury and SCN infection. Crop rotation and tillage may be of value where disease has been severe. Some seed treatment fungicides may reduce Rhizoctonia infection for a few weeks after planting. Soybean cultivars may have different levels of tolerance, but none are fully resistant to this disease.
Reviewed in 2018