Figure 1. Soybean plants dropping leaves prematurely due to charcoal rot. Credit: E. Byamukama
A few fields scouted last week had soybean plants dropping leaves prematurely. Upon checking these plants, they were found to be infected with charcoal rot (Figure 1). Areas in a field with plants droppings leaves earlier than normal should be scouted to rule out diseases such as charcoal rot, brown stem rot, or sudden death syndrome.
Causal Organism, Symptoms, & Signs
Charcoal rot is caused by a fungal pathogen Macrophomina phaseolina. This pathogen survives both in the soil and in plant residues and also infects the plant through the roots. The pathogen clogs the roots which leads to a blockage of water and mineral salts being transported. Infection takes place early in the growing season but symptoms develop later in the season when plants are under stress. The presence of soybean cyst nematode in a field exacerbates charcoal rot symptoms.
To differentiate charcoal rot from other diseases which may cause early leaf drop such as sudden death syndrome, uproot a symptomatic plant in the middle of the patch and peel off the tap root epidermis. If small black speckles are observed the plant is infected with charcoal rot (Figure 2). Also splitting the stem will show the black speckles resembling charcoal powder, hence the name charcoal rot .
Figure 2. Charcoal rot sign on a taproot. Notice the small black spots embedded in the taproot tissue. Credit: E. Byamukama
Charcoal rot can be managed by avoiding practices that increase plant stress. Reduce plant stress by planting at recommended seeding rates, using no-till which helps to conserve moisture, rotating to non-host crops, and practicing weed control in order to avoid competition. Crop rotation can help to reduce the amount of inoculum; however, the charcoal rot pathogen has many hosts. For fields with a history of charcoal rot, select a good rated cultivar. No resistance is available for charcoal rot but there is tolerance among cultivars.