Nathan Kleczewski, Extension Specialist – Plant Pathology; nkleczew@udel.edu
Over the last two weeks I have been hearing more reports of charcoal rot showing up in soybean fields in Delaware and Maryland and Nancy Gregory at the UD Diagnostic Clinic confirmed charcoal in a soybean sample last week. Charcoal rot is a soilborne stem and root disease that tends to develop after pod fill when plants are under heat or drought stress. Premature death occurs but the foliage will remain attached to the stem. The stem and root system may turn grayish in color and one may observe black dots or specks inside the lower stem or underneath the outer stem tissue (Figure 1 a; Figure 2). Note that the appearance of black zone lines in stems is not diagnostic of this disease as suggested in the past (Figure 1 b). Recent research indicates that these zone lines are caused by at least one, if not two, different species of fungi not associated with charcoal rot.
Charcoal rot is caused by a fungus, Macrophomina phaseolina, which can infect over 500 plant species. The fungus overwinters in the soil via overwintering structures. These structures germinate and infect roots. After pod set, the fungus colonizes the stem more aggressively, choking off water movement from the roots to the foliage. This results in wilt and plant death.
Figure 1. Black dots/specks and a grey appearance within stems or roots is diagnostic of charcoal rot (a). Black zone lines in the stem are NOT associated with charcoal rot (b).
Figure 2. Black flecks indicative of charcoal rot may also be observed under the epidermis.
The best management for charcoal rot is to avoid conditions that will stress the plant. Do not plant at excessive populations or overfertilize. Planting under irrigation will help in hot, dry seasons. Rotation or tillage will have negligable impacts. Seed treatments and foliar fungicides are not effective for managing this disease.