Nathan Kleczewski, Extension Specialist – Plant Pathology; nkleczew@udel.edu
Over the past 1-2 weeks several growers have asked about the black/brown spots they are seeing on their corn stalks (Figure 1). In many instances these spots may be a result of Purple leaf sheath or Physoderma brown spot. In both cases, the black spots do not affect the stalk. Check this out for yourself by removing a sheath with brown spots. The discoloration is limited to the inside of the leaf sheath and the stalk should be healthy.
Purple leaf sheath develops when weak bacterial or fungal parasites grow underneath the leaf sheath. Here the microbes grow on pollen and other particulate matter lodged between the stalk and the leaf sheath. Eventually they grow into the leaf sheath, resulting in a purple/brown/black spot. Lesions are darker on the inside of the leaf sheath and this should make sense because this is where infection occurs. Insect damage may contribute to occurrence of purple leaf sheath.
Figure 1. Brown spots on corn stalks commonly seen in Delaware corn fields this year.
Physoderma Brown Spot is caused by Physoderma maydis, a fungal like organism. Unlike purple leaf sheath, Physoderma infection results in brown to black “pinhead” spots on leaf midribs and leaf sheaths (Figure 2). Small, oblong, yellow spots may occur on the leaf blade (Figure 3). The organism overwinters in infected tissue and infects corn in the whorl under warm, wet conditions. Because of this, Physoderma lesions often occur as bands of disease, not independent spots as in Purple leaf sheath.
Figure 2. Close up of pin head black/purple spots caused by Physoderma maydis.
Photo: Clemson University –USDA Cooperative Extension Slide Series, Bugwood.org
Figure 3. Symptoms of Physoderma maydis on the leaf.
Photo: Clemson University –USDA Cooperative Extension Slide Series, Bugwood.org
A third spot you may come across looks different than Purple leaf sheath and Physoderma. These spots are large, oval, tan to gray, and often have a black border. These spots are likely caused by Gray leaf spot. Gray leaf spot lesions on leaf sheaths are not rectangular as they appear on foliage because leaf sheaths are more succulent, so the veins do not impede fungal growth. The presence of Gray leaf spot lesions on the sheaths should not cause any direct issues with stalk health.