This past week I have been busy with disease diagnostic workshops at the ISU Field Education and Extension Laboratory. For those of you who have attended my sessions before, you know that I usually have a quiz that consists of leaves with various diseases and disease look-a-likes or imposters. I thought I’d try and write a series of blogs pointing out differences between a disease and it’s imposters.
Physoderma brown spot
Look for purplish-brown distinct circular spots (Figure 1). These may occur on the midrib, leaf sheath, ear husks, internodes and nodes of diseased plants. They may coalesce to form large blotches but you should be able to make out distinct circular spots on the edge of the blotch or nearby.
Purple leaf sheath and discoloration on the midrib (Figure 2) are often misdiagnosed as Physoderma brown spot. These symptoms are a result of microorganisms feeding on the pollen and dust that collects in between the leaf sheaths or on the leaves. Notice that the symptoms are usually fuzzy blotches that may be quite large (extend the whole of the leaf sheath). The warm, wet and humid conditions we are having at the moment will favor the development of these "Physoderma imposters".
Physoderma is favored by excessive rain during early corn growth (V5-V8). Some parts of northern Iowa were very wet during June. These areas are likely at high risk for Physoderma brown spot.