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Purple leaf sheath is an abnormality that is commonly found in corn fields; however, it does not threaten yield potential. This condition is characterized by irregular-shaped purplish-brown lesions that appear on the leaf sheath (Figure 1). The leaf sheath is the lower part of the leaf that encloses the stem and is connected to the leaf blade at the junction formed by the leaf collar.
The area between the collar and stalk can be a place where moisture, nutrients, pollen, and anthers collect. Yeasts that commonly live on the corn leaf surface, as well as secondary, saprophytic organisms, can thrive on the nutrients in this environment.1 The result is purplish blotches appear on the corn leaf sheath, but are not considered a threat. The stalk underneath the sheath is not infected or discolored.
Water behind the whorl or behind the leaf sheath also provides an environment for the fungus Physoderma maydis, which is the cause of Physoderma brown spot (PBS). PBS may be confused with purple leaf sheath, however, PBS lesions also appear on the leaves. PBS symptoms initially appear as small round to oblong lesions, yellowish to brown in color (Figure 2). Lesions expand in size by joining with neighboring lesions and tend to darken in color from brown to reddish-brown or purple.2
Purple leaf sheath is a non-infectious disease, which does not have an effect on yield potential. Purple leaf sheath symptoms can be found throughout some corn fields; however, frequency of purple leaf sheath lesions do not warrant a fungicide application.