Alyssa Koehler, Extension Field Crops Pathologist; akoehler@udel.edu
Corn has been growing well and we are beginning to see some foliar diseases. Reside-borne leaf diseases like Grey Leaf Spot and Northern Corn Leaf Blight are common across our area. It is possible to select hybrids with good/excellent resistance to these diseases, which will reduce the amount of foliar disease observed. Resistant hybrids typically have smaller lesions and reduced spread of spores. Continuous corn, no/minimal till systems, rainy weather (like we have been seeing) or heavy irrigation, and high plant populations can all create environments that favor foliar disease development.
So far we have seen some cases of Grey Leaf Spot (GLS), caused by the fungus Cercospora zeae-maydis. Under favorable conditions, this disease can significantly reduce yields. GLS usually begins on lower leaves with small, tan, rectangular lesions with a yellow halo. When lesions are young, they can be difficult to distinguish from other common corn foliar diseases. As lesions mature, they become more diagnostic, making it easier to separate from other common foliar diseases. At maturity, lesions are grey to tan in color, with a long rectangular shape; partially resistant hybrids can have more jagged margins than lesions on susceptible cultivars. Lesions can coalesce to form large necrotic areas. Yield reductions are typically observed when lesions are present on the two leaves below the ear leaf of higher. Multiple fungicides are labeled for control of GLS, you can reference the Fungicide Efficacy for Control of Corn Diseases table (https://crop-protection-network.s3.amazonaws.com/publications/cpn-2011-corn-fungicide-efficacy-for-control-of-corn-diseases.pdf). If applying a fungicide, application is typically recommended at VT/R1 timing for greatest chance of economic return. As a general threshold, if 50% of plants have lesions on the third leaf below the ear or higher prior to tasseling, a spray may be considered. Other considerations when making a fungicide application include hybrid susceptibility, environmental conditions, grain prices, and cost of application.