Alyssa Koehler, Extension Plant Pathologist
University of Delaware
The beginning of this year was very conducive for Pythium species. Seed treatments typically have efficacy for about two weeks. In many cases this year, stands came up well, but then suffered from post-emergent damping off or stunted, slower growing plants following favorable environmental conditions for disease 3-4 weeks after planting (Figure 1). Infected plants typically have a brown to rotted mesocotyl when dug up (Figure 2). In severely infected plants, the top of the plant may become completely separated, resulting in death of the plant. In less severe conditions, the plant is able to set new brace roots and continue growing. Damping off from Pythium is common in low field areas that hold more moisture, but wet, cool spring conditions favored development across entire fields this season.
Over the past few weeks, corn has been growing well and we are now beginning to see some foliar diseases. Reside-borne leaf diseases like Grey Leaf Spot (GLS) and Northern Corn Leaf Blight (NCLB) 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 or heavy and high plant populations can all create environments that favor foliar disease development.
Grey Leaf Spot, caused by the fungus Cercospora zeae-maydis has begun to show up in fields. Under favorable conditions, this disease can significantly reduce yields. GLS usually begins on lower leaves with small, tan, rectangular lesions with a yellow halo (Figure 3). When lesions are young, they can be difficult to distinguish from other common corn 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. Northern Corn Leaf Blight has not been as common yet this year. Symptoms from this disease are long, tan to grey lesions that become oblong to “cigar-shaped” with age (Figure 4). For both of these diseases, lesions can coalesce to form large necrotic areas and yield reductions are often observed when lesions are present on the two leaves below the ear leaf of higher.
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. The Fungicide Efficacy for Control of Corn Diseases table prepared by the Corn Disease Working Group provides information on fungicide efficacy for various corn diseases (https://crop-protection-network.s3.amazonaws.com/publications/cpn-2011-corn-fungicide-efficacy-for-control-of-corn-diseases.pdf).
