Alyssa Koehler, Extension Field Crops Pathologist; akoehler@udel.edu
As tasseling in corn approaches, it is a good time to scout fields to decide if a fungicide will be applied. When considering the economics of a fungicide application, it is important to know your potential for disease based on field history, environmental conditions, and hybrid selection. Many of the foliar pathogens of corn can survive in residue, so corn-on-corn fields carry a higher potential for disease, especially if disease has been observed in previous years. Hybrids with good resistance ratings may not need a fungicide. Resistant hybrids typically have smaller lesions and reduced spread of spores. In dryland fields, hot, dry weather will keep disease pressure low.
Reports of foliar diseases have been minimal so far this season. Irrigated fields keep enough moisture to favor environments for disease and may see development of Grey Leaf Spot (GLS) or possibly Northern Corn Leaf Blight (NCLB). GLS is our most common foliar disease of corn. Symptoms usually begin on lower leaves as 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. At maturity, lesions are grey to tan in color, with a long rectangular shape (Figure 1); partially resistant hybrids can have more jagged margins than lesions on susceptible cultivars. Lesions often join to form large necrotic areas under favorable environmental conditions. Yield reductions are typically observed when lesions are present on the two leaves below the ear leaf or higher, so these are the leaves to pay close attention to when scouting. If over 50% of plants have lesions on 5% or more of this leaf surface (flag leaf or 2 below), you may want to consider a fungicide application. If applying a fungicide, VT/R1 timing has shown the greatest chance of economic return for GLS and NCLB. Last week we shared the 2022 Fungicide Efficacy for Control of Corn Diseases, which provides ratings of product performance across multiple diseases based on trials conducted by Extension specialists across the country.
Figure 1. Rectangular lesions of Grey Leaf Spot on Corn