Jul 18, 2022

How to identify top corn diseases

As corn is beginning to tassel, it is a good time to scout fields to decide if a fungicide will be applied. While you are out scouting, here are some tips from Alyssa Koehler, Extension field crops pathologist at the University of Delaware, for sorting out pathogens.

Grey Leaf Spot

Grey leaf spot (GLS) (pictured at top) 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. The 2022 Fungicide Efficacy for Control of Corn Diseases, provides ratings of product performance across multiple diseases based on trials conducted by Extension specialists across the country.

Curvularia Leaf Spot

Curvularia leaf spot is a new disease that was first observed in the region at the end of 2020. Lesions will have a brown border with a yellow halo that can look very similar to the start of a GLS lesion. However, these lesions will usually stay small and round, while GLS lesions will continue to expand to a rectangular shape (Figure 2). Lesions can be scattered or in dense groups. At present, this disease is not associated with notable yield loss and foliar fungicides are not labeled for management of Curvularia leaf spot.

Northern Corn Leaf Blight

Northern Corn Leaf Blight (NCLB) is present in the regions at low levels, often showing up later in the season. Like many of the foliar pathogens, it is favored by prolonged wet weather and canopy moisture. These lesions will be oblong to cigar shape (Figure 3).

Diplodia Leaf Streak

Diplodia leaf streak can be observed occasionally in the region, most often in fields with corn on corn rotation. These lesions can look similar to NCLB, but inside of the lesions you will see black dots called pycnidia that contain spores of this fungus (Figure 4).

Tar Spot

As you are out scouting this year, you will also want to keep an eye out for Tar Spot, a foliar disease caused by the fungus Phyllachora maydis. It first showed up in northern Illinois and Indiana in 2015 and was found in Lancaster County at the end of the 2020 season and continued to spread to surrounding PA counties in 2021. To date, this disease has not been reported in DE or MD. The fungus produces small, raised, black bumpy lesions that look like specks of tar, giving it the common name of tar spot (Figure 5).

These structures known as stroma can be on the upper or lower leaf surface and do not wipe off the corn leaf. In severe cases, lesions may also be observed on the leaf sheaths, husks, and tassels. Tar spot is most often observed after silking, but can appear earlier, particularly in areas where it is established. If you suspect you have Tar Spot, please contact your county Extension agent or submit a sample to the UD plant diagnostic lab for confirmation.

– Alyssa Koehler, University of Delaware Extension