Corn is moving through growth stages quickly, and the warm, humid weather in many parts of Kentucky has been conducive for foliar disease development. Fungicides are commonly promoted to reduce the impact of foliar disease in corn, but with tight margins and a difficult market, it is important to pay careful attention to the factors that influence the profitability of a fungicide application.

1. Crop Production Factors

Check hybrid ratings for foliar diseases such as gray leaf spot prior to fungicide application. Fungicide applications to hybrids with good foliar disease resistance are less likely to provide economic returns. Hybrids susceptible to foliar diseases are more likely to respond to foliar fungicides, especially if planted in continuous corn or fields under conservation tillage. These fields are at higher risk for foliar disease development since the fungi that cause several foliar diseases survive in residue. Additionally, irrigated fields are at higher risk for foliar diseases since irrigation creates an environment favorable for disease development.

2. Fungicide Timing

University research indicates that foliar fungicides applied at tasseling or early silking (VT-R1) provide optimal foliar disease control and also the best chance for seeing a yield response, compared to applications that occur after "brown silk" (R2). Early vegetative stage applications are less likely to provide an economic return, and applications that occur at "brown silk" or later may be too late to realize the full benefit of the fungicide application.

3. Fungicide Class

Recent University research indicates that fungicide class influences the potential for yield response from foliar fungicide applications occurring at VT. Applications of products containing both strobilurin and triazole fungicide classes are more likely to result in a positive return on fungicide investment compared to applications of products containing only a strobilurin or triazole fungicide active ingredient. Fungicide classes and efficacy of specific fungicide products for foliar diseases like gray leaf spot are described in the updated fungicide efficacy table for management of corn diseases, which is developed by the national Corn Disease Working Group, and posted on the Crop Protection Network website: https://cropprotectionnetwork.org/resources/publications/fungicide-efficacy-for-control-of-corn-diseases

It is important to accurately identify foliar diseases before deciding if a fungicide application is needed. There are several diseases appearing across Kentucky, some of which do not warrant fungicide application. The most common corn diseases observed over the last week are described below.

Gray leaf spot

Gray leaf spot is caused by the fungus Cercospora zeae-maydis. Early symptoms of gray leaf spot are observed on leaves as tiny lesions surrounded by a yellow halo. It is difficult to diagnose GLS when the lesions are very small, but these lesions will elongate into narrow, rectangular, brown to gray spots, expand parallel to the leaf veins, and may grow to over one-inch-long on susceptible hybrids (Fig. 1). Symptoms vary depending on hybrid susceptibility, and hybrids with some level of resistance to gray leaf spot may only have small, jagged lesions rather than the long, rectangular shape characteristic of lesions on more susceptible hybrids. The fungus that causes gray leaf spot survives the winter in residue, and typically lesions are first observed on the lower leaves, and move up the canopy as the season progresses.

Figure 1: Gray leaf spot lesions (photo by Kiersten Wise)

Diplodia leaf streak

Diplodia leaf streak, caused by the fungus Stenocarpella macrospora, can be confused with gray leaf spot in the early stages of development. Small, elongated lesions appear on leaves (Fig. 2), sometimes in the mid-canopy, which can help distinguish it from gray leaf spot, which typically appears in the lower canopy and progresses into the mid-upper canopy. The lesions will expand over time into streaks that are several inches or more long. Small black fungal structures may be visible in the center of the elongated lesions. Diplodia leaf streak is a disease that has recently become more prominent in corn, and the link between disease and yield loss has not been established. Currently, there are no fungicides labeled for Diplodia leaf streak. The fungus that causes Diplodia leaf streak survives in residue and therefore rotation or residue management can help reduce disease in future years.

More information on Diplodia leaf streak can be found in the UK Extension publication:

Figure 2: Early lesions of Diplodia leaf streak (photo by Kiersten Wise)

Holcus leaf spot

Holcus leaf spot is very common in Kentucky corn this year. Holcus leaf spot is a disease caused by the bacterium Pseudomonas syringae pv. syringae, and is characterized by round, discrete lesions that are initially pale yellow to white and then enlarge and turn gray or brown. Lesions have a water-soaked halo and on certain hybrids, the margin of the lesion may appear brown or purple (Fig. 3). Holcus leaf spot is not known to limit yield. Although the disease may cause concern based on symptom appearance, no in-season treatment is available or necessary. Fungicide applications will not have efficacy against this bacterial disease.

Figure 3. Holcus leaf spot (photo by Kiersten Wise)

Common rust

Common rust of corn, caused by the fungus Puccinia sorghi, is present at low levels in most Kentucky corn fields every year. The fungus that causes common rust produces brown to brick red pustules that are present on upper and lower surfaces of the leaves (Fig. 4). Young leaves are more susceptible to rust infection than mature leaves. In most years, common rust does not require management in hybrid field corn in Kentucky, and the greatest concern is that common rust is accidentally confused for the more damaging disease southern rust.

Figure 4. Common rust (photo by Kiersten Wise)

Southern rust (not currently in Kentucky)

Southern rust of corn, caused by Puccinia polysora, has been a concern of farmers for the last few years. This disease is characterized by orange pustules that form on the upper surface of the corn leaf (Fig. 5) The fungus that causes this disease does not survive winters in Kentucky, but moves north each year from Mexico, and states in the southeastern U.S. As of July 1, 2019, southern rust has been confirmed in a few counties and parishes in Georgia and Louisiana. If corn can reach milk stage (R3) before southern rust appears in Kentucky, there is likely to be little to no impact on yield. Real-time monitoring of southern rust can be observed by checking https://corn.ipmpipe.org/southerncornrust/

Figure 5. Southern rust (photo by Kiersten Wise)

For more information contact the Pulaski County Extension office.