Corn is advancing through growth stages quickly. Dr. Kiersten Wise, extension specialist for plant pathology, prepared the following discussion for fungicide consideration in corn.
Fungicide applications should be considered first for hybrids with lower foliar disease resistance. Hybrids susceptible to foliar diseases are more likely to respond to foliar fungicides, especially if planted in continuous corn or fields under conservation tillage.
University of Kentucky 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) for most foliar diseases in corn. Early vegetative stage applications are less likely to provide an economic return and are not recommended before V-10 in typical growing conditions. Applications that occur at brown silk or later may be too late to realize the full benefit of the fungicide application for diseases like gray leaf spot (GLS).
Recent UK research indicates that fungicide class influences the potential for yield response from foliar fungicide applications occurring at VT. Applications of products containing both strobilurin (QoI; FRAC group 11) and triazole (DMI; FRAC group 3) 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 GLS 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 at https://cropprotectionnetwork.org/.
It is important to accurately identify foliar diseases before deciding if a fungicide application is needed. GLS is the primary disease of concern in the absence of southern corn leaf rust and is caused by the fungus Cercospora zeae-maydis. Early symptoms 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 more than 1-inch-long on susceptible hybrids. The fungus that causes GLS survives the winter in residue. Lesions are typically first observed on the lower leaves and move up the canopy as the season progresses.
Diplodia leaf streak, caused by the fungus Stenocarpella macrospora, was seen last year and also overwinters in residue. It can be confused with GLS in the early stages of development. Small, elongated lesions appear on leaves, sometimes in the mid-canopy, which can help distinguish it from GLS that 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 longer. Small black fungal structures may be visible in the center of the elongated lesions.
Holcus leaf spot (HLS), a disease caused by the bacterium Pseudomonas syringae pv. Syringae, was very common in Kentucky corn last year. It 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. HLS is not known to limit yield. Fungicide applications will not have efficacy against this bacterial disease.
Curvularia leaf spot was also very common in Kentucky last year and can be confused with Holcus leaf spot and gray leaf spot. Curvularia is a relatively new disease in Kentucky, caused by the fungus Curvularia lunata. Small, tan-to-pale yellow lesions can be observed on the leaf and the lesions are often surrounded by a brown or tan halo. Curvularia is not thought to consistently limit yield and no fungicides are labeled for the disease at this time.
Common rust of corn, caused by the fungus Puccinia sorghi, produces brown to brick-red pustules that are present on upper and lower surfaces of the leaves. Young leaves are more susceptible to rust infection than mature leaves. In most years, common rust does not require management. The greatest concern is that common rust is accidentally confused with the more damaging disease southern rust.
Southern rust of corn, caused by Puccinia polysora, does not overwinter in Kentucky but has managed to make its way into the Ohio Valley in recent years. This disease is characterized by orange pustules that form only on the upper surface of the corn leaf. Yield can be significantly reduced by southern rust if it appears before milk stage.
June 20, 2021 at 12:00PM
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Corn Disease Update: Time for Foliar Fungicide Application Decisions in Corn - messenger-inquirer
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