Corn planting was delayed in Kentucky in 2022, and this has prompted questions about how growers should be thinking about corn disease management this year.
Weather will play the biggest role if disease develops and becomes a concern. Remember that favorable environmental conditions are needed for disease to develop. For diseases like gray leaf spot, caused by Cercospora zeae-maydis, and southern rust, caused by the fungus Puccinia polysora that means warm, humid days, and frequent moisture. Frequent rains are needed to result in optimal conditions for the development and spread of many diseases, and that has been a limiting factor in many areas of Kentucky over the past few weeks.
Other factors that increase disease risk are planting hybrids that are susceptible to foliar diseases, planting continuous corn, and/or planting corn into high-residue fields. Fungi that cause diseases such as gray leaf spot survive the winter in residue, and fields with higher levels of residue may be more likely to have problems with these diseases.
The exception to this is southern rust of corn. The fungus that causes southern rust does not overwinter in Kentucky, but spores of the fungus move north on wind currents and weather each summer. We can track the movement of southern rust by watching the map on the CornIPMpipe website. On the map, red counties/parishes indicate that southern rust has been confirmed by university/Extension personnel.
The potential impact of any disease in Kentucky will depend on the crop growth stage of a field once disease is confirmed in an area. If conditions are favorable for these diseases as corn approaches tasseling and risk factors exist in a given field, a fungicide application may be needed. Research at the University of Kentucky, and research from other states indicates that fungicides may be needed to protect yield while corn is in the tasseling through blister stages (VT-R2) for diseases like gray leaf spot and from tasseling through milk (VT-R3) growth stages for southern rust.
The efficacy of specific fungicide products for foliar diseases 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.
Tar Spot Update
Finally, there has been much discussion about tar spot, caused by Phyllachora maydis, in Kentucky. In 2021, tar spot was confirmed in two counties in Kentucky, Todd County and Ohio County. To date, these are the only confirmed locations with tar spot in the state.
As a reminder, tar spot is usually first observed when the causal fungus produces small black structures called stromata on leaf tissue. These structures protrude from the leaf surface, and affected areas of the leaf feel rough or bumpy.
At this point in the season, it is easy to confuse insect frass with tar spot. If the black spot on the leaf can be rubbed or washed off, it is not tar spot, and it is likely insect frass. Later in the season when plant tissue senesces, fungi that use dead or dying tissue as a food source colonize the tissue, leaving a black sooty appearance that can be confused with tar spot. Saprophytic growth does not harm the plant, and will not produce the raised or bumpy fungal structures produced by the tar spot fungus.
Although you may continue to hear about tar spot, it is important to remember that UK plant pathologists cannot yet accurately predict the impact of the disease in our Kentucky climate. A few points to remember:
- Tar spot was first confirmed on corn in the United States in 2015. Since 2015, it has been reported in multiple Midwestern states and Ontario Canada, and as far south as Georgia (confirmed 2021) and Florida. It has caused significant yield loss in two of the seven years (2018 and 2021) since it has been in the United States.
- The most significant yield losses have been in areas surrounding the Great Lakes. The fungus prefers cool (60-70°F) temperatures and frequent rainfall or moisture. Few areas of Kentucky have these optimal conditions for tar spot when corn is tasseling through early grain fill. Yield loss due to tar spot varies, and depends on hybrid susceptibility, infection timing, and environmental conditions. Even if tar spot is confirmed in a field, conditions may not be favorable enough for it to develop into a yield-limiting disease.
- Research on tar spot is ongoing and has primarily occurred in northern states. UK researchers are working to understand the disease in Kentucky, so that if needed, they can make more specific management recommendations that fit Kentucky's climate and production practices.
- It is important to alert your county Extension agent if you think you have tar spot so distribution can be documented in the state. Please contact your county Extension agent to submit a sample to the Plant Disease Diagnostic Lab for confirmation.
Categories: Kentucky, Crops, Corn