ASK THE AGRONOMIST BLOG

Last year, we saw the disease tar spot in alarming number of corn fields for a variety of reasons, and it’s likely we’ll cross paths with it again this year because of the extra moisture we’ve received (and continue to receive) across the Corn Belt and South. Get to know the signs of tar spot and how you can help mitigate the disease so that you’re prepared for its impending arrival this summer.

What is tar spot?
Tar spot is a fungal disease that affects corn leaves and husks. According to Crop Protection Network, the disease was primarily located in parts of South America, Central America, the Caribbean and Mexico until 2015. The disease has since spread to the United States, affecting areas such as Illinois, Iowa, Indiana, Wisconsin, Ohio, Michigan and Florida. The spores survive on top of the soil during the winter and resurface the next growing season. The fungus, also known as Phyllachora maydis (or P. maydis), forms stromata on the leaves of the plant that can cause defoliation, which can directly affect grain fill around the development stage in corn. The spores are known to travel via wind or rain, putting your entire corn crop at risk of infection if detected.  

How to detect tar spot
This foliar fungus produces small, irregular raised black spots (stromata) on the leaves of the corn plant, affecting both lower and upper surfaces. If wet conditions persist, some growers may even see a gel-like mass form on top of the spores. In some instances, brown spots that resemble fish eyes may appear. These are called fisheye lesions and occur because of a different fungus called Monographella maydis, also referred to as tar spot complex. Michigan State University Extension states that “growers are often describing the visual symptoms of the disease as leaves that have been speckled by black paint or motor oil.”

Tar spot can be mistaken for southern rust, so enlisting assistance from a local agronomist or university expert can help correctly classify the disease. One obvious distinction is tar spot stromata cannot be rubbed off the leaf, whereas corn rust pustules can.

Methods for management
Growers should consider fungicides if tar spot is detected in their fields. For example, Lucento is a newer fungicide that can help control tar spot and can be applied when the disease first appears through the R4 growth stage in up to two applications. According to FMC, the creator of Lucento, the fungicide “attacks disease from the inside out by delivering active ingredients through translaminar and acropetal movement that provides uniform leaf distribution, disease protection and keeps working long after application for control that lasts.”

Fungicide applications are difficult to time with this disease. Inoculation can occur without visible symptoms for 10–14 days. The best indicators of when to apply fungicides for tar spot are:

• Disease present in the area in previous year
• Warm, humid conditions followed by wet periods (cloudy days that allow leaf surfaces to stay moist into mid-morning)
• Tassel through R5 or dent. Fungicides protect photosynthetic tissue up to and through flowering and grain fill. These are critical times to protect with fungicides.
• Protection from secondary infections from fungicides can be beneficial (stalk rots and mycotoxins)

I also encourage growers to respect the rotation. Tar spot can thrive in corn, but it’s not known to survive on soybeans. If tar spot is a serious issue for you in 2019, make sure you give that field a break in 2020, or swap it for another crop to help reduce the risk of tar spot in coming years. Also consider tillage to help reduce the tar spot that may be overwintering in your soil.

If you think you’ve detected tar spot in your fields, don’t delay. Reach out to your local extension office with a sample to confirm the disease. The good news is there are ways to manage tar spot before it devastates your crop.

Last week we focused on issues found in corn during the 2018 growing season. This week, we’re turning our focus on issues found in soybeans throughout the year and how we can prepare for those same scenarios in 2019.

Phomopsis Complex Phomopsis is not a commonly observed disease in soybeans; however, because of the weather conditions that occurred during harvest, in some areas the disease was able to cross the barrier of the pod wall in soybeans and infect the seed within. In severe cases it leads to chalky white, shrunken seeds with a wrinkled appearance. In the case of seed production, infected seed will need to be “scrubbed” prior to planting by applying a fungicide that kills any of the disease that may remain viable on the seed surface. Stine® XP seed treatment products offer excellent control and scrubbing ability for the Phomopsis complex.

Cercospera
Cercospera, or “purple seed stain,” is another disease not regularly seen in the upper Midwest.  However, because of environmental conditions, it has been prevalent in some Midwestern areas. Seed treatments are recommended in the areas that have experienced the disease.

Extended Node Abortion
This phenomenon describes “blank” areas on soybean plants. Many growers reported areas of fields with “blank” nodes or no pods at certain sections in the plant. Perhaps it was mid-canopy or in the upper canopy. We sometimes see one node that will not have pods, but rarely do we see several nodes missing pod set. This situation was caused by extended periods of heat without adequate soil moisture. The plants, in an attempt to preserve existing yield, would abort new pods in favor of filling existing pods. The best management strategy to mitigate this issue is to choose varieties that have the highest yield capability for the maturity zone. Stine soybean seeds are known in the industry to be some of the highest-yielding material, so our portfolio had an exceptional year as we checked yields across the Midwest this fall.

If there was a disease, pest or other area of concern in your fields this year, consult with your local Stine sales agronomist to learn how you can mitigate the issue in 2019.

Part of my job as Stine’s technical product agronomist is to work with our regional sales agronomists throughout the Midwest. This gives me an excellent opportunity to hear about the management systems and tips that are working well across all regions. It also puts me in a unique position to hear of regional issues that affect crop performance, from weather to insects to diseases. Over the next two weeks, we will focus on significant issues noted throughout the 2018 growing season. To start, we’ll focus on corn.

Significant Issues Noted in Corn
In 2018, several unique phenomena have been observed:

• Summer heat early in large parts of the Midwest, followed by cooler, wetter weather patterns.
• Higher than average minimum or overnight temperatures affecting plants.

These weather patterns caused unique diseases to infect earlier and in more widespread areas. They also caused unique responses from all plants including those infected. In corn, those issues included:

• Tar Spot (affecting large areas in Indiana, Illinois, Wisconsin and Iowa)
• Nitrogen cannibalization in corn causing deteriorating stalks
• Apical Dominance Abortion (low ear syndrome)

Tar Spot
Many areas in the upper Midwest experienced tar spot in corn, otherwise known as tropical disease of corn. This disease is prevalent in equatorial regions and is rarely seen in the Midwest. However, with early heat and tropical storms in the South, the disease was carried North. Best practices for managing this disease moving forward will be to select corn hybrids with tar spot tolerance. The University of Illinois and the University of Wisconsin have been collecting samples for rating tolerance to this yield robbing disease. I do not expect an outbreak in 2019; however, we cannot rule it out, as Mother Nature is very unpredictable.

Nitrogen Cannibalization
Parts of Iowa and Illinois experienced strong wind events in July and August, which led to large areas of green snap. In addition, drought was also a major factor because it made nitrogen scarce near tassel time. Tassel is a critical time for nitrogen, so the plants were forced to take it from the stored reserves in the stalks, causing the stalks to become less rigid and more brittle. Conducting soil tests to know both phosphorous and potassium values mitigates this problem. In addition, having adequate sulfur available with nitrogen applications enables more efficient uptake, and utilization of the existing nitrogen can also help in this area. The final component is planting hybrids specifically developed to withstand adverse conditions with exceptional standability and green snap scores.

Apical Dominance Abortion
Growers in certain areas reported hybrids with extremely low ear placement. This phenomenon occurs with unusually hot, dry conditions just prior to tassel when the apical dominance of the corn plant is being determined. The weather conditions caused the primary node to abort in favor of lower node, which is supposed to occur later during plant development. There is no management tip to help with this condition; this is a function of environmental conditions during a specific growth stage in corn.

Check back next week when we focus on prevalent issues found in soybeans in 2018.