Learn How to ‘Spot’ Two Soybean Diseases This Season

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Summer is almost here, and with it comes extended periods of rain and increasingly high humidity. Many diseases develop under these conditions in the Southern United States, especially target spot and frogeye leaf spot.

Before you scout fields with your customers this season, here are some key pieces of information to refer to as you help educate them on proactively managing these two soybean diseases.

Target Spot

Target spot is caused by the fungus Corynespora cassiicola. “This fungus overwinters in soybean residue and soil for up to two years,” according to the University of Arkansas Division of Agriculture Research and Extension. “Therefore, growers should also take into consideration that this disease may be problematic in fields with a history of the disease in cotton and soybean.”1

Early symptomology can be visible in the lower canopy with circular shaped lesions about 1/4 inch to 1/2 inch in diameter with concentric rings that look like a target, and give this disease its name. Defoliation will also progress from the lower canopy to upper canopy as the disease develops through the season.

Extended periods of warm temperature in conjunction with canopy wetness, either by frequent rains or high humidity, during the

reproductive growth stages are prime conditions for increased risk of target spot development. When these conditions are present, disease severity can increase and severe defoliation can be observed in fields. It is important to scout fields when these conditions occur because defoliation starts low in the canopy and may not be noticeable without looking between the rows. Timely applications at the R3 growth stage or at the very start of defoliation are critical to maximize yields when this disease is present.

Frogeye Leaf Spot

Cercospora sojina is the fungus that causes frogeye leaf spot. “Young expanding leaves are extremely susceptible to infection, while fully expanded leaves are relatively resistant,” according to the Oklahoma Cooperative Extension Service.2

Target Spot
Target spot on soybean leaf.
Photo courtesy of BASF


Early symptoms growers should look for include small yellow spots on the leaves, which may develop after long periods of high humidity.

Frogeye leaf spot is known as a polycyclic disease, which means that the disease will undergo multiple cycles of infection as long as favorable conditions persist. According to Purdue Extension, “the greater the number of lesions, the less green leaf area on the plant, and the greater the reduction in yield. If favorable conditions for infection persist until late in the season, the fungus will infect pods and seeds.”3

It’s important for growers to feel confident in their disease diagnosis as oftentimes this disease can be confused for herbicide drift, the Purdue Extension noted.

How can foliar fungicides help?

When it comes to disease management, it’s important to educate your customers on the importance of being as proactive as possible, which means making planned foliar fungicide applications.

Applications of Priaxor® Xemium® brand fungicide can help protect against target spot and frogeye leaf spot by providing long-lasting disease control from chemistry that is continuously distributed throughout the soybean’s leaves. Additionally, the added triazole in Priaxor D fungicide can help provide protection against strobilurin-resistant frogeye leaf spot by providing three modes of action. These two products provide proven Plant Health benefits through long-lasting disease control, increased growth efficiency and increased stress tolerance.

With disease pressure managed, the soybean plant can focus its energy on nutrient uptake and producing yield. In 2014, BASF conducted a research trial in Seymour, Illinois, where it measured the rate of photosynthesis in soybean plants after an application of Priaxor fungicide at R3 as an indicator of growth efficiency. Three days after the soybeans were treated, there was an 8.7 percent increase in photosynthesis which resulted in an 8.8 percent increase in yield compared to the untreated group.4

With applications of Priaxor fungicide, growers have seen consistently healthier, more productive plants that equate to better yields come harvest. On over 80 on-farm trials conducted by BASF between 2012 and 2017 in the southern U.S., including Alabama, Arkansas, Kentucky, Louisiana, Missouri, Mississippi and Tennessee, it was found that soybeans that received an application of Priaxor fungicide saw a positive yield response over the untreated check 95 percent of the time, by an average of 5.6 bushels per acre.5

Before you go out in the field with your customers, help them plan ahead now for disease pressure by developing a strategy for their foliar fungicide use, and educating them on the key disease conditions of target spot and frogeye leaf spot.

Always read and follow label directions.
Priaxor and Xemium are registered trademarks of BASF.
1 Faske, Travis. The University of Arkansas Division of Agriculture Research and Extension. (2016, November). “Target Spot on Soybean: What do we know?” Retrieved from. http://www.arkansas-crops.com/2016/11/02/target-spot-of-soybean-what-do-we-know/
2 Damicone, John. The Oklahoma Cooperative Extension Service. (2017, February). “Foliar Diseases of Soybeans.” Retrieved from. http://factsheets.okstate.edu/documents/epp-7672-foliar-diseases-of-soybeans/
3 Abney, T. Scott; Shaner, Gregory; Westphal, Andreas; Purdue Extension. (2006, August). “Diseases of Soybean: Frogeye Leaf Spot.” Retrieved from. https://www.extension.purdue.edu/extmedia/BP/BP-131-W.pdf
4,5Results may vary.


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