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CropTalk: Fungicide Timing in Soybeans

July 2021

Published on Thursday, July 1, 2021

When appropriately timed, fungicide applications in soybeans provide one of the most consistent returns of all the things tested through Beck’s Practical Farm Research (PFR)®, even in the absence of significant foliar disease pressure. Beck’s 4-year, multi-location research suggests that the optimal timing is the R3 reproductive stage, which is identified by having a 3/16 in. pod at one of the uppermost four nodes.

To better explain why an application at the R3 stage is most effective, we must first understand how a soybean plant develops and which yield component(s) can be influenced by a fungicide application.

 

 

Yield Components – Soybeans

  1. Nodes/A.
  2. Pods/Node
  3. Seeds/Pod
  4. Seed Size

Yield component #1, the number of nodes produced per acre, is primaril influenced by planting date, another PFR Proven™ strategy for success. However, yield components #2 through #4 can all be influenced by a properly timed fungicide application. If more pods are present than the photosynthate can support, or if the plant is under moisture stress, it will abort individual seeds and/or whole pods. The plant health benefits from specific classes of fungicides (FRAC code 7 and 11) provide additional value other than disease suppression. These benefits include improved water use efficiency, overall stress reduction, and a “greening effect” that increases photosynthesis, all positively influencing on pod/seed retention and seed size by keeping the plant alive and functional later into the grainfill process.

Approximately 70% of a soybean plant’s yield comes from nodes 6 to 13. Many of the uppermost nodes of this region of the plant do not exist at R2 (full flower); therefore, a fungicide application this early has a limited impact on pod/seed retention and seed size. Conversely, at R4 (full pod), many of the lower nodes of this region are too far along in pod/ seed development for a fungicide to have a meaningful impact. Fungicide applications at R3 (beginning pod) work because the nodes that do the heavy lifting (6 to 13) are beginning pod development and are present at the top of the canopy at this time. Most years, the R3 growth stage will last approximately 10 days. It is crucial to hit this window to experience the consistent response year in, year out.

FROGEYE LEAF SPOT

There are exceptions to the R3 fungicide application timing, which include instances of heavy foliar disease pressure. The most common yield-limiting foliar disease that plagues Midwest soybean growers is Frogeye Leaf Spot (FLS). Symptoms typically occur from growth stage R1 to R6 in soybeans, though symptoms can occur at any time. FLS causes small (up to ¼ in.) lesions that are circular or slightly irregular in shape. The lesions begin as small, gray, watersoaked spots and develop into tan or gray necrotic lesions with red to brown borders. Sometimes, the lesions can be surrounded by a chlorotic halo. Lesions are thin and can leave holes in the leaves. In highly susceptible varieties or cases of severe infections, the lesions may join to form larger lesions on the leaves.

In rare and extreme situations, pod and stem infections have been observed. Severe infections may result in premature leaf drop. Seeds may be infected when lesions occur on the pods. Gray blotches may appear on infected seeds, or the disease may be asymptomatic on the seed. Since FLS infects new tissue growth, it is common to find symptoms in one layer of the canopy (i.e., on the sixth trifoliate) but not on leaves above or below that point if favorable conditions for disease development did not continue. When favorable conditions persist, symptoms will continue to appear on new leaves and exist in multiple layers of the canopy.

FROGEYE LEAF SPOT MANAGEMENT

CULTURAL PRACTICES: Frogeye leaf spot overwinters in infested soybean residue. Crop rotation out of soybeans for at least one season will reduce inoculum. Tillage will bury residue and decrease inoculum; however, neither of these will completely mitigate the risk of infection. Since seed can be infected, it is important to plant certified seed that is free of infection and to use a comprehensive seed treatment.

RESISTANCE: Resistance genes have been identified for FLS. Planting resistant varieties is one of the most cost-effective means of controlling FLS. If FLS is a known problem, plant a variety that is rated high for resistance to FLS.

FUNGICIDES: Applying fungicides that are labeled for control of FLS is an effective strategy to control this disease. FLS has shown widespread resistance to the Qol or strobilurin group of fungicides (FRAC code 11). It’s recommended to apply a fungicide with at least two effective modes of action to prevent resistance development.

For more pest and agronomy articles like this: beckshybrids.com/Agronomy/Agronomy-Resources

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Travis Burnett
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Travis Burnett

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