As soybean fields are blooming, disease scouting needs to start to guide fungicide application decisions.
To determine if a disease will become a significant PEST, that will effect yield, four main factors need to be considered…
- Pathogen presence – can be influenced by field history
- Environment – the right conditions for disease to develop
- Susceptible host – a variety that is susceptible to disease
- Time – regarding the growth stage of the plant and when the disease develops
Scouting for and correctly identifying disease during soybean reproductive growth stages is critical to determine how necessary a fungicide application may be in protecting yield from disease. Some diseases have characteristic symptoms that are easily identified, where others may not be specific and may be mistaken for weather or chemical damage. Pattern in the field where symptoms are being seen and entire plant samples should be examined and when needed can be taken to your local extension county agent for identification.
Some common soybean diseases found in Tennessee include Frogeye Leaf Spot, Septoria Brown Spot, Target Spot, Southern Stem Canker, Charcoal Rot, and Sudden Death Syndrome. Descriptions of symptoms, pictures, and management options for these and other soybean diseases can be found on the mobile friendly field guide at guide.utcrops.com. Many diseases can be avoided by planting a disease resistant variety, regularly rotating to non-host crops, and when warranted fungicide applications can be applied. The searchable variety database at search.utcrops.com can be used to look up disease ratings on varieties as well as yield response to fungicide in different environments (high, moderate, and low disease pressures).
UT research has previously indicated that under circumstances of high disease pressure and conducive environmental conditions, timing a fungicide application at R3-R5 (beginning pod develop to beginning seed development) often provides the most consistent yield protection.
A foliar fungicide efficacy table (Soybean Fungicide Efficacy Table found at UTCrops.com) is developed by The North Central Regional Committee on Soybean Diseases (NCERA-137) and modified as needed by each state to provide information on foliar fungicide efficacy for control of major foliar soybean diseases. The table lists major soybean diseases and rates the different fungicides efficacy for each disease.
Specific fungicide efficacy trials that I, along with my graduate students, have conducted over the past years investigating if different fungicides pair better with different varieties have found that regardless of variety, the same fungicide products protected yield better than others (Figures 1 and 2). The results in the figures are from 2 different studies, one focusing on varieties with different susceptibility to frogeye leaf spot (FLS) – Figure 1; and the other focusing on varieties with different susceptibility to target spot (TS) – Figure 2. Both trials were conducted with 3 different varieties (differing in their susceptibility), 5 different fungicides compared to a non-treated check (NTC) and conducted at 3 different locations that varied in disease pressure (high and moderate pressure, full season locations) and one location planted after wheat (low to no disease pressure).
In the FLS trial, little FLS developed but Septoria brown spot (SBS) was present and could have caused some of the yield loss in the NTC in 2020 (Figure 1). While there were not significant interactions between fungicide product and different varieties on yield (i.e. same responses across all varieties), location was highly significant and lower disease pressure locations did not see any differences in yield from fungicides compared to non-treated checks (Figure 3). Hence, fields that are continuous soybean and disease is present during early to mid-reproductive growth stages have the greatest potential to benefit from a fungicide application; whereas fields that are regularly rotated and free of disease may not benefit from a fungicide application.