Alyssa K. Betts, Extension Field Crops Pathologist; akoehler@udel.edu
The earliest planted full season soybeans across the area are flowering and may be approaching or at R3. The R3 growth stage or “beginning pod” starts when any one of the four uppermost nodes on the main stem with a fully developed leaf has a pod 3/16 inches long (Figure 1). Fungicide applications in soybean are most economical when disease is present and fungicides are applied during R1-R6 growth stages, with R3 being the optimal timing. Most soybean diseases are favored by humid, wet conditions. With the recent dry weather, disease pressure has remained low. When foliar disease is absent or low, the probability of economic return from fungicide application is reduced.
Figure 1. Soybean plant at the R3 growth stage
I have observed low levels of Septoria Brown Spot Leaf symptoms begin as small brown spots that can have a yellow halo around them, as spots coalesce the leaf may turn yellow (Figure 2). This disease usually stays to the bottom of the canopy, limiting yield impact. If you are in an irrigated setting with higher disease pressure and are considering a fungicide application, continue to scout fields and monitor the weather. The 2024 National Fungicide Efficacy Recommendations for Foliar Diseases of Soybean provides an overview of product ratings based on trials conducted by Extension specialists across the country.
Figure 2. Low canopy Septoria brown spot
For dryland fields that have been under a lot of stress, this may be a year where we see earlier leaf drop or more prominent differences in field areas with Soybean Cyst or Root-Knot nematodes. Charcoal rot is another disease that is most visible when plants are stressed by hot, dry conditions and one we will be scouting for this season. Charcoal rot gets its name from small, black survival structures called microsclerotia that will form on and in stem and root tissue of infected plants (Figure 3). Charcoal rot can be first spotted in the driest areas of the field that could include compacted field entrances, sandy areas, etc. The top leaves of infected plants will have premature yellowing and leaf drop and upper pods may fail to fill out.
Figure 3. Microsclerotia of the fungus Macrophomina phaseolina that causes charcoal rot in soybean.