Updated November 6, 2019:
Across all sites, the only rateable disease was Soybean Vein Necrosis Virus (Wye). Thus, no disease data are presented. After assessing the greenseeker data, it was evident that utilizing an early and late rating provided the same amount of information as all four ratings. Thus, we present ratings from 9/15 (Greenseeker 1) and 10/25 (Greenseeker 2) here. Data showed that fungicide timing significantly impacted greenseeker ratings at 9/25, and percent green stem. For the greenseeker rating, NDVI was significantly greater (thus more canopy closure or healthy tissue at 9/15) compared to untreated controls or treatments applied at R3. The percent green stem of plots treated with Priaxor at R1 or R3 were approximately 7% greater than untreated control plots. Spacing affected percent green stem and yield. Double cropped soybeans planted on 7.5” rows averaged approximately 10% more green stem than plots planted on 15” spacing. Yield was significantly greater in the 7.5” spacing, when compared to the 15" spacing.
The first year of this study provided insight into the impacts of fungicide use and spacing on disease and productivity in double cropped soybean systems commonly used in Delaware and Maryland. Despite ample residue, the use of a variety with susceptibility to common foliar diseases, and significant rain during several periods of the growing season, there was not enough foliar disease to rate. Profitable fungicide use is best attained when using a susceptible variety and the environment is conducive for disease. Although this was only a single season, our results indicate that other aspects of double crop production may limit foliar fungal disease and subsequent yield impacts associated with fungicide use. In double crop systems, soybean residues have more time to decompose due to the presence of corn and small grains, as well as additional residue sizing resulting from light tillage. Consequently, this can reduce the amount of residue borne foliar diseases such as Septoria brown spot, frogeye leaf spot, and pod diseases such as phomopsis pod and stem blight. Other aspects of the double cropped system are at a disadvantage for foliar fungal disease development. For example, later planting dates result in canopy closure during hotter, drier periods of the year, which may not favor disease development. Additional years and sites would give us more confidence as to the likelihood of fungicide need in double cropped soybean systems, as we expect variability in disease between growing seasons. Regardless, the lack of any disease across four sites despite a susceptible variety and weather is telling.
We did not see any additional yield benefit from the use of Priaxor in these conditions. Although this product, and several others with a strobilurin active ingredient are touted as having plant health benefits that can improve yield in the absence of the disease, we did not observe this to be true. However, we did notice that the Priaxor treatments did result in greater amounts green stem near harvest, compared to untreated controls. This may be a direct consequence of the strobilurin active ingredient, which may promote tissue longevity and thus carbohydrate production. When carbohydrate production exceeds demands by carbon sinks such as pods, tissues remain lush for a longer period. This can result in delayed harvest or harvest issues.
Narrow rows, as expected, resulted in greater yields when compared to 15” rows. This is likely due to less weed pressure and increased light interception of the canopy. As mentioned previously, at least one year of additional field research would be needed to improve our dataset and generate more robust management recommendations for growers. At present, we have no evidence that spacing influences foliar disease in double cropped soybeans produced around the Chesapeake Bay. Our data indicate that fungicide use may have a potential harmful effect if used in double cropped soybeans. Again, we utilized a susceptible variety, and this was what we would consider a disease favorable year. Based on the current dataset I would not recommend using a fungicide in double crop production in this region.
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Results of the project "Assessing the Impacts of Row Spacing and Fungicide Timing on Disease Control and Profitability in Double Crop Soybean Production Systems" indicated that aspects of double crop production may limit foliar fungal disease and subsequent yield impacts associated with fungicide use. In double crop systems, soybean residues have more time to decompose due to the presence of corn and small grains, as well as from light tillage. Consequently, this can reduce the amount of residue-borne foliar diseases such as Septoria brown spot, frogeye leaf spot, and pod diseases such as phomopsis pod and stem blight. Also, later planting dates result in canopy closure during hotter, drier periods of the year, which may hamper disease development. Narrow rows, as expected, resulted in greater yields when compared to 15” rows. Spacing affected percent green stem and yield. Double cropped soybeans planted on 7.5” rows averaged approximately 10% more green stem than plots planted on 15” spacing. Yield was significantly greater in the 7.5” spacing, when compared to the 15" spacing. This is likely due to less weed pressure and increased light interception of the canopy.
No additional yield benefit from the use of Priaxor in these conditions was observed. Although this product, and several others with a strobilurin active ingredient are touted as having plant health benefits that can improve yield in the absence of the disease, we did not observe this to be true. However, it was observed that the Priaxor treatments did result in greater amounts green stem near harvest, compared to untreated controls. Fungicide timing significantly impacted percent green stem. The percent green stem of plots treated with Priaxor at R1 or R3 were approximately 7% greater than untreated control plots. Based on the current dataset using a fungicide in double crop production in the Delaware and Maryland region is not recommended.