Some early planted soybeans are starting to flower (R1 growth stage). One soybean disease that needs to be managed at this growth stage is white mold (Figure 1).
The white mold pathogen infects soybean through flowers that are senescing after pollination. Unfortunately for this disease, by the time symptoms are seen, it is too late to apply a fungicide.
Growers need to be aware of the risk factors for white mold to decide the need for applying a fungicide to manage white mold in flowering soybeans.
What are the risk factors for white mold?
- Field history: Has soybean been infected with white mold in this field in the past? White mold pathogen can survive in the field for several years especially if tillage is a common practice.
- Row spacing: Are soybeans planted <20 inches’ row spacing? The narrower the row spacing, the higher the risk for white mold.
- Cultivar susceptibility: Was the cultivar planted, rated moderately susceptible to susceptible for white mold? While there is no complete resistance to white mold, cultivar differences in tolerance to white mold exist.
- Fertility level/yield potential: Does the field have elevated fertility level – has animal manure been applied this current season or past few seasons? Animal manure or excessive fertilizer tend to promote quick growth leading to early canopy closure. This increases risk for white mold.
- Planting population: The higher the plant population (>150,000 seeds per acre), the faster the canopy will close and hence providing the microclimate for white mold to develop.
For fields that have some or all the conditions above, white mold risk may exist. Although white mold infection is driven by rainy weather and cooler temperatures throughout soybean flowering, it is hard to predict if these conditions will be favorable for white mold to develop 4 to 5 weeks out. Therefore, consider the above factors when making a decision to apply a fungicide against white mold at R1. A number of fungicides are registered for managing white mold and these can be found on this publication under the white mold management column.
A recent white mold publication by a group of plant pathogologists in the North Central region on fungicide efficacy indicated that a return on investment varied across various fungicides due to fungicide prices and white mold pressure. Consider the efficacy and the cost of the fungicide when deciding on the choice of fungicides. The same report also showed that R1 timing gave the best control of white mold.