Written collaboratively by Emmanuel Byamukama and Shaukat Ali.
The prolonged winter conditions and rainy cool spring have resulted in delayed spring wheat planting and as a result delayed plant progress. According to the May 13 USDA-NASS Crop Progress Report, only 28% of spring wheat has emerged compared to 81% during the same time last year and the 5-year average of 58%. Delayed plant development can have implications for plant disease development.
The main diseases that are likely to develop due to delayed crop progress are leaf rust and Fusarium head blight (FHB). Although plant disease development is primarily driven by conducive weather, plant growth stage also influences the level of disease severity. Sometimes a crop may escape disease development because by the time the weather conditions are conducive for the pathogens to infect the plant (warmer humid conditions), the crop would be nearing physiological maturity and diseases would have a minimal impact on yield.
Leaf rust pathogen prefers warmer temperatures (68 - 77 F) and typically infects wheat after heading. However, with the delayed plant progress, leaf rust could develop in spring wheat before heading. Leaf rust pathogen does not overwinter in South Dakota but spores are blown into South Dakota from southern states where the pathogen survives.
Fusarium head blight (scab) is another disease that develops under warm and humid weather conditions. The FHB pathogen, Fusarium graminearum, infects the plants through the flowers and infection can result in part or the entire head to be bleached. This pathogen also leads to poor seed quality due to production of mycotoxins, mainly deoxynivalenol (DON). Delayed spring wheat progress could mean coinciding of warmer humid conditions and flowering which might increase the risk for FHB.
Growers are encouraged to scout regularly to keep an eye on diseases that are likely to be more elevated due to delayed crop development. The United States Wheat and Barley Scab Initiative FHB risk forecasting tool (Figure 1) and the SDSU Small Grains Disease Forecasting tools can help to track conditions for disease development. Growers are encouraged to consult these tools in order to decide on the need for applying a fungicide to control leaf and head diseases. Leaf rust and FHB can be effectively managed by fungicide application. The best fungicide timing for FHB management is at flowering. Triazole fungicides are the recommended fungicides for FHB management. These fungicides are also effective against foliar fungal diseases.
Figure 1. An output of the United States Scab Initiative Fusarium head blight forecasted risk. This tool can help growers decide the need for a fungicide application based on the FHB risk in their areas when wheat is at flowering.