Written collaboratively by Ruth Beck and Febina Mathew
Producers of field peas may need to scout for powdery mildew this year. Powdery mildew is a late-season fungal disease that can impact peas if weather conditions are conducive. Peas that are planted late are usually more susceptible to powdery mildew than those planted early. However, this disease can also occur in early planted fields in South Dakota under the right environmental conditions and when the crop canopy is heavy.
Symptoms of powdery mildew begin with white powdery spots that occur first on the upper surface of the oldest leaves and stipules and then on stems and pods. Warm dry weather accompanied by cool nights with dews is conducive to the spread of this disease. Many varieties are resistant to this disease. Producers should select and grow resistant varieties if possible to avoid any chance of this disease. However, this is not always an option for producers.
If producers are growing varieties which are susceptible to powdery mildew and weather is conducive to disease development they should scout the canopy every 2 days beginning at early pod development. If conditions are high risk for disease development scouting should start when the first pods are at full length, otherwise scouting can start when the first pods are filled. Fungicide applications should be made when the first symptoms of powdery mildew are detected at low levels in the lower canopy.
Producers may want to consider a preventative fungicide treatment if they are not able to monitor the fields as needed. However this is only necessary if the weather is conducive to disease development and varieties do not have resistance. Powdery mildew does not occur every year in South Dakota. Fungicide timing is important.
Most fungicides provide protection for up to two weeks. The goal is to apply the fungicide before disease exceeds low levels but not too far in advance of initial disease onset, as you want high levels of fungicide residual remaining in the plant when disease begins to develop. Preventative fungicides such as Proline, Quadris, Headline, and Priaxor can be applied during bloom to late pod fill. These fungicides have shown efficacy in field trials conducted in previous years.
It is important that this disease is properly identified. Powdery mildew can affect the entire field when varieties are susceptible. Peas naturally can have a white film on their leaves during growth and this can be mistaken for powdery mildew.
A good resource for labelled fungicide options on field peas is the NDSU Field Crop Disease Management Guide. For more information on field pea disease concerns, view the publication, “An Identification Guide of Common Diseases of Dry Peas in South Dakota.”