When to Watch for Lima Bean Downy Mildew? New Risk Assessment Tool Can Help

Kevin Brinson, Associate State Climatologist and DEOS Director; kbrinson@udel.edu

It’s getting wet again; time to keep an eye on downy mildew in lima beans.

The theme for July’s weather was feast or famine. What started off looking like it would be a dry July turned into an above normal rainfall month. The map below shows that some locations in Delaware saw as much as 7 or 8 inches over the 5-day period of July 21st through 25th, with most other areas receiving at least 2 inches of rain. The wet weather has continued this week and the outlook for the next 7 to 10 days looks wet as well. When it gets wet and humid in the middle-to-end of summer, disease risk in lima beans start to become a concern. Lima bean growers know this well, and while many fields were planted in the last few weeks, some were planted in June and are either in the flowering or pod setting stage, which are particularly vulnerable stages for downy mildew.

Four years ago, a team of researchers and extension staff at the University of Delaware developed a risk tool to allow growers to know when downy mildew conditions are favorable. The goal was to give field personnel, consultants, and growers a tool to better time their field scouting efforts and pesticide applications and thereby reduce costs by spraying less often. The tool works by combining the disease history of the field, as provided by the grower, with weather information from the Delaware Environmental Observing System (www.deos.udel.edu) to provide a risk score for each field using different downy mildew risk models. Higher risk scores suggest the need for growers to step up scouting efforts, particularly during times when the plants are more susceptible to disease. Accessing and using the tool (http://dims.deos.udel.edu/limabeanrisk/) is simple, and only requires an e-mail to Kevin Brinson at kbrinson@udel.edu to setup up a free account. From there, you can add any lima bean fields to the system with just a couple of additional pieces of information about each field, such as its last occurrence of downy mildew (if known) and the culitvar’s susceptibility to downy mildew. The model only describes the risk of downy mildew formation within the field and does not account for the transfer of downy mildew spores between fields. The system automatically updates every day and shows the disease risk both for the day and throughout the growing season, as well as a few other pertinent weather variables.

Fungicide recommendations for downy mildew are in the Bean Section of the Mid-Atlantic Commercial Vegetable Production Recommendations.

Please contact Kevin if you’re interested in using the risk tool or have weather and climate questions in general. For questions about downy mildew in lima beans or lima bean production in general, reach out to Gordon Johnson, Emmalea Ernest, or Tom Evans.