Gordon Johnson, Extension Vegetable & Fruit Specialist; gcjohn@udel.edu
Much of the information below was taken from the bean chapter of the 2022-2023 Mid-Atlantic Commercial Vegetable Production Recommendations Publication https://www.udel.edu/content/dam/udelImages/canr/pdfs/extension/sustainable-agriculture/BFP_2022-2023_Beans.pdf
Lima bean fields are flowering and setting pods currently and should be scouted in the next 4 weeks for the presence of downy mildew as well as white mold and Phytophthora capsici.
As we move into late August and September, cooler temperatures, heavy dew and fogs, continued rainfall in some locations and the potential for heavy rains from tropical storms can be favorable for development of downy mildew in lima beans. Wet conditions also favor the development of other diseases, such as white mold, P. capsici, and Pythium.
Conditions for downy mildew are most favorable when fields receive 1.2 inches or more of rain within a 7–day period. Heavy dews and fogs reduce the amount of rainfall necessary to start infection. In 2014, 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. The Delaware Downy Mildew Lima Bean Risk Tool is a web-based system that requires a user account to gain access. To obtain a user account, please e-mail deos-info@udel.edu with “Lima Bean Risk Tool” in the subject line. 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.
Race F was the only race of downy mildew identified in baby lima beans in the past 12 years in the region and most of our acreage is being planted to susceptible varieties (Cypress, Meadow, C-elite Select, Maestro, and 184-85 are all susceptible to race F). The M-15 variety that was planted on significant acreage in the past is resistant to race F of downy mildew (but not race E); however, it is not being planted currently to any extent. The more recently released variety ‘Emperor’ is listed as having race F resistance.
Since environmental conditions vary from field to field and in different locations within a field, use the above information as a guideline. Fields that are not rotated and planted to susceptible varieties should be scouted regularly for disease occurrence.
Under favorable environmental conditions the following preventative fungicides are recommended for Downy mildew Forum – 6.0 fl oz 4.18SC/A, Headline – 6.0 to 9.0 fl oz 2.09 EC/A, Ranman 400 SC 2.75 fl. oz/a Omega 500F 0.5-0.85oz/A or Ridomil Gold Copper–2.0 lb 65WP/A. Application at flowering or when pods are first forming is recommended if weather is favorable for disease.
If disease pressure is high, use Ridomil/Gold Copper, or the phosphite (also called phosphonate) fungicides (ProPhyt, K-Phite, Phostrol, and others).
Ridomil/Gold Copper and phosphite fungicides provide some curative activity if applied at the first signs of disease. Another product that is labeled on lima beans for white mold control is Omega, which has shown excellent control of downy mildew at the 8.0 fl oz/A rate when used as a preventative application where control of both white mold and downy mildew are targeted.
Phytophthora capsici will most commonly be found in low lying wet field areas, irrigation wheel tracks, and shaded field borders. In fields with a history of P. capsici, applications of Ridomil Gold Copper 2.0 lb 65WP/A, Forum – 6.0 fl oz 4.18SC/A, Ranman 400 SC 2.75 fl. oz/a Omega 500F 0.5-0.85oz/A or phosphite fungicides when applied for downy mildew may suppress the disease.
See the 2022-2023 Mid-Atlantic Commercial Vegetable Production Recommendations for more information on fungicides for lima beans.
Downy mildew on limas is characterized by white downy growth on the pods, petioles and racemes. A reddish-brown border often surrounds the infected area on the pods.
Downy mildew on petioles and stems often produces the distorted “crooks” seen in this picture.