Gordon Johnson, Extension Vegetable & Fruit Specialist; gcjohn@udel.edu and Nathan Kleczewski, Extension Specialist – Plant Pathology; nkleczew@udel.edu
Much of the information below was taken from the Commercial Vegetable Production Recommendations Publication http://extension.udel.edu/ag/files/2012/03/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 dews and fogs, continued rainfall in some locations during this wet 2013 season 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 and when average daily temperature during this period is 78°F or less. Heavy dews and fogs reduce the amount of rainfall necessary to start infection. Temperatures over 90° F have broken the infection cycle in the past and an additional 7-day period with the above weather conditions is necessary to start infection again. Research is underway to verify temperature and moisture conditions favorable for downy mildew disease development and to develop improved predictive modes.
Race F was the only race of downy mildew identified in baby lima beans in the past 4 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.
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: copper, fixed (Champ DP, 2.0 lb 58DF/A, Kocide 3000 1.25 lb DF/A or other labeled fixed copper product), Forum – 6.0 fl oz 4.18SC/A, Headline – 6.0 to 9.0 fl oz 2.1 EC/A, ProPhyt -3.0 to 4.0 pts/A, K-Phite -1.0 to3.0 qt/A, Rampart -1.0 to 3.0qt/A, Phostrol -4.0 pt/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 phosphonate fungicides (ProPhyt, K-Phite, Phostrol, and others).
Ridomil/Gold Copper and phosphonate fungicides provide some curative activity if applied at the first signs of disease. If downy mildew is present in the field do not use copper fungicides alone for curative control. 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 or Forum 6.0 fl oz 4.18SC/A when applied for downy mildew may suppress the disease.
See the 2013 DE Commercial Vegetable Production Recommendations for more information on fungicides for lima beans.