Gordon Johnson, Extension Vegetable & Fruit Specialist; gcjohn@udel.edu
The following was modified from a 2011 article from Bob Mulrooney, Extension Plant Pathologist (now retired), University of Delaware
As we move into late August and September, cooler temperatures, heavy dews and fogs, and the potential for heavy rains from tropical storms can be favorable for development of downy mildew in lima beans. Conditions are most favorable when fields receive 1.2 inches or more of rain within 7 days 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 will break the infection cycle. Lima bean fields should be scouted in the next 4 weeks for the presence of downy mildew as well as white mold. Race F of Phythophthora phaseoli was the only race of downy mildew identified from 2008 to 2011.
Preventative applications of 2 lbs fixed copper such as Kocide 3000 (1.3 lbs/A), Champ DP, or other coppers; 2 lbs Ridomil Gold/Copper; or 3- 4 pts Prophyt have provided control of downy mildew in the past. The best controls continue to be Ridomil/Gold Copper, and Prophyt, or other labeled phosphonate fungicides when disease pressure is high. Application at flowering or when pods are first forming is recommended if weather is favorable for disease. If disease is present Ridomil/Gold Copper and phosphonate fungicides have shown to provide some curative activity if applied when downy mildew is first seen. If downy is present in the field do not use copper fungicides alone for curative control, they will not provide control. Another product that is labeled on lima beans for white mold control is Omega and while not specifically labeled for downy mildew, three years of data has shown that it has excellent control of downy mildew at 5.5 fl oz and 8.0 fl oz/A as a preventative application before disease is found in the field. In DE this would be a 2ee use since the fungicide is labeled on lima beans and is appropriate where a mix of white mold and downy mildew are in the field. Omega has a 30 day preharvest interval. Headline from BASF is also labeled for downy mildew. It has been tested in Delaware and it has provided good control of downy when applied on a 10 day schedule at 6.0 fl oz. /A. It does not give as good disease control as Ridomil Gold/Copper or the phosphonates preventatively but the yields have been comparable. It is also labeled for anthracnose which the other products do not control. See the 2012 DE Commercial Vegetable Production Recommendations for more information on fungicides for lima beans.
We would appreciate samples and reports of lima bean downy mildew this season. Samples should be fresh and packaged in a zip lock plastic bag with dry paper towels. Samples can be dropped off at any of the county Extension offices or delivered directly to Nancy Gregory at Townsend Hall in Newark. Reports should be made to Nancy Gregory at ngregory@udel.edu.