Downy Mildew on Lima Beans

Bob Mulrooney, Extension Plant Pathologist; bobmul@udel.edu

When cooler nights and heavy dew begin, along with heavy rainfall associated with tropical storms and hurricanes, downy mildew can threaten the fall lima bean crop. Last season downy mildew appeared sporadically. Race F of Phythophthora phaseoli was the only race identified in 2006, which may predominate if downy mildew should appear again. Preventative applications of 2 lbs fixed copper, 2 lbs Ridomil Gold/Copper, or 3-4 pts Phostrol have provided control of downy mildew in the past. The best controls continue to be Ridomil/Gold Copper or Phostrol, especially 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 Phostrol have been shown to provide some curative activity if applied when downy mildew is first seen. Be sure to have a copy of the label on hand since Ridomil/Gold Copper and Phostrol have 24c labeling in DE.

Last season Headline from BASF was also labeled for downy mildew after the Vegetable Recommendations went to press so it is not in the current edition. I have tested it 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 Phostrol preventatively, but the yields have been comparable. It is also labeled for anthracnose, which the other products do not control. If soybean rust had become a problem in other legumes it would have been another fungicide in the toolbox for lima beans, since it is labeled for soybean rust on limas as well.

Fungicide Rates and Intervals for Control of Downy Mildew of Baby Lima Beans

Fungicide

Preventative

Curative1

High Disease2 Low Disease3 High Disease2 Low Disease3
Ridomil Gold/ Copper 2 lb 2X
7 to 14-day interval
2 lb 1X 2 lb 2X
7 to 14-day interval
2 lb 1-2X
10 to 14-day interval
Phostrol 3-4 pt 1-2 X
7 to 14-day interval
2-4 pt 1-2 X
7 to 14-day interval
3-4 pt 2 X
7-day interval
4 pt 1X
or 2-4 pt 2X
7 to 14-day interval
Copper4 2 lb 4X
7-day interval
(may not control)
2 lb 2-3X
7 to 10-day interval


Not Recommended

1Curative – when disease first seen, very low incidence, less than 1% of pods and/or racemes infected

2High Disease – conditions very favorable for infection and spread, i.e. ample rainfall, dews, fog and cool temperatures

3Low Disease – conditions less favorable for disease, i.e. low humidity and rainfall and/or temperatures too high (high 80s and above)

4Copper fungicides include Champ DF, Kocide, Cuprofix Disperss and other labeled coppers.

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