Downy Mildew Identified on Lima Beans

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

Downy mildew on lima beans was identified this week on the variety ‘Cypress’ near Harbeson in Sussex County. This is pretty early, but the increased rainfall and cooler than normal temperatures in this area were favorable for infection. Not all lima bean growing areas have had the same amount of rainfall, but growers and crop consultants need to be scouting for downy mildew. Last season downy mildew appeared very late and infected only a few fields. Race F of Phythophthora phaseoli was the only race identified in 2006 and 2008. 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 newest formulation of fixed copper from DuPont is Kocide 3000 and it performs as well as the other formulations of copper at the rate of 1.3 lbs/A. 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 shown to provide some curative activity if applied when downy mildew is first seen. Ridomil Gold/Copper has a national label now, so no 24c label is needed. Phostrol has a 24c label which needs to be on hand: http://www.rec.udel.edu/update09/Phostrol24c.pdf. Headline from BASF is also labeled for downy mildew now. 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 control the disease as well 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 also labeled for soybean rust on limas.

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 2X7 to 14-day interval 2 lb 1X 2 lb 2X7 to 14-day interval 2 lb 1 to 2X10 to 14-day interval
PhostrolFungi-Phite 3-4 pt 1 to 2X7 to 14-day interval 2-4 pt 1 to 2X7 to 14-day interval 3-4 pt 2X7-day interval 4 pt 1X or2-4 pt 2X7 to 14-day interval
Fixed Copper4 2 lb 4X7-day interval(may not control) 2 lb 2 to 3X7 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 3000, Cuprofix Disperss and other labeled coppers.

 lima bean pod infected with Phytophthora phaseoli

Downy mildew caused by Phytophthora phaseoli

downy mildew on lima bean racemes

Downy mildew on raceme and petiole

lima bean pod infected with Phytophthora capsici 

Phytophthora capsici on lima bean pod

Phytophthora capsici will infect lima bean pods as well and can look very similar to downy mildew. P. capsici or lima bean pod rot is usually found in wet low spots in the field. The fungus growth looks more granulated or “pebbly” than downy mildew, microscopic confirmation is encouraged.

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