Lima Bean Disease Update

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

Downy Mildew and Lima Bean Pod Rot
Two more fields were identified with downy mildew this week so it is important to keep scouting. Phytophthora capsici was also identified on pods from a field close to harvest. This disease we have named lima bean pod rot. It is found most commonly in low spots in the field. I had talked about it in the article titled Downy Mildew Identified on Lima Beans in WCU 17:20 and have included another picture to help you identify it in the field. There is no effective fungicide control for this disease at the present time. Copper applications may help, but have not been very effective on Phytophthora capsici on other crops. Some field populations of this fungus may be resistant to Ridomil Gold/Copper as we discovered several years ago, so that product cannot be depended on to provide control unless the fungus population is susceptible to Ridomil (mefanoxam).

P. phaseoli on upper pod, P. capsici on lower podDowny mildew on the upper pod and lima bean pod rot on the lower pod. Note the granular appearance of the fungus on the lower pod and the lack of a reddish brown border on the pod infected with lima bean pod rot or Phytophthora capsici.

White Mold
We have yet to see white mold in limas but there is a new fungicide labeled for white mold in lima and snap beans. Omega 500F (fluazinam) from Syngenta, an excellent white mold fungicide that has been labeled in potatoes and peanuts for a long time, is now labeled on beans (snaps and limas). The good thing is that this fungicide is also very effective against downy mildew on limas if applied preventatively although downy mildew is not on the label. I have one year’s test results under heavy disease pressure and it performed very well at the 5.5 fl oz rate. This label was granted after the recommendations book was printed so it is not in the recs. It is the only product that provides control of both white mold and downy mildew. I am going to get another look at it this fall for downy mildew control so I can have two years of data.

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