Sweet Corn and Lima Bean Disease Update

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

Sweet Corn
See the article on stalk rots in this issue for pictures of Diplodia ear rot on sweet corn. Nothing to do for it except work in the corn stalks at the end of the season. Chalk this one up to wet weather.

Lima Beans
Pythium pod rot can be a problem when we have wet, warm weather, like what occurred last week and the week before. Pythium is soil born and moves into pods that are touching the soil and can then spread to other plant parts under ideal conditions. It can be more of a problem in snap beans but does occur sporadically in lima beans as well. It is extremely hard to predict when and where it will occur unless the field has a history of Pythium problems. Ridomil Gold/Copper has a 24c label for use on snapbeans in DE, MD and VA. Application should be made before the pods develop and touch the soil surface. The phosphorus acid fungicides that are labeded on beans, such as Prophyt, would be another product to try if the situation warranted it.

Pythium pod rot on lima bean

Pythium pod rot on lima bean. Note the white cottony growth.

New Product Labeled for Lima Bean Downy Mildew
I just received notice that BASF labeled Forum (dimethomorph), formerly Acrobat, for use on succulent beans for downy mildew. This was a surprise and I had to look at my data to see how it did. It was tested in 2001 and performed very well. It controlled downy as well as the standard Ridomil Gold/Copper. I have the opportunity to look at it again this year since I just applied the treatments for a downy mildew test in Georgetown on Wednesday and have included Forum. Forum is also very active on Phytophthora capsici, which causes lima bean pod rot and is another problem that has been troublesome for lima bean growers. The rate is 6 fl oz/A for downy mildew which is also the recommended rate for Phytophthora capsici fruit rot control on cucurbits. I’m not sure how much material is available, but Forum is another tool in the toolbox. That gives us Ridomil Gold/Copper, Phostrol and other phosphorus acid fungicides, Headline, and Forum for use on baby limas for downy mildew.