Monday, September 15, 2008

Downy Mildew: Cabbage

Today I went to a field of cabbage, a nice change from all of the time I have been spending in tomato and pepper fields. I noticed that the lower leaves had some spots that were yellow and turning necrotic. When I turned the leaf over I notice that familiar fungal growth- downy mildew.

Symptoms of downy mildew on cabbage include irregular shaped lesions that vary in size and are sunken and whitish-gray (image, left). The tissue around the lesions will turn yellow. When you flip the leaf over, a fluffy, white, downy growth is evident (image, below). One can distinguish downy mildew from powdery mildew because only downy mildew will sporulate (reproduce new spores) on the bottom side of the leaf.

Downy mildew on cabbage is caused by Peronospora parasitica. This organism causes downy mildew on vegetables in the Brassicacea family such as broccoli, cauliflower and radishes, and is different from the organism that causes downy mildew on cucurbits (Pseudoperonospora cubensis). P. parasitica, like most downy mildews, thrives in cool, wet weather (temps. 50-60 degrees F) though it can tolerate cooler and warmer temperatures.

P. parasitica forms two types of spores, oospores and sporangia. Oospores are the overwintering, resistant spores that can survive in the soil (not shown). The other spore type are sporangia. Sporangia are large (relatively speaking for a microscopic organism), round spores that are born on tree-looking structures known as sporangiophores (images left and right). These sporangia are disseminated by wind or rain splash. Isn't this organism beautiful (I know, I know... a face only a plant pathologist could love). Making a successful slide is difficult because P. parasitica is quite delicate. You will notice that some areas of the image to the left are out of focus, this is because P. parasitica is 3-D and taking a 1-D image doesn't do it justice.

Downy mildew can be introduced into a field through transplants, so make sure that your transplants are disease free. Also, if you notice that you have diseased plants, do not dump these plants in a cull pile in the field because they have the potential of infecting the non-diseased plants. Try to eliminate brassica weeds and rotating the field with non-brassica vegetables might help.

There are no downy mildew resistant cabbage cultivars, however there are a few effective fungicides for the control of downy mildew on cabbage. A new fungicide from Syngenta, Revus (mandipropamid) has shown excellent efficacy for controlling this disease. Also, ProPhyt (potassium phosphite) + Manex (maneb) can be used to control the disease.
Reduced efficacy of Ridomil Gold and Bravo has been shown in NC (Adams et al., 2007, Plant Disease Management Reports 2:V119. DOI:10.1094/PDMR02).

As always refer to manufacturers' recommendations on the fungicide label.

**Special thanks to Kelly Ivors and Dreams Milks who assisted with the pictures of P. parasitica.**
*Much of this information was gathered from Univ. of FL Plant Pathology Fact Sheet 33 (Tom Kucharek) available on-line.*