PPDL Picture of the Week for

April 6, 2015

Crucifer Downy Mildew

Tom Creswell, PPDL Director, Purdue University

Spring has arrived and home gardeners as well as commercial growers are planting  cool weather vegetables such as broccoli, cauliflower, kale and cabbage. One threat to high quality production of these crucifer (mustard/cabbage family) crops is downy mildew, caused by the fungus-like organism Hyaloperonospora parasitica.Surviving the winter as thick walled spores in old infected plant material, or on susceptible winter weeds; the fungus spreads to new plants in the spring by means of air borne spores. This specific pathogen causes downy mildew mainly on crucifers. Related pathogens may cause downy mildew on roses, basil, soybeans, sunflowers, impatiens or cucumbers.   However, each downy mildew pathogen is specific to closely related plant species such as those in a plant family.

Infected leaves develop yellow spots with areas of gray or tan. Broccoli and cauliflower heads may also become infected, producing a gray to brown internal breakdown. Similar symptoms can be produced on crucifers by bacterial diseases like black rot (Xanthomonas campestrispv. campestris) as shown in figure 2; so getting confirmation of the diagnosis by a diagnostic lab may be needed.

Downy mildew can often be prevented by removal or deep plowing of all previous crucifer debris, starting with healthy transplants and rotation with non-crucifer crops. Controlling nearby crucifer weeds may also help.

​Click image to enlarge

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Figure 1: Downy mildew symptoms on leaves of cabbage. Photo by Gerald Holmes.
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Figure 2: Black rot of cabbage, caused by the bacterium Xanthomonas campestris pv. campestris, can produce symptoms similar to downy mildew. Photo by Tom Creswell
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Figure 3: Spores of the pathogen, Hyaloperonospora parasitica, develop on distinctive brush-like structures called conidiophores.  They are shown here on shepherd’s purse (Capsella bursa-pastoris), a winter weed in the mustard family. Photo by Bruce Watt.
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Figure 4: Highly magnified view of conidiophores of Hyaloperonospora parasitica , appearing on arugula (Eruca vesicaria ssp. sativa), also in the mustard family. Photo by Bruce Watt.​