Note the reddened areas on the sepals and stem.
Downy Mildew
There are many types of Downy Mildew, some are very specific- infecting only one type or Genus of plants (such as Peronospora sparsa on Rose), others are have a broad spectrum of hosts. This time of year is especially important in recognizing the symptom
Downy mildew is being found commonly in the nursery trade on roses, perennials, some annuals and shrubs, brambles, vegetables, grapes, and other vines. It is a form of water mold fungus. Sporangia are produced on the undersides of leaves under humid conditions (like this spring weather or in covered greenhouses) at cool-moderate temperatures, and in some cases has caused defoliation or dieback to plants when left untreated. Looking at the tops of the leaves, there is a discoloration of the leaves in angular blotchy patterns (often yellow or brown on some perennials and shrubs, tending to be purple- reddish color on roses and brambles), sometimes looking like it is delimited by veins. If conditions are right, the sporangia can be easily seen under the leaves while on the plant, but sometimes a leaf can be removed and stored in a sealed plastic bag at room temperature for a day or two for confirmation of the disease. It usually is found on the lower leaves first, as it can overwinter in infected stem and leaf tissue, or plant duff remaining in the pots. There are a number of forms of DM that are exotic to the US, coming in on imported plants (may or may not show symptoms right away). It can be confused with Grey Mold or Powdery Mildew, but both of these usually produce fungi on upper leave surfaces, where Downy Mildew's fungi are seen on the undersides of the leaves. If DM is suspected, you should remove infected leaves/tips. Plants in production that are losing leaves due to the infection are usually bad enough to consider disposal. Improve air circulation by spreading plants out, removing plant duff, by use of overhead fans (in greenhouse) and by avoiding overhead irrigation. Preventative fungicide spray applications should be made to cleaned plants and surrounding plants as the spores can be moved by splashing water or by light winds.
downy mildew
Peronospora parasitica Tul
Symptoms
Photo by David B. Langston
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