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Downy Mildew

Posted: September 5, 2012

A few weeks ago I wrote to you in a vegetable pest update about downy mildew moving into Northeastern Pennsylvania. What you may not know is that downy mildew infests a wide range of garden plants...
Purplish-gray sporulation of downy mildew on the underside of a cucumber leaf. Photo: Tianna Dupont.

Purplish-gray sporulation of downy mildew on the underside of a cucumber leaf. Photo: Tianna Dupont.

In addition to several vegetable species, it also affects at least 66 common ornamental species. The most common are geranium, impatiens, rose, pansy, snapdragon, and sunflower.

It did not come on the plants you purchased from local greenhouses. Downy mildew does not survive our winters. It is blown or moved into our area from the south. Since we had such a mild winter, downy mildew over-wintered farther north than usual. So it arrived in Pennsylvania earlier than normal. It thrives in wet and humid weather. The disease infects the leaves. Within a few days of infection, spores are produced that move through the air. These spores land on new plants infecting them. As the summer goes on, more and more spores are produced and more and more plants are infected.

There are considerable differences between species as to their susceptibility to downy mildew. For example, there are five strains that attack plants in the cucurbit family. Cucumbers are susceptible to all five strains while pumpkins are only susceptible to one of those five strains.

So what can a gardener do? Keep your plants healthy and minimize stress. Plants that have been stressed are more susceptible to all diseases. Eliminate weeds. I know, easier said than done. Weeds compete with your desirable plants for water and light. The weed canopy makes the environment around the plant more humid which is what downy mildew likes. Keep the soil moist but not wet. Water the soil not the foliage. Give plants plenty of space so there is good air circulation around each plant. Finally, if you apply a fungicide, apply it before the downy mildew shows up. Fungicides containing chlorothalonil or mancozeb are effective. Applying fungicides after the infection takes place does nothing but waste your time and money.

For more information contact your local extension office. In Lackawanna County call 570-963-6842 or email LackawannaMG@psu.edu

John Esslinger, Extension Educator
Penn State Extension