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Mildew Diseases In The Garden

Sep 02, 2015

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By Carl John Esslinger

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. 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.

Downy mildew is a disease that can only live on healthy green leaf tissue. Downy mildew does not survive our winters. It does not come with the plants you buy from local greenhouses. It is blown or moved into our area from the south each year. Its movement up the coast is tracked each year by Extension. It arrived in here at the time it normally arrives, mid to late July. What is unusual about downy mildew this year is that when it arrived it quickly infected a wide range of plants. It infected winter squash and pumpkins back in July. These crops don’t normally get downy mildew until the end of August. The first crop to get downy is typically cucumbers. Most gardeners do not plant late season cucumbers because it is almost guaranteed that they will get downy mildew.

It thrives in cool and wet 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. Because it turned hot and dry this summer, downy mildew did not spread quickly.

The disease that has thrived in the dry weather is powdery mildew. It is the white fungal disease that is common on summer squash (especially zucchini), pumpkins, winter squash, grapes, and a host of other edible and ornaments plants.

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 will help prevent downy mildew. Washing powdery mildew off the foliage with a garden hose will slow the progression of this disease. Give plants plenty of space so there is good air circulation around each plant. Finally, if you apply a fungicide, apply it before the mildew shows up. Fungicides containing chlorothalonil or mancozeb are effective at preventing downy mildew. Applying fungicides after the infection takes place does nothing.

Source: psu.edu

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