Downy Mildew Disease

Overview

Downy mildew disease is an extremely destructive and widespread disease. The disease is caused by downy mildew. It is extremely harmful for grapevines located all over the world. Downy mildew disease is especially problematic where spring and summertime rainfall takes place at temperatures that are higher than 50 degrees F.

Identification

Downy mildew disease is caused by downy mildew, which is a variety of oomycete microbes that function as parasites to plants. Oomycete microbes are obligate parasites. Downy mildew is part of the Peronosporaceae family. Agriculturally, the mildew is particularly problematic for grape, crucifer and vine vegetable cultivation.

Common Symptoms

Downy mildew disease can be a major cause of destruction in greenhouses, nurseries and outdoors. Some extremely common symptoms of downy mildew disease include distortions, blights and leaf spots appearing on plants. However, the symptoms can vary depending on the hosts, with wide variations existing between lesion sizes and colors.

Hosts

The signs of downy mildew disease can vary depending on their hosts. Some of the most frequent downy mildew hosts include aster, alyssum, dead nettles, kale, violets, veronica, sage, snapdragon, rosemary, geraniums, crane's bill, coreopsis, phlox, pansies and butterfly bushes.

Development

The first sign of downy mildew disease tends to be the appearance of yellow to pale green spots, which are known as "oil spots" due to their greasy appearances. Below the surfaces of the lower leaves are downy and white masses of spores, which are dispersed by the wind. The lesions gradually turn brown; at the same time, the tissue that is infected dies. Leaves that are severely infected drop off prematurely, which can decrease the vine's wintertime hardiness. In times of humidity, clusters of flowers that are infected become covered with many white spores or dry up entirely. Berries that are infected become purplish-red or dull green in color and fall out of clusters.

Humidity and Temperature

Humidity and temperature have crucial roles in the development of pathogens of downy mildew disease. In times of wet and cool conditions and high humidity, outbreaks of the disease happen when the germinating oospores create sporangiophores, which look like a cluster of grapes coming out from the stomate of the plant. Every sporangium is full of many zoospores, which travel to plants that are susceptible and then infect them.

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About this Author

Isabel Prontes is a freelance writer and traveler residing in Manhattan, NY. She has traveled to five continents and counting. Her work has appeared on a number of websites, such as Travels, eHow.com and "Happy Living Magazine." Prontes has a professional background in public relations; she received a bachelor's degree in communication studies from Pace University.