Description
Plasmopara viticola, also known as the grape downy mildew is considered to be the most devastating grape pathogen in North America and Europe. This fungus was originally observed in the United States about 1834 and is most abundantly found in the northern and midwestern areas of the United States. Shortly after this first observation the pathogen was introduced to the European countries where it played a devastating role in the yield and production of their grapes, and consequently their wine. France was among the first of the European countries to gain experience in dealing with the pathogen. Within just a few years of the pathogen's introduction the French attempted to graft American root stock to their own vines in order to produce a more resistant strain of grape. Depending on the year, production of grapes in France has been estimated to be at a loss of 50% or more. Because of numbers and results like these, downy mildew has been considered the most devastating fungal disease to affect European vineyards.
Symptoms cover a fairly large range depending mostly on the host. Common symptoms include necrosis of the stem or shoot, discoloration including brown spotting (lesions) and yellowish-green tips of the leaves. Grapes may exhibit sporangium or mycelium, appearing as a white to gray coat on the outer surface.
Downy mildew requires optimum conditions to reproduce and infect. A warm, moist, and humid environment is required. Studies in Sicily have shown optimum time for oospore germination is between the end of February and the middle of March. With this understanding, if fungicides are used just before optimum conditions occur, they have proven to be an effective control method of the pathogen. Other control methods include propering watering, and a good location where the plant can receive continual sunlight.
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