Plasmopara viticola
Plasmopara viticola, the causal agent of grapevine downy mildew
Downy mildew
Downy mildew refers to any of several types of oomycete microbes that are obligate parasites of plants. Downy mildews exclusively belong to Peronosporaceae. In commercial agriculture, they are a particular problem for growers of crucifers, grapes and vegetables that grow on vines...

, is a heterothallic
Heterothallic species have sexes that reside in different individuals....

 oomycete that overwinters as oospore
An oospore is a thick-walled sexual spore that develops from a fertilized oosphere in some algae and fungi. Also the result of plasmogamy/karyogamy in oomycetes, which in turn leads to the development of hyphae, then mycelium....

s in leaf litter and soil. In the spring, oospores germinate to produce macrosporangia, which under wet conditions release zoospore
A zoospore is a motile asexual spore that uses a flagellum for locomotion. Also called a swarm spore, these spores are created by some algae, bacteria and fungi to propagate themselves.-Flagella:...

s. Zoospores are splashed by rain into the canopy, where they swim to and infect through stomata. After 7–10 days, yellow lesions appear on foliage. During favorable weather, the lesions sporulate and new secondary infections occur (Kennelly et al., 2006).

Plasmopara viticola, also known as the grape downey mildew is considered to be the most devastating pathogen to grapes 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 in which it played a devastating role in the yield and production of their grapes, and consequently their wine. France was amongst the first of the European countries to have experience with the pathogen, just a few years after they attempted to graft american root stock. Depending on the year, production of grapes in France has been estimated to at a loss of 50% or more(Hesler, 1917). Because of numbers and results like these, downey mildew(plasmopara viticola) has been considered the most fungal disease to affect european vineyards. (Perez-salas, 1998; Perez Martin, 1989; Salazar, 1994)

Symptoms cover a fairly large range depending mostly on the host. Common symptoms will include necrosis of the stem or shoot. Discoloration including brown spotting (lesions) and yellowish-green tips of the leaf portion. Grapes may include sporangium or mycelium, appearing as a white to gray coat on the outer surface.

This downey mildew requires optimum conditions to reproduce and infect. A warm, moist, and humid environment is required of the fungus. Studies in Sicily have shown optimum time for oospore germination is between the end of February and the middle of March (Burruano and Ciofalo, 1990) With this understanding, if fungicides will be used just before optimum conditions, they have proven to be an effective control method of the pathogen. Other control methods would include propering watering, and a good location where the plant can receive continual sunlight.

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