Downy Mildew

I'm still in my self education mode regarding the fungi that can infect our vines. Earlier, I blogged about powdery mildew in Strategies to Control Powdery Mildew: Part I and followed up with Strategies to Control Powdery Mildew: Part 2. I'm also collecting the blogs that deal with fungal diseases under the tab Fungal Diseases of Grapevines.
Next fungal disease: downy mildew. Downy mildew in grapes is caused by the fungus Plasmopara viticola, an oomycete in the order Peronosporales. Spring and summer rainfall at temperatures above 50ºF releases the oospores which is the primary inoculum. The oospores form a single germ tube terminating in a sporangium that constitutes the means of secondary spread of the pathogen, when they produce zoospores. The zoospore is a small swimming spore that needs water for dissemination and grape leaf stomata for entry and penetration into the inner tissues of the vine.
These zoospores initiate secondary infections which can occur in as little as two hours of wetting at 77°F or up to 9 hours at 43°F. Infections are usually visible as lesions in about 7 to 12 days, depending on temperature and humidity.
I found a very well annotated life cycle of downy mildew at The Ohio State site. The site credits the New York State Agriculture Experiment Station. Figure taken from Grape IPM Disease Identification Sheet No. 1.

Symptoms on leaves are small yellow lesions also known as oil spots. Young berries are also highly susceptible to downy mildew. They appear grayish when infected causing gray rot and become covered with a downy felt of fungus sporulation. Berries become less susceptible as they mature, but rachis infections can spread into older berries causing brown rot, having no fungus sporulation.
More information can be found in the references listed below.

References:
1. APSnet Downy Mildew of Grapes.
2. Downy Mildew Plasmopara viticola Michigan State University AgBioResearch.
3. UC Davis IPM Online Grape Downy Mildew.