Downy Mildew
About
Downy mildew is the name given to several types of oomycetes microbes from the Peronosporaceae family of obligate parasites that infect plants.
Biology
These fungi come from the water mold family and are all closely related to one another. Though there are many different downy mildew species, all appear during times of high humidity levels, around 85% or higher. They also prefer cool temperatures around 54°F to 72°F.
If leaves remain wet for prolonged periods of time, the spores can infect the leaves and mature to produce more spores. The process only takes about five to ten days before more spores are released.
Preferred Host
Downy mildew is known to infect crucifers, grapes, and vegetable crops. The most common vegetable crops infected are basil, cucurbits, and spinach.
What to Look Out For
Look under the leaves of your crops and look for any white or gray spots forming on them.
During times of cooler temperatures, look out for any thick-walled gumball-shaped structures on crops.
Leafy crops, especially basil, will also develop a yellow color and start to curl up.
For cucurbits, it is common to see misshapen fruit and cause sunburn.
To prevent any outbreaks, it is recommended to have sanitary practices in place and to keep leaves dry, especially if they are infected.
Economic Impact
Due to the number of different species and varieties of downy mildew, it can be difficult to treat properly with pesticides.
This also means some crops may be affected by an infection while others remain untouched in the same field.
Downy mildew is unable to overwinter in the Northern U.S. and typically overwinters in the gulf states. Every season, downy mildew migrates north while never establishing a multiple-season colony.
Scientific classification
Clade: SAR
Phylum: Oomycota
Order: Peronosporales
Family: Peronosporaceae