Downy Mildew (Plasmopara viticola)

DAMAGE AND SYMPTOMS

  • Attacks any green tissue on the vine.

  • Destroys tissue in a few days.

  • Symptoms include large yellow spots on the tops of leaves, called “oil spots,” and white, cottony (“downy”) colonies on the undersides.

  • Infections also attack shoots, shoot tips, petioles, berries, and rachises.

 

LIFE CYCLE

  • Oospores overwinter in fruit and leaves on the vineyard floor.

  • Oospores can be viable for several years.

  • Primary infections begin in the spring at 52°F or higher and after a rain event of at least 0.1”.

  • Secondary infections of sporangia form after humid nights followed by a rain event.

  • Sporangia then release zoospores which move onto or around the vine via wind or rain (splashing).

  • Zoospores enter the plant through the stomates.

  • Downy mildew infections spread rapidly after this point.

  • Under ideal conditions the time between generations is 4 to 5 days.

  • Ideal conditions are night and morning temperatures between 65-77°F.

  • Limiting temperatures are below 52°F and above 86°F. 

 

MANAGEMENT

 

"Oil spot” on the surface of a leaf. (Mark D. Welch)

Colony of sporangia on underside of a leaf. (Mark D. Welch)

 

SOURCES:

Wilcox, Wayne. 2017. “Downy Mildew.” Grape Disease Control for 2017. Geneva, NY: Cornell University.

Gubler, W. Douglas, George M. Leavitt, and Larry J. Bettiga. 2013. “Downy Mildew.”  Pp. 117-119 in Grape Pest Management, 3rd. Edition. Larry J. Bettiga, Technical Editor. Oakland, CA: University of California Agriculture and Natural Resources: