Downy Mildew of Grape: An Overwintering Pathogen

Downy mildew (Plasmopara viticola) is a major disease of grape here in Kentucky. Infection by this pathogen can weaken plants, infect fruit and buds, and reduce photosynthesis.

Downy mildew disease symptoms develop in spring during warm, wet weather when temperatures range from 65 to 75 degrees. Fungal masses then begin to grow across plant surfaces, infecting tissue along the way. Symptoms usually subside when summer temperatures become hot and dry, but the disease quickly returns when temperatures begin to drop again in the fall.  

The pathogen, however, does not disappear in winter. It can survive mild winters in infected dormant canes and begin to grow again in spring (sporangia).
During this time of year, the pathogen also generates specialized overwintering spores (oospores). Leaf litter on the vineyard floor creates the ideal environment for winter survival of these spores.

It is a misconception that downy mildew is a late season disease. Very early in the spring, when temperatures begin to creep into the 50’s, the overwintering spores germinate on the ground, new spores (zoospores) splash up onto vines, and infection occurs. This infection can remain inactive until temperatures rise above 65 degrees, and the disease cycle begins once more.

 Control of downy mildew begins with removal of leaf litter. This helps eliminate early sources of the fungus. Fungicides can be applied after harvest to destroy spore-producing fungi, and dormant sprays can be used before bud break to help eliminate any pathogen that may have overwintered in canes.

 Understanding a pathogen’s life cycle is the first step in maintaining a disease-free vineyard. Control of downy mildew before it becomes a problem is the best way to manage the disease.