grape powdery mildew model

Suggested grape powdery mildew model for enviroweather from Annemiek Schilder

Grape powdery mildew ascospore release warning into the Enviro-weather system?

Ascospores are released from overwintered cleistothecia on the bark of grapvines when > 0.1 inch (2.5 mm) of rain occurs and the average temperature is >50ºF (10ºC) during that period (some publications say at least 0.1 inch rain and 50ºF). Ascospores are released from bud break (Late April-early May) through bloom and early fruit set (late June-early July). The ascospores do not require leaf wetness for infection, so theoretically any time green tissue is present and ascospores are released, infection is possible.

An ascospore release warning could indicate that people need to start scouting the leaves for powdery mildew colonies. Also, the more of these ascospore release events occur, the higher the risk of a severe powdery mildew epidemic, especially if these events occur at bloom and early fruit set, which is the critical time for fruit infection). Wayne Wilcox found a very strong relationship between the severity of fruit infection at harvest and the number of “primary infection periods” (>0.10 inches rain + temperatures >50°F) that occurred from just before bloom until fruit set (see Table below). Similarly, years in which 4 or more such events occurred during this fruit growth stage were typified by severe outbreaks of powdery mildew region-wide.

Year % Fruit surface Number infection periods*
infected Just before bloom to fruit set
1988 <1 2
1989 25 7
1991 1 2
1994 61 7
1995 7 3
1996 31 6

*Infection period = > 0.1 inch of rain and temperature > 50° F.