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This option predicts two infectious stages, an ascospore
stage and a conidial stage (Thomas, Gubler, and Leavitt
1994; Weber, Gubler, and Derr 1996) Ascospores are
released in the spring from the structure in which the
disease overwintered. Conidial spores are the result of an
ascospore infection. Ascospores cause primary infections
and conidial spores cause secondary infections. Your
State Agricultural Extension Service can advise you about
which stage is important in your area.
Ascospore Infection risk is determined using the daily
average temperature and the hours of leaf wetness. A
modified Mills Table (2/3 the original Mills leaf wetness
value) is used to determine the development of a ‘Heavy’
Ascospore Infection, the point at which treatment should
begin.
Three consecutive days with 6 consecutive hours of
temperatures between 70°F and 85°F are required to
initiate the Conidial Index. Thereafter, the index
increases by 20 with each day having six consecutive
hours between 70°F and 85°F. The index decreases by 10
on days with less than six consecutive hours in the range
of 70°F to 85°F and on days with a maximum temperature
greater than 95°F. The index will always be between zero
and 100.
Infection Severity Screen
This screen shows the risk for Black Rot and Powdery
Mildew. The Powdery Mildew ascospore infection is
none, and the conidial index is 10.
Grape Powdery Mildew
BLK ROT=NONE PM:
ASC=NONE CON=010
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