Weather, models & degree-days

Grape Powdery Mildew Risk Assessment Index

The grape powdery mildew risk assessment index (RAI) is useful for determining disease pressure and how often you need to spray to protect the vines. For information on how to use the RAI, see the pest management guideline. RAIs are based on actual weather data for stations that take appropriate readings.

County Active Weather stations RAI* for
04-22-2025
Disease pressure Pathogen status
Amador

Based on bud break, April 9, in Zinfandel, may need to adjust for other cultivars that emerge earlier than the indicated date.

Rombauer Sierra Foothills 100 high reproduces every 5 days
Calaveras

Based on bud break, February 1, in Cabernet Franc, may need to adjust for other cultivars that emerge earlier than the indicated date.

Ironstone (IST) 100 high reproduces every 5 days
West Point 100 high reproduces every 5 days
El Dorado

Based on bud break, February 1, in Sangiovese, may need to adjust for other cultivars that emerge earlier than the indicated date.

Fair Play 40 N/A Not Present
Gold Hill (SRL) 100 high reproduces every 5 days
Lava Cap (LAV) 100 high reproduces every 5 days
Fresno

Based on bud break, March 16, in Thompson Seedless, may need to adjust for other cultivars that emerge earlier than the indicated date.

Caruthers 80 high reproduces every 5 days
Del Rey 80 high reproduces every 5 days
Kerman 40 moderate reproduces every 15 days
Kingsburg 80 high reproduces every 5 days
Lake

Based on bud break, March 20, in Sauvignon Blanc, may need to adjust for other cultivars that emerge earlier than the indicated date.

Kelseyville (KEL) 60 high reproduces every 5 days
Middletown (MID) 100 high reproduces every 5 days
Scotts Valley (SVL) 100 high reproduces every 5 days
Upper Lake (UPL) 100 high reproduces every 5 days
Madera

Based on bud break, March 16, in Thompson Seedless, may need to adjust for other cultivars that emerge earlier than the indicated date.

Firebaugh 90 high reproduces every 5 days
Mendocino

Based on bud break, March 27, in Chardonnay, may need to adjust for other cultivars that emerge earlier than the indicated date.

Ukiah-Ruddick (RUD) 60 high reproduces every 5 days
San Joaquin

Based on bud break, March 17, in Chardonnay, may need to adjust for other cultivars that emerge earlier than the indicated date.

Collegeville (COL) 70 high reproduces every 5 days
Escalon (ESL) 100 high reproduces every 5 days
Linden (LIN) 40 moderate reproduces every 15 days
Lockeford (LOC) 100 high reproduces every 5 days
Lodi (LangeTwins) 70 high reproduces every 5 days