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 05-12-2025 |
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 | 90 | 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) | 90 | high | reproduces every 5 days | |
| West Point | 90 | 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 | 80 | high | reproduces every 5 days | |
| Gold Hill (SRL) | 90 | high | reproduces every 5 days | |
| Lava Cap (LAV) | 90 | 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 | 70 | high | reproduces every 5 days | |
| Del Rey | 70 | high | reproduces every 5 days | |
| Kerman | 50 | moderate | reproduces every 15 days | |
| Kingsburg | 20 | low | present |
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) | 60 | high | reproduces every 5 days | |
| Scotts Valley (SVL) | 80 | high | reproduces every 5 days | |
| Upper Lake (UPL) | 80 | 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 | 50 | moderate | reproduces every 15 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) | 40 | moderate | reproduces every 15 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) | 20 | low | present | |
| Escalon (ESL) | 80 | high | reproduces every 5 days | |
| Linden (LIN) | 60 | high | reproduces every 5 days | |
| Lockeford (LOC) | 80 | high | reproduces every 5 days | |
| Lodi (LangeTwins) | 50 | moderate | reproduces every 15 days |