Research and IPM
Disease: Powdery Mildew
Model 1 of 2
Thomas, C. S., Gubler, W. D., and Leavitt, G. 1994. Field testing of a powdery mildew disease forecast model on grapes in California. Phytopathology 84:1070 (abstr.).
Weber E., Gubler, D., and Derr, A. Powdery mildew controlled with fewer fungicide applications. Practical Winery & Vineyard, January/February 1996.
In the fruit zone in the canopy.
Environmental: Hourly average temperature, daily maximum temperature; hourly leaf wetness duration (ascospore model only).
Calculated: Daily average temperature; daily value of number of hours that 70<=T<=85 F, amount of time that temperature is above 95 F.
This model contains two stages based on pathogen biology, an ascospore and a conidial stage. Check with your local Cooperative Extension agent to determine if both stages are important in your area.
To determine ascospore infection risk levels, the model calculates the daily average temperature and measures the hours of leaf wetness. The model modifies the Mills table developed for apple scab ascospore infection by predicting infection based on 2/3 of the hours of required wetness (column 2 in Table 1).
|Daily Average Temperature (F)||Hours of Leaf Wetness required for heavy ascospore infection (2/3 original Mills value)||Leaf Wetness Hours required for heavy infection (original Mills value)|
According to the model, for ascospore infection treat when the conditions required for heavy infection have occurred (at the daily average temperature, treat when the number of modified Mills leaf wetness hours have occurred, column 2 of Table 1 (2/3 of the number of Mills' hours of leaf wetness for severe infection). For example, at a daily average temperature of 55 F, 16 hours of leaf wetness are required for heavy infection (2/3 of 24 hours of leaf wetness for severe infection from original Mills Table). At a daily average temperature of 65 F, 12 hours are required for heavy infection (2/3 of 18 hours of leaf wetness from original Mills Table).
For conidial infections, the timing of treatments is based on temperature, fungicide type, and spray intervals (see Table 2). An index of 30 or less indicates that a spray interval can be stretched to the label maximum. An index of 40 to 50 indicates that a spray interval can be of intermediate length. An index of 60 to 100 indicates that there is high pressure for powdery mildew and spray intervals should be shortened to the label minimum. After treatment, the index is reset to zero.
|Powdery Mildew Risk Index||Spray Material||Spray Interval|
|0 to 30||sulfur dust||14 days**|
|micronized sulfur||18 days**|
|DMI fungicides*||21 days**|
|40 to 50||sulfur dust||10 days|
|micronized sulfur||14 days|
|DMI fungicides*||17 days|
|60 to 100||sulfur dust||7 days|
|micronized sulfur||10 days|
|DMI fungicides*||14 days|
|* Demethylation inhibitors such as Bayleton, Rally, Rubigan.|
|** Or label maximum.|
Since 1995 validation work has been conducted in multiple growing regions of California. In 1997, California PestCast is sponsoring validation projects in additional table and wine grape areas and in raisin grapes. The model is also being validated in New York, Washington, and Oregon; Germany, Austria, and Australia.
This model is being implemented by University of California Extension Plant Pathologist Doug Gubler, growers, Cooperative Extension advisors, and licensed consultants in several grape-growing counties in California.
Model 2 of 2
R. L. Snyder, La Vine, P. D., Sall, M. A., Wrysinski, J. E., and Schick, F. J. Grape Mildew Control in the Central Valley of California Using the Powdery Mildew Index. U. C. Cooperative Extension Leaflet 21342 (April, 1983).
At regional CIMIS weather stations.
Environmental: measured or forecasted daily high and low temperatures, rain of greater than 0.10 inches.
This model was developed for wine and raisin grapes. It is based on the assumption that host tissue growth and weathering of sulfur are the two main reasons why sulfur must be reapplied in vineyards. Daily mildew indexes (DMI) are calculated from daily minimum and maximum temperatures (see Table 1). DMI are accumulated to determine the powdery mildew index (PMI). DMI begin accumulating twelve days after initial leaf appearance or 6-inch shoot growth (whichever comes first) and continue throughout the season until the berries contain 12-15% sugar.
|Daily low temperature||Daily high temperature --- F°|
|* Under high temperature conditions, he amount of sulfur applied per acre should be reduced to avoid excess leaf burn. If possible, sulfur should be applied in the evening after temperatures drop below 90° F to allow slow oxidation during the night, thus avoiding some leaf burn which occurs during the hotter daytime hours.|
|*PMIs are forecasted for dates July 5-8 on July 5.|
Last dusting - 6/25
Next dusting - 7/5
Last dusting - 6/27
Next dusting - 7/7
Last dusting - 6/29
Next dusting - ?
According to the model, initiate first dusting twelve days after initial leaf appearance or 6-inch shoot growth (whichever comes first).
Subsequent dustings should occur when the difference between the current PMI and the PMI on the last dusting date equals or exceeds 1.0 (see Table 2 for an example). When precipitation exceeds 0.10 inch, the vineyard should be re-dusted.
Sall, M. A., Wrysinski, J. E., and Schick, F. J. 1983. Temperature-based timing method of sulfur dust applications. California Agriculture 37, #5 & 6.