(Information in this article has been gleaned from a career of research by Dr. Doug Gubler, University of California, Davis plant pathologist. Dr. Gubler is recognized worldwide as a leading expert on powdery mildew.)
Powdery mildew has been a persistent and enduring problem on California grapes for decades. This is especially true in California Vitis vinifera vineyards.
The impact powdery mildew infection has on a vineyard often depends on the timing of the first infection. Early fruit infections cause stunted berries, scarring, and off-flavors in wine. Powdery mildew also reduces the storage life of table grapes and can affect the photosynthesis rate, thus reducing berry sugar content.
It can affect all succulent tissues on a grapevine, including the stem, fruit and leaves. In some vineyards, especially Carignane, young shoots entirely covered with mildew can be found shortly after bud break. In some varieties — Thompson Seedless, Ruby Seedless, Chardonnay, Chenin Blanc, Cardinal, and Cabernet Sauvignon — initial infection symptoms can be seen after the first spring rain, with individual colonies on the underside of basal leaves.
Fruit can become infected at the beginning of development until the sugar content reaches 8 percent. This makes control essential during the early part of the season. Established infections will continue to produce spores until the berries reach 12 to 15 Brix. As colonies age, they become inactive and berries become less susceptible after sugar content exceeds 15 Brix.
The degree of susceptibility to mildew varies between varieties varieties. Carignane, Thompson Seedless, Ruby Seedless, Cardinal, Chardonnay, Cabernet Sauvignon and Chenin Blanc are the most susceptible. White Petite Sirah, Zinfandel, Semillon, and White Riesling are less susceptible.
Fungus life cycle
U. necatorsurvives the winter as dormant mycelium in buds or as cleistothecia. Cleistothecia mature in late summer and fall and are washed onto the cordons and spurs with fall and winter rainfall. Ascospores are released in the spring with rainfall, sprinkler irrigation or fog. Dormant mycelium emerges when shoots begin to grow, and at temperatures of 64 to 86 degrees infection occurs immediately. Conidial production occurs in 7-10 days after primary infection and continues throughout the season as long as moderate temperatures persist.
Mild weather results in increased powdery mildew growth. Spores germinate at leaf temperatures between 43 and 91 degrees; the optimum temperature for growth is 75 degrees. At 70 to 86 degrees, rapid germination and mycelium growth take place. During favorable temperature periods, the time between spore germination and production of spores by the new colony takes only 5 days.
High temperatures that do not harm the plant can harm the fungus; spores and mildew colonies can be killed at extended duration of temperatures above 91 degrees. The fungus is destroyed completely when air temperatures rise above 95 degrees for 12 hours or more if colonies are directly exposed to UV light.
When temperatures are moderate for extended periods of time, even the best control program often fails. With moderate temperatures, the pathogen’s reproductive rate is greatly increased. In addition, these same conditions increase germination and infection efficiency, resulting in increased disease pressure and poor disease control. On the other hand, during periods of extended heat (above 90 degrees) the reproductive rate of U. necator is slowed, germination and infection efficiency are decreased, and virtually any control program works effectively.
A disease risk assessment model using temperatures as guidelines is now widely used in all California grape production areas to aid growers and Pest Control Advisers in timing powdery mildew control sprays.
The University of California Risk Assessment Index (RAI) can help quantify disease pressure by measuring how favorable weather conditions are for powdery mildew development and reproduction. By monitoring and using this model to anticipate disease development, growers and PCAs can make smart decisions about spray timing and frequency, based on disease “risk” and proven treatment intervals for individual fungicide materials.
The RAI model measures an accumulation of disease risk index points based on accumulative in-canopy temperatures and leaf wetness over a measured period of time.
“This model was built to function easily using in-canopy temperatures, and it has all the necessary parameters built into it for an accurate prediction of disease pressure,” Gubler emphasizes. “The model is based on information and knowledge derived from epidemiological studies in all California grape production areas. The model has also been validated in all California production regions as well as in other states and countries.“
It has been so successful that a recent survey by Gubler found that 50 percent of respondents use the model either for spray timing or to dictate what product to use.
To trigger the RAI
The RAI correlates in-canopy temperatures with conidial production and assigns disease risk index points accordingly. Once initial infection occurs an epidemic will begin when there are three consecutive days with six or more continuous hours of temperatures between 70 and 85 degrees measured in the vine canopy.
Begin with 0 pointsthe first day,then add 20 points for each day with 6 or more continuous hours of temperatures between 70 and 85 degrees.
Until the index reaches 60 points (three consecutive days), if a day has fewer than 6 continuous hours with temperatures between 70 and 85 degrees, reset the index to 0 and start again. If the index reaches 60, an epidemic is under way. Begin using the spray-timing phase of the index.
Once the index is triggered (by reaching 60 points.), continue recording daily in-canopy temperatures. Adjust the previous day’s index using the steps below, and keep a running tabulation throughout the season. This is made easier by using a weather station that has the RAI programmed into it.
If fewer than 6 continuous hours of temperatures between 70 and 85 degrees occur, subtract 10 points. If 6 or more continuous hours of temperatures between 70 and 85 degrees occur, add 20 points.
If temperatures reached 95 degrees for more than 15 minutes, subtract 10 points. If there are 6 or more continuous hours with temperatures between 70 and 85 degrees AND the temperature rises to or above 95 degrees for at least 15 minutes, add 20 points and subtract 10 points. Once the RAI reaches 100, don’t add points. If the index is already at 0, don’t subtract points. Never add more than 20 points a day. Never subtract more than 10 a day.
The RAI neverexceeds 100 points or goes below zero. If the index is 0-30, the pathogen is present, but level of disease pressure is nil. If the index is 40-50, disease pressure is intermediate, i.e., the pathogen is reproducing every 15 days. If the index is 60 or above, disease pressure is high and the pathogen is reproducing every 5 days.
Disease severity can be calculated by using the RAI model as described above, or a grower and PCA can use weather equipment that already has the RAI built into its software. Self-monitoring is possible using simple devices such as Hobo units that sell for $50 to $100, or more sophisticated equipment like data loggers from Spectrum Technologies, which hang in the canopy and download information directly into a computer.
Subscription weather services are also available that monitor and calculate the index. Make sure that all calculations are local and based on temperatures within the vine canopy.
California’s central and northern grape regions are served by companies such as Terra Spase (which privatized the Gubler UC stations on the North Coast), Agrilink and Ag Unlimited. These companies provide GIS maps along with temperature monitoring. Crop Production Services (CPS) (formerly Western Farm Service), which privatized the system in the Central Valley and on the Central Coast in 1995, provides a similar service.
Additionally, the UC PestCast Program continues to support collection of data from two weather networks — the Fresno-Madera Grape Network and San Joaquin WEATHERNET. For more information see: http://www.ipm.ucdavis.edu/WEATHER/wxretrieve.html.
Daily analysis of the UC Davis RAI allows growers and PCAs to predict level of conidial populations about one week in advanceand what potential disease severity will be up to two weeks in advance. Anticipating developing mildew pressure and planning control programs ahead of time allows growers to stay ahead of the disease. It also allows them to decide which product to use based on the disease pressure.
“Use of the model is simple,” Gubler says. “And, if anyone visualizing the index thinks in terms of the way disease increase occurs, they might think of the fluctuating index line as the exact thing the fungus population will be doing one week later. For example, if the index goes up and stays up, the pathogen population will be going up one week later (from when the index hits 60). This gives the grower one week to make a decision on what to do and what to spray.”
Using all available methods of monitoring is added insurance that a grower’s spray program is on target.This is especially important since many growers continue to miss disease onset, optimum spray timing and frequency.
With mildew, being 10 days late could mean a grower may have already missed two life cycles. Or, they may jump the gun and spray too early when the disease isn’t even present — and that can be expensive. Use of the model generally reduces the number of applications per year by two or three, with some saving up to six or eight.