Last year, many growers in California reported heavier disease pressure than usual. After a winter of heavy rain and some late spring rains, this year growers may see even greater rates of powdery mildew infection. However, there are actions you can take to reduce powdery mildew in your vineyard.
There are two important strategies for control of powdery mildew: reducing initial inoculum and preventing infection.
In most areas, powdery mildew overwinters as fruiting bodies called chasmothecia, which contain the spores (called ascospores) of powdery mildew. In warm climates, it can also overwinter as mycelium in dormant buds. An early season application of lime sulfur or horticultural oil (such as JMS Stylet Oil) should be applied at budbreak if temperatures are optimal for ascospore release: between 70° F to 80° F. This application significantly reduces spores released from chasmothecia and delays initial infections.
After the initial period of ascospore release, preventing further infection entails a season-long control strategy which includes fungicides (conventional or organic) and cultural controls. Further fungicide types and application intervals may be timed using the powdery mildew risk index.
Powdery Mildew Risk Index
Grape growers can use the powdery mildew risk index model to micro-adjust their spray interval (http://ipm.ucanr.edu/calludt.cgi/GRAPEPMVIEW1). The model correlates the pathogen biology with the canopy temperature and leaf wetness to assess the disease pressure during the growing season.
The model contains two stages: an ascospore and a conidial stage. The ascospore model is used to determine the risk of ascospore release and primary infection. Daily average temperature and duration of leaf wetness are put into the calculation.
Once ascospore infection has occurred, growers can switch to the risk index (RI) to determine the potential of secondary infection by conidia. At this stage, pathogen population increase is mainly based on the canopy temperature. There are two stages for calculating the risk index: initiating the risk index and adjusting the spray timing.
Initiating the risk index: Once there are three consecutive days with six or more continuous hours of canopy temperatures from 70°F to 86°F, it means the index reaches 60 and an epidemic is under way. Growers should begin using the spray-timing phase of index.
Adjusting the spray timing: After the index reaches 60, the calculation of index is simply based on the daily canopy temperature. Basically, 20 points are added to the index when six or more continuous hours of canopy temperatures occur from 70° F to 86° F. Similarly, 10 points are subtracted from the index when fewer than six continuous hours of canopy temperatures occur from 70° F to 86° F or temperatures reach 95° F for more than 15 minutes. According to the RI, local conditions and field scouting, growers can tighten or loosen the spray interval to confront the different levels of disease pressure in their vineyards (Table 1).
Growers can use the RI to micro-adjust spray intervals and reduce unnecessary sprays when the disease pressure is low. However, alternating fungicides with different modes of action is critical to reduce the risk of developing resistance. Growers should avoid applying two sequential sprays of any fungicide without alternating with a fungicide of a different mode of action.
Fungicide formulations that mix more than one active ingredient may be useful to delay the development of resistance and improve disease control by broadening the spectrum of activity. Whenever possible, tank mix conventional or biological fungicides with sulfur. Because the risk of resistance to sulfur is minimal, tank mixing with sulfur reduces the chance that resistant isolates of powdery mildew will successfully infect plants and spread.
The Importance of Coverage
Like any spray program, coverage is one of the most important factors that determine the success of the powdery mildew fungicide spray. Vine vigor, trellis types, irrigation/fertilization, canopy management and sprayer/nozzle types can all be managed in order to achieve better coverage. Water-sensitive paper and proper sprayer calibration should be used as tools to validate and improve spray coverage. Canopy management, such as shoot thinning, leaf removal, and hedging, are commonly applied, when appropriate, to increase the fruit quality, and these practices can also open the canopy for better spray coverage and improved disease control.
Powdery mildew is one of the most common disease issues grape growers face. However, with a comprehensive and well-timed management program, your risk of crop loss can be significantly reduced.
As always, follow label guidelines for application rates, minimum spray intervals, and pre-harvest intervals. Rotate fungicides between different modes of action to prevent resistance. If you have any questions, consult your local farm advisor or Extension agent.