Managing grapevine powdery mildew

The 2009 grape growing season has commenced—Thompson Seedless budbreak occurred on March 17^th—and warm weather is predicted in the days ahead. The predicted temperatures are optimal for quick spring growth and for the development of powdery mildew. To improve powdery mildew control, Valley grape growers should consider incorporating the UC Davis powdery mildew risk assessment index (RAI) into their management programs. The advantage of using the RAI will be during periods of low mildew pressure when spray intervals can safely be lengthened thereby saving grower’s time and money as well as eliminating fungicide applications. When disease pressure is high, a grower following the RAI recommendations may apply more fungicides than a grower not following the model, but those following the model would be expected to achieve better mildew control. To get the most out of the RAI, growers should consistently monitor canopy temperatures, recognize differences among cultivar in susceptibility to mildew, and understand how trellis systems may affect mildew growth.

 

Powdery mildew growth is temperature dependent and requires careful monitoring to optimize fungicide applications. The RAI helps determine grapevine powdery mildew growth based on optimal canopy temperatures (70-85^oF). The RAI cannot be initiated until budbreak occurs and there are 6 continuous hours between 70-85^oF for three consecutive days. Once these parameters have been met, growers have approximately 5-7 days to apply a fungicide. The RAI is calculated by adding 20 points each day in which there are least 6 continuous hours with optimal growth temperatures. If there are less than 6 hours or if the maximum temperature in the canopy that day is greater than 95^oF, then 10 points are subtracted from the index. The RAI never deviates outside the 0-100 point range. Growers can calculate the RAI themselves, access the calculated RAI from the UC IPM website or follow us on Twitter (http://www.twitter.com/grapetweets) to receive weekly updates via the internet, your cell phone, or other mobile device.

The RAI can be used to determine disease pressure and how often fungicides need to be applied. Spray intervals can be shortened or lengthened depending on disease pressure, as indicated in the table.

 

Spray intervals based on disease pressure using the RAI

Similar to last year, the RAI threshold is forecasted to “kick-off” late March or early April, which gives growers some time to consider a season long management program. Limited foliar growth should be treated with wettable sulfur or a systemic fungicide. Dusting sulfur should be saved for late May applications when the canopy is more fully developed. Growers are strongly encouraged to use multiple classes of fungicides during the season to minimize resistance to any one class of fungicide. Fungicide resistance will be further discussed in a future blog.

 

Two other factors play a key role in mildew growth. First, grape cultivars have varying degrees of susceptibility to powdery mildew infection. Thompson seedless is less susceptible to powdery mildew infection than Fiesta, so Fiesta growers should begin mildew monitoring earlier in the season and they should do it more frequently. Fungicide programs will also be different, given the ability of powdery mildew to grow exponentially on Fiesta when conditions are optimal. The second factor that growers need to consider is trellis type. Growers using an overhead trellis frequently see more mildew. This could be due to the fact that a lush, dense canopy on an overhead trellis could lower canopy temperatures and increase humidity, both of which favor mildew growth. Also, it can be more difficult to achieve good spray coverage on an overhead arbor trellis than on a conventional trellis. Thus, growers will need to consider trellis type when developing their powdery mildew management program.

Additional sources of information:

For a more detailed discussion on grapevine powdery mildew and the use of the RAI go to:

 

For a cultivar susceptibility chart go to: