I recently was contacted by a journalist regarding the more difficult growing conditions for grape farmers in the foothills this year. Her question was about mildew on the grapes and the potential need for wineries to throw out a lot of fruit. All wineries sort fruit when they are destemming and crushing, but mildew can be a real problem, often in the interior of the bunch where one doesn’t see it.
In any case, here is my response to her question, and our particular situation. By the way, all the work I mention about opening up the vine is also useful now when we get late rains. It helps dry the vine dry out quickly and makes rots and molds (like botrytis) less likely. One last thing. Once the grapes go through veraison (where they soften, turn color and start accumulating sugar quickly), they are no longer susceptible to mildew. The vine is (on the leaves, etc.), but the actual grapes are not! Super important when you’re growing grapes!
Thank you for your interest in our region and its challenges.
The sense out there regarding your question is that there was more mildew pressure on the vines this year. Essentially, the quantity of rain in the spring (and late spring) helped the vines to continue growing more canopy. We’ve also had a relatively moderate summer in terms of temperatures. This in itself is great for wine quality as it lengthens the growing season and gives the fruit more hangtime. But in combination with more leaf area, it can make a thick canopy where the interior area of the vine can almost have a micro-climate of its own (and perfect for mildew). And we must add one more aspect to this, and that is that a dense canopy prevents mildew sprays from getting good coverage in the vine.
The answer to all of this is to do copious amounts of leaf-thinning at the right time. By removing some of the leaves, this allows airflow through the vine. This in turn disrupts the micro-climate inside the canopy while giving sprays better access to the interior.
I have heard of some farmers struggling some with mildew, although we were spared the issues. Our most susceptible varieties such as Chardonnay came through clean with no mildew. (P.S. We had a little mildew in Marsanne, another canary in the coal mine, and one small block of vertical cordon Zin at the top, but we just dropped the fruit (and the crop still is heavy).)
We are blessed that our vines are older and, in this particular case, weaker. There was less canopy growth to deal with. In addition, we have a year-round crew working with us. We jumped in early to do the necessary leaf-removal to open up the vines. At the 3,000-foot elevation, we are used to high mildew pressure since temperatures here don’t get hot enough to kill it (like other regions). We also farm naturally (organically) on all 85 acres which limits our mildew arsenal essentially to sulfur and mineral oil. (Conventional farming has more choices although the best eradicant is organic!) Thus each year we have a set program for spraying that has been successful every year in controlling mildew.
Lastly, the Integrated Pest Management Program with the University of California has a great mildew model that helps growers see the mildew pressure out there. There is a weather station at Lava Cap Winery just slightly lower in elevation than our own that helps me remain vigilant on a successful program. Here’s the link to the site: http://ipm.ucanr.edu/calludt.cgi/GRAPEPMVIEW2?Y=2019&STN=Lava_Cap-02.P You’ll see the pressure we had for the season as well as other locations in the foothills.
The only successful program is one that keeps mildew out before it ever starts. Once a farmer has mildew, it’s challenging to eradicate it and keep it under control. And mildew will overwinter as well, bringing more early season pressure the next year.
So was it a challenging year? Potentially yes due to the rain in the spring and the moderate temperatures. We didn’t have issues, but others may have.
I hope this helps give you some background. Please let me know if you have any other questions.
And so far, even with these fall rains, the crop load and quality are looking and tasting great. Now if we can keep the snow at bay until we finish harvesting.