One of our vineyard consultants -- who will remain nameless today-- believes spraying for mildew where we live may not be necessary very often because we have wide spacing between our rows and good wind flow. Our neighbor (Sunrise Vineyards) has the vines really packed in, and sprays quite often. On the other hand, our other neighbor at Coyote Oaks with 3 year old vines (who uses this same consultant) has never sprayed. Neither have we.
Our friend Gerry -- who grew the excellent Petit Verdot we have in the barrel -- operates a weather station and sends out weekly mildew reports. I had to ask him, in a polite way, is our consultant nuts? Here's Gerry's answer:
"Hi Craig, [the consultant] is not nuts but he does not have all the technical facts about mildew. It depends on grape variety , air flow, temperature, and moisture at the start of the season.. but I use the Davis system (used to be a secret). Some folks will make a big deal of the moisture being important but that is only the thing that gets the mildew started in your vineyard. Once it exists even in a small area it exists and then you have to proceed with protecting your vineyard from getting a major infestation developing. I just assume at the start of the season that I already have some mildew (it stays in the vineyard over the winter by the way). So I am in protective mode from day one. Thus It really depends on temperature and time, Mildew likes its temperature between 70 and 85 deg F. (just like us human beings). I would be happy to describe it to you in person or over the phone. It is simple to do if you have a recording weather station which I have.
The simple version is: 1. do your first spray using Thiolox when the buds just appear... maybe 1" long. The rest of the sprayings depend on your grape variety and the calculated risk level that develops after your first spraying.
2. Each day you have temperature between 70deg and 85deb for 6 continuous hours you add 20 to the risk level. ( If it goes above 85deg for 45 minutes that breaks the cycle and you subtract 10 from the risk level. If it goes above 95 for 15 minutes you get to subtract 10 from the risk level for that day. (That is why the Temecula folks don't worry much about mildew... because it is very hot during their season.
3. the risk value you compute never goes above 100 or below 0. A mild level is 30 or below. 60 is very high 100 is as high as the system will compute it.
4. The day you spray puts the risk level down to zero and you start over. The interval between sprayings depends on the material you spray with. If that first spraying was Thiolox (micronized sulphur) and the risk level goes up to around 40 to 60 or above you only get to wait 7 days til the next spraying..... so you see it depends on the type of spray. I use Thiolox for the first 2 or 3 sprays (or til verasion starts)... I am now using Rally and Pristine... you have to alternate the types of spray unless you are using sulphur which does not need to be alternated with anything except you do not to want to use sulphur near harvest (your wine is affected by it negatively) This is simpler than it seems to be if you can make a good stab at the risk level. If you do not do that just read the label on the material and it will tell you the interval days/weeks...sort of and just do what the label says forget the fancy risk level....you will probably just add one or 2 sprayings to your vineyard for the year. I usually spray about 10 times per year. Gerry."