Hi Foothill Fodder Family! This week temperatures warmed up enough to begin the powdery mildew index in all of our 7 foothill PMI (powdery mildew index) stations-the information is publicly available on our UCIPM website here, with links on my webpages here. What does the index mean and how can we best use this information to begin our powdery mildew treatments?
I like to start by thinking of the disease "triangle": the 3 things needed for disease to happen. The key here is that all 3 of these parameters need to be in place for mildew to infect.
1. HOST. In the case of powdery mildew, infection only occurs on GREEN tissue. Shoots, leaves, flowers and fruit are all susceptible. This is why we start the mildew index at budbreak-before budbreak, infection can not occur because there is no green tissue to infect. Once budbreak has occurred, all green tissue and fruit is susceptible to infection until after fruit verasion. Fruit is not susceptible once it reaches 8 degree Brix (lucky for us!). Another thought about host is grape cultivar susceptibility to the mildew pathogen, which is a measure of how easily a variety can be infected. There are big differences among cultivars as I'm sure anyone who grows Carignane, Chardonnay, Chenin blanc, Muscat or Rousanne- all considered highly susceptible varieties- has noticed. See a list of susceptible varieties based on visual observations by former Fresno UCCE Advisor Stephen Vasquez.
2. PATHOGEN. For infection to happen the pathogen needs to be present. There are 2 possible scenarios for every vineyard in spring:
Scenario 1: Mildew was present in the previous year AND/OR I have a highly susceptible variety. If you had mildew last year, or you have a highly susceptible variety (see above), then I always advise you start early-within 4-6 inches of shoot growth with an eradicant (i.e. horticultural oil at 1.5%), or apply a dormant or delayed dormant treatment (before budbreak) of lime sulfur, oil or micronized sulfur. If the problem was really bad, it may be a good idea to do both a dormant and an early treatment. The idea is to kill any of the overwintering chasmothecia spores and for this you need good coverage to contact those spores that are in the trunk and cordons crevices. This will greatly
reduce the spore load once environmental conditions favor disease.
Early season spraying requires good coverage to be effective. How much spray is really hitting the target?
Scenario 2: Mildew was NOT present in the previous year and varieties are not highly susceptible. The
"reward" for keeping a clean vineyard free of mildew in the previous season is that it should make the next season easier (ahh) and requiring fewer sprays (yeah!), especially, I think, safely eliminating the need for an early first spray. Why do I say this? Think about the biology of the fungus. It overwinters as a special dormant type of spore structure-chasmothecia-in the trunk and cordon crevices. This spore sac needs to germinate, land on susceptible green tissue, and grow hyphae (hyphae are the vegetative or feeding strands of the fungus) and then grow the secondary, asexual spore called conidia, and then these conidia need to blow over into your vineyard and infect tissue and grow in order for you to have infection. If you have no overwintering spores present, (the KEY), it will likely take at least a couple of weeks of optimal environmental conditions (see 3 below) for the conidia to develop in some other neighboring block, mature, and blow into YOUR block. The conidia are the spores produced for the rest of the season and do not require rainfall to germinate, just warm temperatures. The chasmothecia are produced only in fall and are the overwintering spores and spores that first germinate after 2 mm of wetness and optimal temperatures of 50-80F.
Powdery mildew disease cycle-taken from Grape Pest Management UCANR pub. 3343. Bettiga, editor.
By the way, there is new technology out there that uses lab sensing of fungal DNA (called "PCR" for polymerase chain
reaction)-basically the lab provides the enzymes for the DNA to start replicating, and if it is there, it replicates enough that we can "see it". In order to capture spore DNA to test for its presence with PCR, you need to "trap it". Just think about how we use traps for insect monitoring all the time, but we always have to ASSUME fungus is present because we haven't been able to trap for fungus. Until now! Come to my Foothill Grape Day on May 18 to hear Bryan Rahn of Coastal Viticulture Consultants talk about how they use this new technology. Cool stuff!
Horticultural oil successfully keeps fruit clean; the reward is fewer sprays next year.
3. ENVIRONMENT. Powdery mildew is a living organism, even though we may not think about it that way. It grows at optimal temperatures, and at some temperatures-optimal is 70-85oF- it grows more quickly than at others. Mildew growth slows down or stops at high and low temperatures. That is the crux of the powdery mildew index, which accumulates points on a scale of 0-100 to indicate weather conditions that are optimal for mildew growth. The higher the score, the more quickly the fungus will grow as
long as the temperatures remain in that range. To know what temperatures we have, we need to monitor temperatures in grape canopies. That is the job our 7 powdery mildew stations are doing for us across the foothill microclimates. With the cool mild spring we've had so far, this past week has been the first week where we've had temperatures high and consistent enough (at least 6 hours per day in the 70-85 degree range) to accumulate PMI points. So some spores were likely released this past week.
Doug Gubler inspects the Amador Eagle station in 2013. Hosted by Pat Rohan, this station was the first in the foothills. Now we have seven stations!
To use the index well though, we really also need to look at our weather forecast-because this will help us predict how the index will change in the next week or so. Looking at the National Weather Service forecast, my preferred weather site, my crystal ball says the PMI will drop 10 points a day for several days this weekend and into next week; while rain with temperatures above 50 degrees will allow spores to germinate. I would GUESS that this next week will be our primary spore production period. If we had spore traps placed, we would be able to know better, so this is, again, only my best guess based on the weather forecast. Since weather can always change, I like to start my day (like every good farmer) looking at the NWS site to see our weather forecast.
I'll discuss more about current mildew conditions here in the foothills at Foothill Grape Day. I hope to see you there!