protectant such as Kumulus or Purespray will have to be applied more frequently to accommodate the rapid growth rate of spring, probably every ten days. Keep doing it. After bloom and fruit set, the growth rate declines. You can relax a bit and spray less frequently. Watch the temperature and modify your intervals accordingly. Mildew grows most rapidly in the mid 20s. There’s a lower risk of infection in the low 20s or in the 30s. The upper 30s or higher are either inhibitory or lethal to mildew. There are a number of ways to resist a mildew infection, independent of sprays. This won’t eliminate the need for sprays unless you’re growing a mildew resistant hybrid, but it may allow you to increase the interval between sprays.
The first strategy occurs when you plant the vineyard. You can play “catch up” later, but the best time to act is at planting. Add a mycorrhizal inoculum to your planting mix. If you have a loamy soil, it may already be present, but it typically isn’t present in the sandy soils of the Okanagan. Some nurseries inoculate plants during propagation, but I prefer to inoculate at planting, even if the plants have a nursery inoculum. There are many actions of mycorrhyzae but the benefit that affects mildew is that grape vines respond to a fungal infection (such as mycorrhizae) by releasing phytoalexins.
These are a component of the plant’s immune system. The most familiar of these is resveratrol. Grapes grown in humid regions have higher resveratrol levels, stimulated by the constant threat of mildew. Pre-infection with mycorrhizae prepares the vines prior to a mildew infection.
Another management strategy to reduce infection risk is to control temperature, light, and moisture. Ultraviolet light is lethal to mildew. By maintaining a grape canopy that allows light penetration and drys out quickly, you are inhibiting mildew growth.
This can be difficult if you have a vigorous site. A little bit of clay or silt in your soil can hold enough extra water that vines will explode in early summer. Probably the best way to handle this is to leave a lot of extra
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buds when you prune and minimize or even eliminate watering, which can be pretty scary. After fruit set, you can do a quick run-through and instead of hedging, remove the canes that grew from the extra buds you left. This will open up the canopy without having to hedge. Topping and hedging doesn’t open up the middle of the canopy and typically stimulates the vine to push laterals that create more shade. My final bit of advice is site selection.
I’ve looked at a lot of sites during the past 40 years and I know the arguments — south-facing slopes to get more sun, west-facing slopes for warm afternoon sun, north-facing slopes to extend season length for short-season varieties.
I like east-facing slopes. They warm up earlier in the morning and dry out the canopy to inhibit mildew. They are not so hot in the afternoon and have more uniform temperature throughout the day. What’s not to like?
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