Here on the west coast of North America we are lucky to have long, dry summer days which reduce the amount of maladies that can afflict our grape vines. European grape vines did not evolve with many of these diseases so they have little resistance unlike their North American counterparts. So, we need to resort to chemical intervention to prevent or remove these problems. The two biggest problems for growers in the Puget Sound region are Powdery Mildew (PM) and Botrytis.
Powdery Mildew is by far the biggest problem we need to address. It can effect vines at almost anytime during the growth cycle from April until October, but luckily it’s fairly easy to control. Many backyard growers tell me after planting a new vineyard that they haven’t had a PM problem and haven’t had to spray anything. I just tell them wait, it will catch up with you. It always does. The spores float in the air for miles and miles. While the PM that affects other plants like Big Leaf Maples and squash is similar, it does not infect grape vines, but it’s a good indicator because if those plants are getting it, it’s more than likely your wine grape vines will get it. But, we have a lot of weapons in are arsenal to deal with effectively and the good news is that spraying Organic is very easy.
There is a simple way around this, plant interspecific hybrids that have been bred to resist this problem. Grapes like Regent were bred with this in mind. Plant Regent or a similar white grape and no spraying for PM or botrytis. You can skip this whole section if you’ve done that, but if you are glutton for punishment, read ahead.
Powdery Mildew
Powdery Mildew (PM) is a fungus that grows on many different plants. Roses, squash, big leaf maples are many notable examples around the Puget Sound region. Most plants can co-exist with it without a lot of problems but many can have severe problems with it. Vitis vinifera vines are greatly affected by PM. Native American grapes and many hybrids have high tolerance levels against PM and are not affected by it. PM may not be present in your vineyard until it get’s established after a couple of years. I hear from many new growers telling me they haven’t gotten PM and they haven’t sprayed anything. Unfortunately, it will catch up with you as the spores will blow in from some distant vineyard. It always happens.
Powdery Mildew forms on leaves in the spring and summer time, although the spores will be in the bark and soils around the vines during the winter months so you never really get rid of it. When it is visible on the leaves of the plant, it looks like someone sprinkled flour on the leaves. It will also attack green grapes but after the grapes turn color, PM will no longer attack the berries but continue to be a problem on the leaves until it’s too cold for the spores to form fruiting bodies. A little PM on the leaves is almost always going to happen in the Puget Sound region no matter what you do. Getting it on the fruit can cause scarring and give fruit that is not perfectly formed and can lead to splitting problems as the berries swell up.
PM likes warm, humid conditions. Temperatures above 70 degrees and below 88. Ideal conditions are foggy mornings; dry afternoons with little or no wind. You will start to see it on your leaves starting in June or so if you don’t spray. If you are seeing leaves that look like the picture below, it might be too late to save the harvest for that year, but you will be fine for the next year. You will have to spray before the problem looks like this picture.
There are many different chemicals to prevent or eradicate PM. By far the most common is Sulfur. There are other methods like Stylet Oil, biofungicides and other systemic chemicals. Once you get PM on your vines, you can eradicate the problem, if it’s not too severe, by spraying Sodium Bicarbonate (Baking Soda) or Potassium Bicarbonate (Kaligreen). I usually spray Sulfur and Potassium at the same time to eradicate any unseen problems and prevent future problems at the same time. You can also find potassium bicarbonate easily at many garden shops or on the internet.
In the past I would spray micronized sulfur mixed with a spreader sticker like NU Film 17 every couple of weeks until veraison when I switched to biofungicides for Botrytis, but working with sulfur does have it’s downsides. While it’s considered “organic” it can irritate your breathing passages and cause a allergic reaction. Then the smell of sulfur is hard to get rid of. But it is very effective and cheap. Recently have switched to 100% horticultural oils, mainly JMS Stylet oil or PureSpray Green.
I can’t emphasize this enough, keeping on top of PM is an ever present task that changes frequently with the weather. Once you have a large scale infection, it is hard to eradicate so early prevention is key.
Botrytis
Like PM, Botrytis is another fungus that attacks the grapevines, but it mainly forms on ripe grapes and can ruin your crop if you don’t keep a good eye on the problem and may not be able to prevent it in the wettest of times. It can be a heartbreaking problem because around here in the Puget Sound you can do everything right and at the very end of the season watch your grape rot away in very wet weather. But hopefully, prevention is key here too.
There are two ways to prevent Botrytis, cultural and chemical. Cultural which means it’s prevented by the use of mechanical means rather than chemical although chemicals can be used, the main way to prevent Botrytis is by pulling leaves from around the cluster of grapes after veraison (or the color change). In the picture above, ripe white grapes are being attacked by botrytis. You can see individual rotten grapes and once you see rotten grapes, it usually means that grapes in the middle of the cluster have been rotting for a while. If you see a cluster like this, it’s probably too late to save that cluster.
Botrytis can have a beneficial effect if you want to see it through to the very worst. At Bainbridge Vineyards they have successfully made botrysized wine, like Sauterne style wines from Siegerrebe grapes with excellent results, but it’s not for the faint of heart because too much rain can make the grapes unusable for wine and rot them completely.
To prevent botrytis you need to have a two pronged approach. Leaf pulling and spraying. Start pulling leaves from around the fruting zone before the fruit has started to turn color. Since rain is your enemy, having the fruit exposed to winds and sunshine help dry them out when they get wet. At the same time you need to stop spraying sulfur on the fruit (Still spray it on the leaves), and start spraying something like Serenade or Sonata which are organic biofungicide controls for botrytis. Keep your fingers crossed because even the wettest harvest times can be trying for botrytis!
Viruses of the grapevine
I won’t spend a lot of time discussing this issue. There are quite a few viruses that are floating around WA state. The best way to protect yourself is to get vines from a well known nursery that grows virus free plants. Don’t bring equipment, especially pruners, that have been in other vineyards. Don’t share cuttings for vines with other vineyards. The big viruses that are spreading around WA state are Grape Leafroll virus, Rugose Wood, Grape fanleaf virus and Red blotch virus. Sometimes vines can live for years with these viruses and have no problems and then sometimes vines will die pretty quickly. Unfortunately, there are no cures for any viruses except to pull out the effected vine and replant. If you think you might have a virus problem in your vineyard, contact the WSU extension office. They would probably help you figure out what it is.
Spray Schedule
Here is a typical spray schedule that I have adopted the Oregon Vineyard Supply spray schedule for small growers. You can adopt this to sulfur instead of horticultural oil. Micronized sulfur in 5 gallons should be around 5oz.. I use a Solo 433 gas powered backpack sprayer. While it’s a hard to spray an acre with this sprayer, it’s very effective. A tractor mounted sprayer would be best for 1+ acres.
The time difference between sprays should be calculated based on the Powdery Mildew Formula for grapevines, but generally 10-14 days between intervals. Maybe 7 days if the pressure is really high.
