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Written by davethetreecenters • January 30 Pest & Disease Control on Grape Vines in Home Gardens

Few things are as satisfying as harvesting your own vegetables and fruits. Vegetable growing is generally easy, but many gardeners want to ‘upgrade’ to fruits. While some, like strawberries, are pretty easy, things get more complex with tree fruits, and success takes a little effort and learning – making it all the more satisfying when you succeed. Perhaps the ultimate fruit crop is grapes, for eating fresh, making juices and jellies, and even for home winemaking. The plants themselves are quite easy to grow, but if you Google “pests and diseases of grape vines”, the long list is likely to turn even the most optimistic gardener away from the idea. So lets take a look at some of the most important potential problems, and see how, in a home garden, you can deal with them, and succeed.

Learning to Live with Imperfection

There has been an enormous in attitudes of gardeners, since the middle-years of last century. There was a time when ‘perfect’ plants and flowers were the goal, and if that meant extensive use of chemical sprays, then that is what was done. This approach didn’t take into account the resilience of nature, and pests quickly evolved new defenses against one chemical after another. As well, the natural ‘balance’ that existed was disrupted, with natural predators being killed too, leading to bigger outbreaks.

Since then things have gone in several complementary directions. Many of those old chemicals are no longer available for home gardening, or people are no longer willing to use them. Plant breeding has raised resistance to disease to a priority, and many plants, including grape vines, have degrees of resistance – the problem is solved if the plant isn’t affected by the disease anymore. An emphasis on preserving predators has help to keep pest outbreaks small.

Most of all, though, gardeners have learned to relax a little, and accept that plants with a little damage are just fine; weeds in a lawn are OK; some damaged leaves and a few flowers lost is not a problem; vegetables that are a bit deformed, or need trimming before eating, are still great; and fruit that isn’t perfect is still wonderful.

So it’s important to learn to accept that your plants might not always look picture-perfect, and, returning to grape vines, some of the bunches you harvest might contain a few moldy or buggy grapes, but that doesn’t mean you haven’t succeeded in producing some great food from your garden. Once you can do this, then problems become more controllable, and if you get 10 pounds of grapes, instead of the potential 15 pounds, you have still succeeded.

Important Grape Vine Diseases

Botrytis Bunch Rot

So you did everything right, and your vines are filled with growing bunches. Then you start to see this – and your crop begins to rot away. This is botrytis, a common fungal disease. It is especially common during periods of high humidity and temperatures in the 60sF. It can quickly damage a crop just when it is beginning to ripen.

Humidity can be in the air, but it will also be higher is vines are watered overhead, or if a dense canopy of leaves keeps things damp around the fruit. Periods of rainy weather will make it worse too, so it can vary from year to year.

Botrytis rot on a ripe bunch of grapes

Your first line of defense is to reduce the canopy, firstly by not encouraging lush growth with lots of rich compost or fertilizers. If growth is adequate, avoid overfeeding. Secondly, remove the leaves and shoots directly before and after each bunch of grapes. Not only does this let more light in, helping to sweeten and ripen your crop, but the improved air circulation will keep the fruit drier, and safer.

Next, break the chain of the disease, which overwinters on left-over bunches of fruit, leaves and debris. Make sure your fall clean-up if thorough, removing shriveled bunches, dead leaves and debris, and remove it completely, or burn it, rather than leaving it piled up nearby.

Finally, there is a safe, organically-approved method of control for botrytis, and some other diseases, called lime-sulfur spray. We’ll talk about it further down.

Powdery Mildew

Perhaps the most damaging disease of grape vines, powdery mildew is a common problem with many garden plants, but doesn’t damage flowers the way it does grape vines. Fortunately grapes with American heritage, like ‘Concord’, are less susceptible, and it is possible to find varieties with good resistance to this disease – always your first line of defense. This disease causes dry whitish powdery on the leaf surfaces, but more importantly can cause a stunting of the developing grapes, so that they never sweeten properly, remaining small and sour.

Powdery mildew begins to develop and spread when the temperatures are between 70° and 85°F, and is inhibited at both lower and higher temperatures, especially periods over 95o. So the severity will be affected by where you grow, and seasonal differences. The coating on grapes shouldn’t be confused with the natural ‘bloom’ that most grapes develop.

Powdery Mildew (left); Natural ‘Bloom’ (right)

Lime-Sulphur spray, in spring when buds are swelling, is one of the traditional organic treatments for powdery mildew (and helps with botrytis too). The stems and bark are sprayed thoroughly, but there is evidence that this can also reduce beneficial predators, by killing them while they are overwintering in the bark. This is also a treatment for less-common diseases like anthracnose and black rot, and if you have had issues with these diseases it is valuable. Since powdery mildew multiplies during summer, it is possible this spray gives only limited control.

Another useful and perhaps surprising treatment during the season is watery milk. Simply dilute milk 1 part to 9 parts water, and spray every two weeks, on leaves and developing fruit. For home gardens this is probably the best – and certainly the simplest – control. It doesn’t taint the fruit in any way.

Pests of Grape Vines

Leafhoppers

This pest can reach large numbers on vines, with young stages feeding on leaves, and weakening plants – although it isn’t going to threaten your crop directly. It is often controlled by natural predators, and keeping rough patches of grass and ‘weeds’ near your vines is a helpful natural refuge for them. Yellow sticky traps are used to monitor numbers, and trap adults. If you use the best organic treatment option – pyrethroid sprays – then once you are catching 10 leafhoppers over a few days of trapping it is time to spray.

Leaf hoppers in their juvenile stage

Thrips

These are extremely small pests, that cause ‘silvering’ of leaves  when they are in large numbers, and can also damage developing fruit clusters. The feed by sucking sap from the undersides of the leaves. Spraying with water of the foliage and fruit after flowering is helpful, and usually biological control from natural predators helps keep numbers down. You might not get a photo-perfect bunch of grapes, but they will taste fine.

In Conclusion. . .

In general, pests can cause quite a lot of visual damage without causing major problems to your harvest. There are a whole lot more pest and disease possibilities for your vines, but in most years it isn’t so hard, with balanced growing, keeping some wild areas nearby, and acceptance of ‘less than perfect’ results, to produce a satisfactory and satisfying crop of grapes without resorting to harsh chemicals and major toil on your part. Good harvest!