Erin Doman on October 7, 2015 24 Comments For winemakers, grapevine diseases can be devastating. Unfortunately, there are many kinds of vine diseases that thrive in all sorts of conditions. Bacteria and fungi cause the most common grapevine diseases. Insects can also spread disease and damage roots. Environmental conditions can trigger fungi development that wreaks havoc on grapevines in vineyards. Below, we have created a list of the 7 most common grapevine diseases that can spoil your favorite would-be wine. 1. Pierce’s Disease (Xylella fastidiosa) The Xylella fastidiosa is spread by the blue-green sharpshooter in California and by sap-feeding insects in Europe. Insects feed on a plant and transmit the bacteria inside the plant, where it travels through the xylem vessels and spreads the infection. Infected vines appear stressed, with leaves turning red or yellow in the summer, berries shriveling, and dead and dried leaves falling off the vine. There is no cure for Pierce’s Disease. However, current research involves using bacteriophages (viruses that kill bacteria) to stop and prevent the spread of Pierce’s Disease on wine grapes. More research is being conducted to learn precisely which bacteriophages can kill the toxic bacteria. A phage cocktail, consisting of four different phages, is used on plants that have already been infected and on plants that have not been infected. The result has been the same so far, that infection is prevented and the existing disease is controlled and not allowed to grow and spread. 2. Phylloxera (Daktulosphaira vitifoliae) Phylloxera are microscopic insects that suck sap and feed on the leaves and roots of grapevines. The roots become deformed and fungal infections occur there, which blocks the flow of nutrients and water to the vine. There is no cure and no chemical control or response available. However, some vine species, such as the American type, have evolved and have natural defenses, such as emanating a sticky sap that repels insects by clogging its mouth as they feed from the vine. Some preventative or curative solutions have involved grafting phylloxera-resistant roots with more susceptible vines. Another innovation being tested is training dogs to smell and detect pests and diseases in the fields. 3. Downy Mildew (Plasmopara viticola) This fungi vine disease can destroy grapes and vine vegetation. Between mid-May and late autumn, the aerial parts of the plant can become infected, particularly when the temperature is below 65°F. The symptoms are oil spots and a moldy cover or bright green spots found on the leaves. A white moldy cover is also found on the shoots and the inflorescences. The infection can also reach the berries, particularly the thriving ones, and cause them to dry out and look like raisins. They may also soften and turn a violet-brown color. Chemical applications, such as fungicides, are used to control the fungus. Also, measures are taken to eliminate humidity and moisture around impacted plants. Drip watering systems and selective pruning to improve air circulation help to do this. Enclosed environments, such as houses or greenhouses, aid in reducing humidity and thus reducing the chance of this mildew from harming the grapevines. 4. Powdery Mildew (Uncinula necator) All aerial parts of the plants can get infected by this disease. Development happens through the entire growing season until late fall. It favors dry years and climates with low humidity and little rainfall. Moldy coats on leaves, inflorescences, and rachis are telltale signs. Berries can become infected in the early stages of their development, with a gray coat covering them and then they dry out afterwards. Infected half-ripe berries usually crack and dry out. The plant’s canes can also become covered with a gray coating with gray-brown spots underneath. To control this pathogen, genetic resistance, chemical methods, and careful farming are used to manage conditions. Fungicides are used as well as silicon, which assists plant cells in fighting against the fungus and strengthening the epidermal cells of the plants. 5. Grey Mold (Botryotinia fuckelina) Fruit crops and aerial parts of vine organs are susceptible to grey mold or botrytis. This disease can strike at any time throughout the year, especially when the temperature ranges between 60 and 75°F. Rainy conditions and overly fertilized vineyards can cause the botrytis to appear. The first signs of the disease are spots on leaves that turn brown and begin to rot. The leaves then die, dry, and fall off the vine. Inflorescences can also rot and dry out. On berries, the most devastating effect of the disease is when the fruit becomes covered with the grey mold and begins decaying. Large-scale field monitoring has been used to test rain shelters to control this and other grape diseases. Rain shelters are more effective than leaving the plants out in an open field. 6. Black Rot (Guignardia bidwellii) All aerial parts of the plant can become infected with this disease. The fungus grows in warm and humid environments between mid-June and late August. The infection likelihood increases when the temperature ranges between 60 and 90°F with a rainy climate. There are first round brown spots on the leaves and the shoots, which die and become dried out. The fungus then reaches the berries while they are still developing. Reddish brown to gray spots appear and then they lose their moisture and shrink into raisins that are brown-black or black-blue in color and have black dots. 7. Vine Trunk Diseases Two grapevine diseases in this category, Petri disease and Esca (black measles), are caused by fungal pathogens. They tend to affect old vines that are at least 10 years old but young vines are not immune. Wood and leaves that have contracted these diseases develop unusual discoloration patterns of stripes or sports. The leaves, and then the stems, eventually shrivel in the middle of growing season and grapes end up falling to the ground. The vine then suffers sudden death, which could happen within days after the initial symptoms appear. The likelihood of these grapevine diseases occurring is highest in dry weather that follows after a wet season. This is a worldwide problem and research is still being conducted on how best to handle the issue. One theory is based on how vines grafted by machine as a cost-cutting and labor-saving measure has created weaker vines than hand-grafted vines. In the meantime, some vintners have tried new pruning techniques to control the fungi spread or splitting vine trunks in half to dry out the fungi and encouraging the sprouting of a new vine branch from below the join. Others inject disinfectants into the trunks. Vine diseases affect vineyards throughout the world. They have devastated plants and ruined crops, causing economical and land distress to wineries. Scientists and winemakers continue their ongoing research and experimentation to keep these diseases under control.