Grapes are excellent sources of water and antioxidants. The vines upon which grapes grow, however, are prone to numerous diseases. Those who experience problems with their grape plants should familiarise themselves with the most common grape plant diseases and treatments in order to keep the plants healthy and encourage maximum grape yield.
Some of the diseases that are seen most often include downy mildew (Plasmopara viticola), powdery mildew (Uncinula necator), bunch rot (Botrytis cinerea), black rot (Guignardia bidwellii) and eutypa dieback (a combination of Eutypa armeniacae and Phomopsis viticola).
Most grape vine diseases are the result of moulds or fungi. They appear when mould and fungi spores are given prime growth conditions of warm temperatures and high moisture. The spores are transported by both wind and splashing drops of rain. You should not assume that your grape vines are free from disease if those conditions have not been present in recent months or if the disease does not appear for a year or twosome grape vine diseases like downy mildew can be dormant on grape plants for up to 5 years.
Symptoms of grape vine disease vary depending on the disease that is affecting the vines. Downy mildew is characterised by "oil spots" on the leaves, while powdery mildew is characterised by a powdery growth of mildew on both sides of the leaves, which tend to shrivel. Both types of mildew affect the shoots and bunches. Other diseases, like rot, can cause the grapes to dehydrate and shrivel (in some instances, this actually can be a good thing because the sugar content in the grape increases for sweeter fruit and wines). Dieback can cause stunting, yellowing and curling of the leaves as well as smaller shoots.
Applications of fungicides are the standard treatment for grape vine diseases. Depending on the specific disease, copper oxychloride, phosphorus acid, wettable sulphur and chlorothalonil are some good choices. Some types of disease are resistant to sprays, so those diseases need to be treated in a preventive rather than reactionary way. If you do not know what type of disease is infecting your grape vines, you can have cultures and other tests done in a laboratory.
The primary effect of grape vine diseases is lower crop yield. In some cases, even a small percentage of infection will result in rejection of crops from wineries, because the diseases change the flavour of the wine. Additionally, because some diseases can be dormant for a number of years, vineyard owners can be financially devastated by an unexpected infestation when yield is predicted to be high.