I have a 12 year old grape vine that shades the patio in my back yard. It has produced fruit for 11 years and then last year the grapes all turned black and fell off. I can see that it’s starting to happen again this year. Should I be pruning the vine? I haven’t been doing anything to this point.
Sounds like the grape vine on your arbour is infected with Grape Black Rot caused by the fungus Guignardia bidwelli. Grape Black Rot affects both cultivated and wild grapes. The leaves are more resistant – only the young, rapidly growing leaves are affected. As you’ve observed, the most damaging effect is to the fruit. The infection starts when the fruit is half to almost full size. A small spot appears and within a few days the entire grape becomes black, hard and mummified. The Missouri Botanical Garden has some good information on grape black rot including pictures of infected leaves and fruit. Click on the link below to check out the pictures to see if they match what you are seeing on your vines.
Warm, wet weather in the spring and summer promotes the disease. The mummified berries on the ground from last year or those still clinging to the vine are the major source of infection. When the weather is wet, the infection will continue for the entire spring and summer. Cool weather slows the growth of the fungus.
On a more positive note, Grape Black Rot can be controlled. Remove all mummified grapes and rake up any fallen leaves in the fall. Dispose of any infected material by sending it out with the garbage. You mentioned that your grape is 12 years old and you haven’t done any pruning. The vines need good air circulation and full sun exposure to reduce the relative humidity and ensure rapid drying of the vines. So pruning will certainly help. At this point, you can prune out any dead material to open up the canopy and then prune every spring. The spring pruning is particularly important to remove fungal spores overwintering on the vines.
There are fungicides available but they are only useful if applied repeatedly and may be a challenge to apply to grapes on an overhead arbour . I think the methods mentioned should bring the problem under control.