I need help in treating my grapevines

Asked October 11, 2017, 9:30 PM EDT

We have had a Concord grapevine for many years and have enjoyed the grapes. In 2016 and again in 2017 the vines were loaded with new grapes. However, both years, the grapes grew to middle size and then started dropping off the vines. By the time we would usually have been picking grapes, they were all on the ground, though they hadn't ripened. A guy with quite a bit of plant knowledge looked at them and told us the vines have a fungus and that we should trim them way back and treat them with something. What do you think? If we should treat them, what should we use and when should we use it?
Thanks for your help!

Newaygo County Michigan

9 Responses

Hi Becky,
It is impossible to guess which of the usual fungal diseases might be at work, causing the berries to drop. The ones that are most common on concord grapes would be black rot, anthracnose, downy mildew and powdery mildew. The best fungicides and proper application timing would depend on which of these diseases is present. Could you provide me with any photographs of the fruit, both the fallen berries and berries that are still on the clusters? yous may send them directly to elsner@msu.edu

I don't have any pictures of the grapes. I can tell you that within any one cluster of grapes the grapes were at different stages of ripeness, with some green and others purple or in between, some smaller than others. All of the fruit is on the ground and dried up by now. Do you want a picture of that?

A picture of that might still help.

It is impossible to say for sure, but the fruit in the images looks like what happens when black rot infects the berries. Black rot starts as leaf infections early in the growing season. Spores are released from the leaf infections, an these can cause the fruit infections. See https://grapesandwine.cals.cornell.edu/newsletters/appellation-cornell/2014-newsletters/issue-17/managing-black-rot

and http://msue.anr.msu.edu/news/banning_black_rot_and_phomopsis_from_young_grape_clusters

Thank you. I will try a spray in the spring. Is Phomopsis the same thing as black rot? There were many sprays listed in the article. It sounds like Rally and Elite are costly, with generic options, with Orious and Tebuzol generic options that might do as well. Rebus Top and Inspire Super also sounded good. Are Luna Experience and Quadris Top equally good? And Abound, ProPhyt and Phostrol? I'm not sure which one I should try for. And where do I go to buy these sprays? Should I trim the vines way back? If so, when? This fall? or in the spring?

Phomopsis and black rot are completely different. Finding the spray materials in the articles in small, affordable quantities will be difficult. I apologize for not reading through the articles completely before sending you the links-- I should have noticed that small-scale/homeowner pesticide options were not there. I would suggest a fungicide called Captan. Check with garden centers and hardware stores to see if they have it or a close substitute. It us useful against many of the grape diseases. Getting sprays on at the correct timing is very important.
As for pruning, it can be done any time the vines are dormant, but it still works if you wait all the way to bud swell next year. Grapes need to be pruned pretty hard to maintain the desired growth and productivity, but remember that the vast majority of the fruit is produced by buds on one year old canes. Be sure to leave some canes that grew out in 2017 when pruning the vines.

This helps my understanding a lot. Thank you!