grapes dropping off before ready

Asked August 8, 2018, 12:17 PM EDT

I believe my vine is a concord grape. Was here when bought house 50 yrs ago. Noticed 3 yrs ago all were gone in August so I have covered it the last 2 yrs so birds could not get at them. Looked like a good crop this year and I had to pick them today because I had already lost most of the grapes. Have had a problem with japanese beetles. The vine has full sun all day. I don't especially water it and never have fertilized it. Has always done fine until last few years.

Grundy County Iowa

1 Response

I am sorry it took so long to get an answer. I see your question has been sitting for almost a month but I just got it today.
It sounds like phomopsis to me. This disease overwinters on infections to this years shoots and then rains down spores from these lesions. The spores are spread by rain splash to new green tissue. You will see black spots and puckering on the leaves and black sunken lesions on the canes and on the fruit rachis in the cluster.
These lesions will girdle the stems of the fruit and the fruit will fall off. Here is a link to a short article on the disease.
Phomopsis is a very frustrating disease because the control windows are early in the season when the flower clusters are first exposed and actively growing and during bloom and soon after. The girdling of the fruit and fruit falling off are symptoms that you see 3 to 4 weeks after you should after sprayed when there is nothing you can do about.
I attach some pictures of symptoms of leaf and shoot lesions on Concord.

Too control the disease you need to do a good job pruning to remove as much of the lesions on old canes as possible. The first leaf and shoot symptoms usually appear below the older wood close to the trunk or canes. The common fungicide captan will give good control if applied at the right time. You need to protect young tissues as they are actively growing. It is best to apply captan before a rain as it works by killing the spores as they germinate and has no effect on established infections. The first critical control window is early in the season as the shoots begin to grow and the flower clusters are first exposed. We recommend a spray when the shoots are 3 to 5 inches long. If there are long rain period you should apply sprays again on a 7 day schedule through bloom. If it does not rain you can stretch the interval but you should put on a spray just before or at the beginning of bloom and another about 10 days later. If it is really rainy then y0u should continue to apply sprays for as long as 4 weeks on a 7 to 10 day schedule depending on the rain. The later post bloom sprays are really to protect the fruit.
We find the per bloom and bloom sprays are most important to preventing the preharvest fruit drop.
I hope this helps and I again apologize that your question got shuffled off and forgotten.
On the attached picture of the diseased leaf, the large brown spot with small black specks in it is black rot.
The small yellow spots with black centers are phomopsis and when it is severe when will cause the leaf to be distorted.
On the one of the girdled fruit you can see the rachis is dead and fruit is getting no nurishment from the vine and the fruit is barely attched.