better pictures of diseased grapes. please help

lycheeluva(6/7)July 10, 2008

Anyone know what disease is killing all my grapes. it would appear to me that it is a fungus or bacteria because of the way it is spreading to every bunch of grapes. its not sunburn because it is effecting the bunches that are covered by leaves.

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jellyman(6/7VA)

Lychee:

Looks like ordinary brown rot to me, common in the humid conditions of eastern climates. That's the main reason I do not grow grapes, except for one vigorous Concord vine.

Even with the Concord, I can get over 90% losses in a humid summer like this one unless I clip off all the leaves above the clusters and allow the sun to strike them directly. This reduces my losses to about 20-30%, which leaves me enough for a lot of grape jelly and juice. If the clusters do not receive good air circulation and dry out very quickly after a rain shower, it's curtains for them.

I suppose you could try applying a fungicide, but that is not a very attractive prospect on fruit that is not far from harvest, and is difficult to wash effectively. I'm willing to use fungicides on many crops, but not on grape clusters.

Don Yellman, Great Falls, VA

    Bookmark   July 10, 2008 at 10:36PM
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lycheeluva(6/7)

Thanks Don:

What fungacide should I use?

    Bookmark   July 10, 2008 at 10:53PM
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kiwinut

Looking at the better pictures now, I'm pretty sure that is the beginning of black rot. The berries will soon start to shrivel up, forming mummies. Treatment with fungicides, unfortunately, needs to be done earlier, before symptoms appear. After infection is visible, it is nearly impossible to control. It is very important to remove the infected berries and particularly the mummies, and eliminate them from the area. The mummies overwinter, and then release spores when splashed by rain the following spring.

See the link below for descriptions and photos of various grape diseases and treatment strategies.

Here is a link that might be useful: Grape diseases

    Bookmark   July 11, 2008 at 11:35AM
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Scott F Smith(6B/7A MD)

LL, I have never heard of brown rot on grapes, it is usually black rot. There are many fungicides that work on it. I use copper with yucca as a sticker. You should look up pictures of black rot to make sure that is what you have; each different grape disease has different sprays that work best. Black rot shows up on the leaves as darkened spots. It looks somewhat different on different varieties and in different weather conditions. The most important time to spray is just before and just after bloom. I covered that really well so I have little rot on my grapes, but I got lazy and didn't spray enough in June and so I now have some black rot on the leaves.

Scott

Here is a link that might be useful: Black rot images courtesy Google

    Bookmark   July 11, 2008 at 11:44AM
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jellyman(6/7VA)

Brown rot, black rot, it's all the same to me from a practical point of view, though I will not dispute there may be different fungal organisms at work. Rot is rot.

Agree fully that control of fungal diseases requires prevention, since cure is far more difficult. But I have had pretty good success with clipping the leaves above the clusters to allow in air and sunlight, even when the black rot is already underway. I don't try to remove all the dried up grapes, since that would be an impossible task on my large Concord vine. I just separate the good grapes from the raisins at picking time, and I do not bother to spray the Concord at all. Perhaps if I did, I would be able to stop the rot before it starts.

Lychee's table type grapes may be even more susceptible to these diseases than the tough old Concord.

Don Yellman, Great Falls, VA

    Bookmark   July 11, 2008 at 1:11PM
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billv(z6 WVA)

I call it black rot as well. There are several fungicides (e.g., Mancozeb)said to "control" but all require 1st application very early (1/4" leaf?) and every 10-d to two weeks after until the withdrawal which is quite long because it has not been properly determined. In other words, I think treating now would be at least a waste of time and money. Jellyman's suggestion of leaf clipping seems about the best course at this point.

    Bookmark   July 11, 2008 at 1:14PM
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lycheeluva(6/7)

thanks for the replies. i'll have to chalk it it down to yet another in the seemingly endless gardening "live and learn' lessons

    Bookmark   July 11, 2008 at 3:19PM
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kiwinut

Concord is an absolute black rot magnet. Not many grapes around that are more susceptible.

Once the fruit builds up some sugar, it is immune to further infection. This usually happens by 3-4 weeks after bloom, so after that period, any fruit that is going to rot, is already infected and there is nothing that can be done. Spraying around bloom up until 3-4 weeks is needed to control fruit infection, but the leaves and shoots can get secondary infections all summer. A better solution is to plant resistant varieties, since black rot is present all over the eastern US. Alwood is a good alternative to Concord, and is claimed to have very good resistance to all the major fungal diseases.

    Bookmark   July 11, 2008 at 5:04PM
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calliope(6)

You often see black rot in old stands of grapes who have not been kept pruned properly or at all. I agree that good ventilation is one of the best ways to keep it under control. when I harvest my grapes, I use snips and take the whole cluster off, and that brings any mummies with it. I also keep the vines well weeded, and keep any residue from gathering under them and overwintering. I've never had enough pressure from it to spray to control it, thank God.

    Bookmark   July 12, 2008 at 2:01AM
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lehua49

Hi Lychee,

Looking at the link to grape diseases I would put my money on it being anthracnose. It looks the most similiar to your picture IMHO. It makes a difference in what fungus you are treating and your grapes sensitivity. Take it to you local ag extension service and get their opinion. Saves on the trial and error process and money too. Good luck and aloha.

    Bookmark   July 12, 2008 at 3:47PM
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