Q. The berries on my ‘Red Flame’ grape are shriveling and
turning gray but the vine looks healthy. Do you know what the problem is?
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| This is probably one of the bunch rot diseases, either powdery mildew or botrytis. Nothing can be done now. |
A. From the pictures you sent it’s one of the “bunch”
diseases. The bunch diseases attack only grape bunches. At the beginning of
summer is when we notice disease problems on the berries of grapes such as the
so-called “bunch rots”. But that’s not the time to treat. Grape fruit (the
berries) develop nicely up until that time and everything appears normal. But you
can’t see the disease yet.
This particular disease is
either botrytis or powdery mildew of grape bunches. The cause was a fungal
disease, and the grape vine needed a chemical spray for protecting the grapes
in about late March or early April. There’s nothing you can do now. Fungicide
sprays must be applied shortly before or right after the berries start
developing.
Fungicides
act differently than insecticides. Disease control using fungicides and
bactericides must be applied months before the disease is seen. For homeowners,
copper fungicides should be applied to these bunches (you must spray the entire
vine) to protect the bunches from this disease. The exact disease name is not
necessary but the fungicide is.
You can apply a fungicide to
your ‘Red Flame’ grapes now, but it won’t do you any good. The disease will
continue and more grapes will turn “gray”.. The infection is already “inside”
the berries but can’t be seen. The graying of the berries is a visual “sign”
the disease is present. Using chemicals to control diseases is usually a
“protective” spray. Controlling insects by applying insecticides are different.
Very seldom are insecticides applied as a “protectant”; a.k.a., applied before
we see the problem. Insecticides are applied usually as a “curative”; after we
see they are present.
How to know when to apply
fungicides? Commercial growers use the weather and understand that if they
don’t apply the right disease control there are risks that they could lose
their fruit. If the weather is right, fungicides and bactericides may be applied
to susceptible plants. Any time windy or wet weather precedes fruit development
or flowering, disease control (fungicides, bactericides, viricides) are applied
to highly susceptible plants.