Why are my Black Grapes Splitting?

Q.I have a old vine in my greenhouse and have obtained good black grapes usually but in the last couple of years the grapes have been covered with a white powder?

This year no powder but all the grapes have split open?

Can you give reason for both problems and any known cure?

(Mr Roy Williams, 8 September 2008)

A.It’s always difficult to diagnose at a distance, without seeing things for yourself – and it can often be a bit tricky even then – but from what you describe, it sounds like a case of powdery mildew. It’s one of the three most common vine problems, the other two being downy mildew and Botrytis.

Powdery Mildew

You don’t specifically say if the powder is found on the leaves too, but powdery mildew usually starts there, eventually spreading to both sides and in bad infestations, the leaves often wilt and die. The splitting of the grapes is another symptom seen in untreated cases of this disease. In fact mildew can attack any above-ground part of the plant, even including flowers sometimes – and it’s not restricted to vines; it can also affect the likes of apple trees, roses, Boston Ivy and Virginia Creeper too.

As to why things have been a little different this year – no powder, but split fruit – it’s hard to say. It might be something as simple as how easy it was to see – in the early stages, infestations are easiest to spot in direct sunlight. As the mildew grows through the season, it changes from its original whitish colour, darkening and turning reddish or brownish, sometimes with mottled dark or black spots, which might explain why it didn’t seem to be present on the grapes this year.

You say the vine is old; like us, as plants age, their natural resistance to disease can become weaker, making them more susceptible to infections which they have shrugged off in their “youth.” It’s possible after years of resisting mildew, your vine has become less immune to the problem simply because of its advancing age.

Prevention and Cure

Once the disease has really taken hold, it’s very difficult to cure in the season, so it pays to be vigilant. Any infected material should be pruned out and burnt and the affected plant dusted with sulphur fortnightly to help stop re-infestation.

A number of measures can help prevent the problem developing. Don’t allow your vine to dry out at the roots – and many gardeners have found that watering during the morning is best, since wet foliage at night seems to encourage mildew growth. It’s also worth mulching the ground to maintain the soil water and ensuring that there’s a good air flow around the plant.

Remove any reservoir of infection by burning diseased leaves and be brutal in your autumn pruning – burning these too. If you can get on top of the disease and the conditions that favour it, your vine could be much happier next year – unless of course it really is just a case of old age.

Now, if I only knew the answer to that one!

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