Q&A: grapevine attacked by pests

Horticultural advice from the experts at the RHS
Question: I have been having some problems with a very old grape vine in my greenhouse. Until about four years ago it always produced a good crop of grapes and I had to do little in the way of looking after it. In recent years the grapes become very hard and produce a furry coat during their middle stage of growing.

At that time I noticed a little limpet-type bug attached to the branches. The bug was brown, slightly smaller than a ladybird and never moved. I knock them off whenever I see them and I also take the loose bits of bark off each winter, but so far I've had no success.

I also have a problem with lots of flies in my greenhouse that seem to live in the grape vine - I have four vents and double doors which are open all summer but I cannot get rid of them. I have the yellow fly traps up but am loathed to use fly spray. Can you suggest anything to help my grapevine?

Answer: Your vine has had a run of bad luck. They are such beautiful plants as well as (hopefully) productive, but in a greenhouse, once a pest or disease gets hold, it can be difficult to eradicate the perpetrator. The furry covering you describe is likely to be powdery mildew, a common fungal disease that you can control by avoiding dryness at the roots and thinning the fruit within the bunches.

Remove grapes when they are tiny and then snip out more when they have increased in size, so that up to half of the fruit is removed. Take out berries that crowd the centre of the bunch and then thin those on the outside. You can use fungicides to help control powdery mildew, such as Westland Plant Rescue Control concentrate or try organic controls such as Vitax 2 in 1 or SB Plant Invigorator. Keeping the vine well watered and the greenhouse well ventilated, as you do, will also help.

The immobile bumps on your vine are brown scale. The actual insect lurks underneath the protective scales and sucks sap from the plant. In June and July the nymphs are mobile and, being unprotected can be sprayed with one of the organic pesticides mentioned above, such as Vitax 2 in 1, SB Plant Invigorator or any product based on fatty acids (Insecticidal soaps).

Happily, the flies are not posing any threat to the health of the vine, although they are unpleasant. All you can do is keep using the yellow sticky traps, a UV fly trap or invest in some carnivorous plants.

Send your questions for the RHS to: gardenproblems@standard.co.uk.

Only a few questions per month can be answered. For further advice on handling problems in your garden, visit www.rhs.org.uk/advice/index.asp.

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