Page 36 - ALG Issue 2 2015

Regulars
Q
Q&A I have tried to select questions that may be
Mike Thurlow...
topical and cover most aspects of the vegetable garden/allotment. Allotment gardening covers a wide range of interests; if you want to include any other
areas of gardening please send your questions in.
Blackcurrants
QLate last autumn I was given a gift of two blackcurrant bushes in pots and purchased from a nursery. I planted them but didn’t cut back. Today when I cut back to half an inch from the ground, I
found I was cutting through a grub in the thicker stem of both plants and that the centre of these stems, and one other thinner stem, had gone black (the grubs didn’t crawl out; I picked them out with a pin).
Please could you advise what I should do?
Many thanks
Kathleen
ABlackcurrants have specific
pests and diseases. The
most common is the virus
‘Big Bud’ which is sometimes
called ‘Reversion’. The virus is
introduced by Cecidophyopsis
ribis, better known as the
blackcurrant gall mite. The
symptoms of an attack are
the reduction in the number
of flowers, a yellowing of the
young leaves combined with
a reduction in their size and
the eventual crop. It is called
reversion because in country lore
it was believed the blackcurrant bush was reverting back to nettles because they share a similar distinctive smell. Of course this isn’t true. The two plant families are not related in any way.
Two other pests of blackcurrants are aphids and midge leaf larvae. The midge leaf larvae are flies that develop from tiny orange white maggots that feed between the folds of the leaves on the shoot
tips. Part of their life cycle is spending the winter pupating in litter or debris on the soil. But they are quite small (only 3mm), one eighth of an inch. The females emerge in the springtime to fly to the top of the plants to lay their eggs.
Potato Rust
QI have recently obtained a new allotment and have been composting peelings, gone-off vegetables etc. Some of the potatoes I have been using show signs of rust.
Do I need to dig all this compost up again and dispose of it? This would be quite a big job.
Jean Ellison
AThank you for your question. Without wishing to cause any offence, are you certain that the potatoes are suffering from potato rust Puccinia pittieriana? I mention this because potato rust is unheard of in the U.K. However, I am prepared to concede that there is always a first time for everything.
Potato rust is confined to South America, reaching as far north as Mexico. Its movement is strictly monitored because it is classified as a quarantine organism for the U.K. and Europe. The only risk posed to crops is through the transport of infected plants or plant material, which is prohibited to prevent contaminated material entering the international market.
Do you think that it is possible that your potatoes could be infected with potato blight Phytophera infestans? If
this is the case, there is no need to worry about digging everything up. Sadly the potential for potato blight is always around. It is always worse in warm, wet summers when humidity is high. If we have a warm and dry summer it doesn’t become a problem.
The main methods of control for potato blight are spraying with an approved fungicide when the conditions for blight are suitable, but before any of the tell-tale dark spots on the leaves are visible. The organic solution is to monitor the weather and cut down all of the haulms to soil level. This denies the spores a breeding ground where they can be washed down into the soil and infect the developing tubers. Although it appears to be a drastic solution it doesn’t reduce the eventual weight of the final harvest too much.
Another weapon in the armoury of the gardener is to grow the modern ‘blight resistant’ cultivars that have been bred. Another thing that you could do is to ask around
the other plotholders to find out which cultivars they grow successfully.
I hope that this reply has been useful. If you want to discuss the problem further or require more information please don’t hesitate to contact me.
Don’t worry about the central brown staining on the older wood for this season. Feed the plants this month with a general fertiliser or mulch them with 75mm/3inch layer of manure/garden compost. If the plants grow looking strong and healthy the larvae you have found are a one off occurrence. I would spray or water regularly with liquid seaweed and inspect the plants regularly for pests. You can control these with an insecticidal soap or use an approved insecticide if you want to.
I hope that this helps you realise what you have discovered is a secondary problem and once the bushes are established under your care they will flourish. If you want to discuss the matter more please don’t hesitate to contact me.
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