Page 14 - ALG Issue 1 2016 No social share

Regulars Q
ike Thurlow... &MA
Potato problems
I have tried to select questions that may be
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.
Chemical solution for onion white rot?
ASclerotium cepivorum or onion white rot, as it is more commonly known, poses the most serious disease threat to all members
of the Allium onion family. But principally it is onions and leeks that are at most risk. It is a soil borne fungus that can survive for up
to 20 years in the soil as a small black dormant structure about 2 mm diameter called sclerotia. Cold, wet soils provide the sclerotia with the perfect conditions to attack allium crops. They are most active when the soil temperature is between 10C/50F-24C/75F.The dormant spores are activated by the scent from volatile oils released by developing plants. This triggers them to attack the roots and eventually enter the base of the plants.
The symptoms of an attack are usually rst seen from mid-summer to early autumn. These are the yellowing of healthy leaves, followed by the plants wilting in dry weather. Also, under wet conditions,
they can be found to be loose in the soil. After a while the mycelium appears as a white uffy growth developing around the base of plants combined with the appearance of small black dots covering the bulbs.
Sadly there is no chemical cure available to growers in the UK. This is because the fungus lives in the soil so it doesn’t need a host plant to survive. Crop rotations combined with strict plant hygiene may prevent the spread but will not cure an outbreak. Do not put any diseased material into the compost heap. Raised beds at least 45cms/18” deep lled with clean soil or compost will allow you to grow onions and leeks but make sure that there is a membrane between the clean soil and the diseased soil.
The most effective way to control the sclerotia is not to grow members of the allium family on known infected sites. Avoid growing overwintering allium crops especially in vulnerable areas. One method that may work is to water the site when the soil temperature is around 16C/60F with a garlic compound to trick the spores
into becoming active. They will ourish for a few weeks before eventually dying. One glimmer of hope is a cultivar of onion that has demonstrated some resistance to white rot and is called ‘Golden Bear’. Avoid purchasing cheap non heat treated sets. Only buy garlic bulbs from growers you know are free from the disease and avoid overwatering.
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Wireworms are the larvae of the click beetle. They have shiny
QNSALG have advised me to ask your advice about possible remedies for onion white rot. I have had this
in my garden for some time and have taken the suggested measures, getting rid of infected plants, trying not to move soil around and rotating my crops. I wanted to ask if there is anything further I can do in the absence, as it appears, of any chemical remedies? Phil Brooks
QPlease can you give us any clues why virtually our whole crop of potatoes has been damaged by worms this year, and how we can avoid this problem next year? John Worsley
Thank you for your question. It is probably wireworm damage.
hard orange/light brown bodies about 3/4 - 1 inch/2-2.5cms long. They will feed on many plants with eshy root systems. With potatoes, they tunnel into the tuber producing 3mm-1/8 inch diameter holes.
The adults overwinter in the soil and emerge between April and August, but the main period of activity is May. The female lays her eggs just below the soil surface on grassy or weedy soil. The foliage of the potato plants provide plenty of shelter for their egg laying activity. The problem is always worse on the later lifted tubers. If it
is possible to grow the earlier maturing cultivars they can be lifted before the main period of larvae feeding.
There are no resistant cultivars or chemical controls available
to the amateur gardener. As already mentioned, wireworms are usually found in grassy, weedy or undisturbed soil. If the soil can be turned over just before planting time, it helps to bring any larvae to the surface where they can either be left for the birds to feed on, or collected up to put on the bird table. An infestation of wireworms will remain in the soil for two to three years. Crop rotation and regular cultivation of the soil will help to eliminate them. The damage is always worse from mid-August and the tunnels make life easier for small keel slugs to enter the tuber, creating a secondary problem.
King Edward, Nadine and Maris Piper have been found to grow successfully where wireworm is present.