Root Crop Pests
Unseen at first, soil-living pests and diseases seriously affect vegetable roots and tubers.
Root vegetable crops, including carrots, onions, parsnips, swedes, turnips, asparagus, Jerusalem artichokes, and especially potatoes, are susceptible to a number of pests and diseases, some quite serious. A couple of other problems — forking or `fanging’ of carrots and parsnips, for instance — are related to cultural defects such as poor soil preparation.
The edible portion of all root crops lies hidden under the ground throughout the crop’s life and so it is much more difficult to catch a pest or disease problem in its early stages than with leaf or pod vegetables. Keep a constant eye on the health of the foliage and stems of root crops, since they reflect the health of the roots.
Discoloration, wilting and general stunting or lack of vigour in the top growth, with no visible signs of a pest or disease, generally indicates a soil or root-borne problem, and this is the time to take remedial action.
Ideally, however, routine preventative treatment should be carried out in advance of any such signs of attack. Soils that are known to harbour pests should be cultivated regularly with a fork or hoe to bring them to the surface and expose them to predators such as birds.
Rubbish and dead growth lying in the vegetable plot harbour pests and diseases, so put them on the compost heap or burn them -always destroy obviously diseased growth. Also, eliminate all weeds in and around the vegetable plot, since many of these form secondary hosts for vegetable pests.
Maintain a three-year crop rotation programme to minimize the build-up of persistent soil-borne pests and diseases. Promote strong, quick growth by feeding and watering regularly — weak growth tends to be more susceptible to pest and disease attack.
Never allow waterlogging problems to persist in the vegetable garden, since these conditions encourage the fungal diseases that cause various root rots. Improve the overall drainage of the bed and incorporate plenty of organic matter — if appropriate for the crops to be grown — when preparing the soil.
Follow the manufacturer’s instructions when applying pesticides and fungicides, especially with regard to the safety interval between spraying and harvesting the crop. Spray crops on a calm, dull day when no rain is forecast, or in the evening, to prevent the chemical from drifting on to delicate plants or scorching foliage.
Other Problems Affecting Root Crops
Bacterial soft rot can occur in the garden or in store, and may be serious, especially after a wet season. It affects swedes, turnips, parsnips, onions and potatoes, and often follows damage to tissues by another pest or disease. Small, water-soaked lesions break out around a wound. These spread and the tubers, stems or leaves rot into an evil-smelling, slimy mass. Improve soil drainage, control other wound-forming pests and diseases, maintain a good crop rotation and don’t use too much manure. Keep lifted tubers and bulbs in a dry place. Don’t store affected crops.
Club root is a serious infection of turnips and swedes, striking during the growing season. Roots swell and distort, and the leaves turn yellow and sickly. Improve drainage, especially on acid soils; rotate the crops.
Apply a liberal dressing of hydrated lime and rake in 4% calomel dust before sowing.
Cutworms may attack the stems of young carrots, turnips, swedes and potatoes, especially on light soils during dry spells. Shoots are eaten through at ground level by fat caterpillars. Control weeds, which encourage cutworms, and protect susceptible plants by working a little diazinon + chlorpyrifos into the soil at the time of sowing.
Eelworms can be serious pests of onions and potatoes, as well as many other plants. The leaves, stems and bulbs of onions become bloated and distorted as a result of attacks by these minute wormlike creatures. The potato cyst eelworm causes pinhead-size yellow or brown cysts to grow on roots, resulting in wilting and death of plants. Where infestations are severe, do not plant the same crop on that site for several years. Dig up and burn badly affected plants. Chemical control of all types of eelworm is very difficult, and no safe and effective compounds are available for use by amateur gardeners. Some potato varieties are resistant to or tolerant of cyst eelworm attacks.
Fanging is a disorder of carrots and parsnips in which the edible root divides into two or more forks. It is caused by too much compost in the soil, or by stony or poorly prepared soil, not by a pest or disease. Use soil which has been manured for a previous season’s crop. Take care not to compress the seed bed too much.
Sclerotinia disease is a fungal disease that over winters in the soil. It attacks many root crops in store, especially carrots and parsnips, and is frequently restricted to the topmost part of the root. It consists of a white, fluffy mass containing hard, black structures of resting fungal growth. The roots soften and decay. Store sound roots only. Check stored roots regularly, and burn all affected material.