Pests & Diseases Affecting Sweetcorn
In tropical and sub-tropical countriesis troubled by a variety of , from rusts and smuts to weevils and ear-worms, but gardeners in temperate climates will find this crop relatively pest and disease free. While the plants are still at the seedling stage, mice, rabbits and birds can be troublesome, and in severe cases, some sort of protective netting may be necessary; once the plants are established and growing strongly, these pests will do little damage.
Frit fly: this is the most serious pest you are likely to encounter. The maggots of this insect bore into the growing point of the young corn, leading to stunted plants with a blind main shoot; mid- to late spring is the time of year they are most destructive. Leaves of infested plants are withered and brown at the tip; irregular holes may appear in the leaf tissue. The best precautionary measure is to spray the newly transplanted sweet corn with gamma-HCH. In the case of direct-sown plants, wait until they are about 15 cm (6”) high, and have two or three leaves.
Botrytis cinerea (grey mould): this is a fungus, the spores of which are always present in the atmosphere; in cold, wet weather, botrytis can be a very serious problem. This is partly because a wide range of fruits and vegetables are vulnerable.
The young, delicate tassels of corn are particularly vulnerable, and can quickly become covered in fluffy, grey growth. Although there is nothing the home grower can do about cold, wet summers, a spray of benomyl is a good preventive measure. Should an infection occur, remove and destroy any badly infected plants, as they are unlikely to recover. Respray the remaining plants with benomyl.
Fusarium rot: this is a-borne fungal infection which thrives in cool soil conditions. If the soil is very wet, so that the sweet corn has difficulty in growing, improve it before planting, and hoe to improve aeration while growing. Applying quintozene to the soil before sowing, if trouble occurred the previous year, is another good preventive measure. Infected plants may wilt and collapse for no apparent reason; when dug up the roots will be brown, stunted, and generally unhealthy looking. If the plant manages to grow to maturity, and produce cobs, these may show secondary symptoms of fusarium rot: misshapen, discoloured kernals, and imperfectly developed silk ends It is best to dig up and burn badly infected plants, and do not replant the site with sweet corn for several years.
Thrips: these are tiny, elongated yellow, dark brown or black winged insects which are very damaging to many garden plants, such as, and ; thrips are sometimes called ‘Thunder flies’ or ‘Thunder bugs’. Although not a serious problem with sweet corn, occasional attacks do occur, particularly in hot, close weather. Thrips damage the leaves by piercing the leaf tissue and sucking out the sap. Infested leaves will have speckling and pale mottling and infested growing points will be checked, resulting in stunting. Small black spots on the undersides of the leaves are excreted by the larva; these minute spots are a sure sign of thrips. Spraying with derris, resmethrin or malathion as soon as the thrips appear is the best method of control, if it is needed, but thrips have a very short life.