Pests and Diseases Affecting Leeks
Leeks are relatively pest and disease free, as long as they are part of a crop rotation plan and are not grown on the same ground several years running.
Onion fly: Leeks are occasionally attacked by this pest, which is particularly troublesome in the larval stage, when the maggots tunnel into the plant tissue. The fly, which looks like a small grey house fly, lays its eggs in spring and early summer, near the base of the leek, on the leaves and in thenearby. Scattering diazinon granules in late spring will help to control the emerging larva. Like , young seem particularly vulnerable. Lift and destroy and infested leeks; the main symptoms are yellowing and drooping foliage.
When lifting the infested leeks, make sure there are no maggots left lying on the ground. Because the flies are attracted to the smell of fresh manure, make sure leeks are not planted on freshly manured ground. Leek moth: this small brown moth lays its eggs on the base of leek plants during mid and late spring. The emerging pale green caterpillars tunnel through the leaves, which are disfigured with white streaks. As more and more tunnels are made, the leeks loose their strength and may collapse and die.
The best method of control is a nicotine wash, applied first when the plants are still in the seed-bed, and again in mid to late summer.
White rot: this fungal infection attacks onions,and as well as leeks, and is most noticeable in hot, dry summers. Plants grown in overcrowded conditions are particularly vulnerable. Infected leeks will have yellow leaves, with a white or grey fungus covering the underground base of the plant, looking like strands of white thread. The spores can remain viable in the soil for at least eight years, so proper crop rotation is the best precaution against white rot. Dusting the soil at sowing time with calomel dust gives some additional measure of protection, and leeks grown on fertile, organically manured soil seem less vulnerable.
White tip: this is another fungal infection, which usually appears from late summer through the winter. Symptoms are a white die back of the leaf tips, white patches appearing on other parts of the leaf and stem, and stunted growth. Infected leaves look watery, thin and papery; eventually they rot away. Besides looking unpleasant, infected leeks wilt quickly and will not keep at all. Badly infected leeks should be removed and destroyed, and the remainder sprayed fortnightly with a copper fungicide until all white patches disappear.
Leek rust: this fungal infection is more prevalent when the autumn and winter are unusually mild. Leeks growing in soil which is rich in nitrogen but lacks potassium are particularly vulnerable. The symptoms of leek rust are orange, powdery spots on older leaves, which later turn yellow. Although they are disfigured, leeks with mild rust infections can still be eaten. Badly infected plants should be dug up and destroyed. To control the spread of the disease, spray the remainder of the leeks with zineb.