Common Leeks pests

Onion thrips are the most frequent insect pest; these tiny slender insects rasp the leaves and leave silvery or whitish streaks and stippling, usually worst in hot, dry weather. Allium leafminer, an invasive fly now established in the mid-Atlantic and Northeast, leaves a telltale row of white dots near the leaf tip where females feed, and the maggots then mine down into the stem. Leek moth caterpillars tunnel inside leaves and bore into the shaft, leaving windowpane feeding scars and frass. Onion maggots (the larvae of a small fly) attack roots and the base, causing wilting and rot, and cutworms can sever young transplants at the soil line.

Common diseases

Leek rust is the signature disease, showing up as bright orange to rust-colored powdery pustules scattered along the leaves, and heavy infection weakens and yellows the plant. Downy mildew appears as pale green to yellow elongated patches with a fuzzy grayish-purple mold in damp weather. Purple blotch (Alternaria) causes sunken purplish-brown lesions with concentric rings, often on older leaves. White rot is the most serious soilborne problem: leaves yellow and wilt, and the base develops a fluffy white mold studded with tiny black resting bodies that persist in soil for years.

Prevention

Rotate alliums so they do not return to the same bed for at least three to four years, since rust and white rot build up in the soil. Space plants generously and keep beds weeded so air moves freely and leaves dry quickly, which discourages rust and downy mildew. Water at the base rather than overhead, buy healthy transplants or clean seed, choose rust-tolerant varieties where offered, and remove and destroy (do not compost) infected leaves and end-of-season debris.

Organic control

For thrips, a strong spray of water and insecticidal soap or neem, applied in the cool of the day and repeated as needed, keeps numbers down; always read and follow the product label. Floating row covers set over the bed at planting and sealed at the edges are the best defense against allium leafminer, leek moth, and onion maggot, since they block the adults from laying eggs during their flights. Hand-pick and crush leek moth caterpillars, and a Bt product labeled for caterpillars can help against them. Encourage beneficial insects such as lacewings, minute pirate bugs, and hoverflies that eat thrips, and for rust or downy mildew a copper or sulfur fungicide labeled for alliums, used according to its label, can slow spread when applied early.

Scouting & monitoring

Scout weekly from spring onward, checking the newest inner leaves for silvery thrips stippling and the leaf tips for the white dotted feeding rows of allium leafminer. Watch for the first orange rust pustules in humid stretches, since early leaf removal keeps it from exploding. Time row covers to the known flight periods of allium leafminer (spring and again in fall) and leek moth, and pull an occasional plant late in the season to check the base for the white mold and black sclerotia of white rot.

Tips

Prevention beats cure with alliums: rotate beds, give plants room to breathe, and use row covers during pest flights to avoid most problems without sprays. Remove rust-spotted leaves promptly and never compost diseased allium material, since rust spores and white rot survive to reinfect next year.

General home-garden IPM guidance, focused on prevention and least-toxic control. Always read and follow the product label for any pesticide, including organic products. Page updated Aug 4, 2026.