Chives

Stages affected: Vegetative · Cut and come again

Tiny slender insects, barely visible, that rasp the leaf surface of onions, leeks and garlic and suck the sap, leaving silvery streaks that turn the leaf tips brown. They explode in hot dry weather, five to eight generations a season, and a heavy infestation gives small distorted bulbs. They also spread iris yellow spot and other viruses.

Visual signs

Silvery or white flecks and streaks along the leaves, worst in the folds where the leaves meet the neck; leaf tips browning and dying back; in bad cases whole leaves bleached and twisted. Part the leaves at the centre and look for pale yellow to brown, 1 mm, cigar-shaped insects that scurry; nymphs are smaller and wingless. Tap a leaf over white paper to count them.

Damage pattern

Starts in the protected leaf axils and spreads outward; builds through hot dry spells in summer and collapses after heavy rain. Plants under drought suffer most. Red onions are the most susceptible, Spanish types the least. Bulbs end up undersized and sometimes distorted; stored bulbs are not affected.

Stages affected: Flowering · Cut and come again

A water mould that attacks onion, chives, garlic, leek, shallot and Welsh and multiplier onions (PNW), spotting and collapsing the leaves in cool, humid weather. It overwinters in perennial alliums such as chive clumps and in volunteer onions, so a chive patch can carry it from year to year.

Visual signs

Pale green, then yellow or light brown to purplish oval patches on the leaves and flower stalks. In humid weather, especially early in the morning, the patches grow a fuzzy greyish-violet mould. Leaf tips bend over and collapse; systemically infected plants are stunted and pale.

Damage pattern

Appears in cool, humid spells with long nights of dew or rain, below about 22 °C; spores form between about 6 and 27 °C, best around 11 to 13 °C (PNW), and dry, warmer days halt it. A full cycle takes about two weeks, so it can sweep a crowded planting in a wet spring or fall.

Stages affected: Vegetative · At planting

The larva of a small grey fly that lays at the base of young onions, leeks, garlic and shallots in spring; the maggots tunnel into the stem base and bulb, kill seedlings outright and let soft rot into older bulbs. Worst in cool wet springs and in beds with fresh manure or last year's onion debris.

Visual signs

Seedlings that wilt in sun, yellow from the tips, flop and lift out with no roots and a rotted base. Inside the base, white legless maggots 6 to 8 mm long with a pointed head. The adult is a slender greenish-grey fly about 6 mm long, like a small house fly, seen around the row in spring. Later generations tunnel into swelling bulbs, which then rot in the ground or in store.

Damage pattern

Appears in gaps along the row in late spring, often several plants in a run as maggots move from one to the next. The first generation on seedlings does the real damage; second and third generations in summer and at harvest tunnel bulbs and invite rot. Worst on sets and seedlings in cool wet weather and on beds carrying rotting onion material.

Stages affected: Vegetative · Cut and come again

A soil fungus that turns the roots of onions pink, then red or purple, until they shrivel and die and the plant stunts. Onion is the main host; garlic is occasionally affected (Oklahoma State) and Utah State lists it on chives. Grasses, cereals and corn carry it between allium crops, and it lasts in soil for a very long time.

Visual signs

Lift a plant: the roots are light pink at first, deepening to red or purple, then shrivel and die while new roots are attacked in turn. Above ground, leaf tips die back and plants are stunted, looking much like drought or hunger.

Damage pattern

Worst in warm soil, most active at about 24 to 29 °C (Utah State), and in heavy, poorly drained ground and stressed plants. It rarely kills outright but cuts growth and yield severely, and builds up where alliums follow alliums.