Garlic

Stages affected: Vegetative · Flowering · Maturation

The one allium disease that permanently ruins a bed. A soil fungus that rots the roots and basal plate of onions, garlic, leeks and shallots in cool soil, leaving white fluff and tiny black resting bodies that survive in the ground for twenty to forty years. There is no cure and rotation barely helps; everything is about keeping it out.

Visual signs

Leaves yellow and die back from the tips, older leaves first, and the plant wilts and topples; pull it and it comes up with no roots, a soft rotted base, a fluffy white mould over the basal plate and bulb, and scattered black specks the size of a pinhead (the sclerotia) in the fluff. Often in a patch that enlarges year on year.

Damage pattern

Appears in cool weather, soil 10 to 24 °C with an optimum around 15 to 18 °C, and stops above about 25 °C: spring on autumn-planted garlic and overwintered onions, early summer on spring onions. Starts as one or two plants and spreads along the row as the mycelium grows plant to plant. Bulbs that look sound at lifting can rot in store.

Stages affected: Vegetative · Flowering

Bright orange pustules that pepper the leaves of garlic, leeks, chives and sometimes onions in mild humid weather. It rarely kills, but a bad attack in late spring strips the leaves garlic needs to size its bulb and can halve the crop; on leeks it spoils the part you eat.

Visual signs

Small raised oval pustules, 1 to 3 mm, bright orange to rust-red, scattered or in rows along both sides of the leaf, rubbing off as orange dust on a finger. Later in the season pustules turn dark brown to black. Heavily infected leaves yellow from the tip and wither early. Starts on older, lower leaves.

Damage pattern

Appears in spring and early summer in mild, humid spells, 10 to 20 °C with dew or light rain, and spreads along the row on the wind. Worst in crowded, overfed or stressed plantings and where alliums have grown year after year. Garlic suffers most when it strikes in the last six weeks before harvest; leeks and chives can carry it all season.

Stages affected: Vegetative · Flowering

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: Germination · Vegetative

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: Maturation · Post-harvest

The grey mould that turns stored onions soft from the neck down six to twelve weeks after harvest. It infects the leaves and neck in the field without any visible sign, waits, and then rots the bulb in store; thick necks, bent-over tops, late nitrogen and a damp cure are what let it in. Garlic and shallots get the same thing.

Visual signs

In store: the neck area softens and sinks, the scales beneath turn water-soaked and brown, a dense grey fuzzy mould grows between the scales and at the neck, and later hard black bodies 2 to 5 mm form at the neck and under the skin. From outside the bulb may look sound until pressed. In the field there is usually nothing to see; occasionally white flecks on the leaves after rain.

Damage pattern

Shows weeks after harvest, in bulbs that had thick or damaged necks, were lifted in wet weather, or were cured badly. Spreads slowly from bulb to bulb in store by contact. Worst in cool wet growing seasons and after heavy late nitrogen. Sweet and white onions are more susceptible than pungent storage types.

Stages affected: Vegetative

Soft-bodied molluscs that feed at night and in wet weather, rasping holes in leaves. Seedlings and low soft foliage are the most at risk, and a single night can take out a row of young plants.

Visual signs

Ragged holes with smooth edges chewed through the middle of leaves rather than at the margins · Silvery dried slime trails on leaves soil and pots, which is the deciding feature · Damage appears overnight and stops in dry sunny spells · Seedlings grazed off at soil level · Slugs or snails found under boards pots and mulch during the day

Damage pattern

Worst after rain irrigation and in humid shaded beds · Feeds at night; plants look untouched by day · Concentrated on low soft leaves near the soil