Onion

Stages affected: Vegetative · Maturation

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 · Seedling

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: Seedling · Vegetative · 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: 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 · Maturation

Not a disease and not a mistake: the plant switching from making leaves to making seed, which is what it was always going to do. It matters because in a leaf or root crop the switch ends the harvest — the plant pours everything into a flower stem, the leaves turn narrow and bitter, and a root goes woody. Lengthening days, heat and any check to growth all bring it forward, and which of those dominates depends on the crop. Cilantro and spinach are driven mainly by day length, lettuce and radish mainly by heat; biennials such as parsley and celery bolt after a spell of cold followed by warmth, so an overwintered plant runs to seed in spring. A dry spell or a transplant check will trigger any of them. Once a plant has committed there is no way back, so everything useful happens before it starts.

Visual signs

A stem lengthening from the centre of a plant that had been low and spreading, often surprisingly fast once it starts · Leaves higher up the stem narrower, more deeply cut or more feathery than the ones below · Spacing between the leaves opening out so the plant looks stretched · Flower buds in a tight cluster at the tip · In lettuce, the head going conical and firm and the milky sap running bitter · In radish and brassicas, a slender stalk with four-petalled flowers and the root turning pithy

Damage pattern

Usually the whole sowing at once, since every plant in it is the same age and met the same weather. That is the clearest way to tell it from a disease: a problem that arrives on all the plants simultaneously and is worse on the oldest sowing is developmental rather than infectious. Scattered individual plants bolting well ahead of the rest are usually the ones that were checked — transplanted, dried out, or left too long in a module.

Stages affected: Germination · Seedling

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

Stages affected: Seedling

The caterpillars of several drab night-flying moths. They live at the soil surface and chew through the stems of young plants at ground level, so you almost always find the damage before you find the caterpillar.

Visual signs

Smooth dull caterpillar up to 40 mm long in grey brown or near black often with faint stripes · Curls tightly into a C shape the moment it is touched or uncovered · Found 25 to 50 mm down in loose soil within a hand's width of the damaged plant · Nothing visible on the plants by day because they feed after dark

Damage pattern

A young plant cut clean through at soil level and left lying beside its own stump · Whole seedlings gone overnight with the root still in the ground · Ragged holes chewed in the leaves that rest on the soil · Damage running along a stretch of row rather than scattered across the bed

Stages affected: Germination

The name covers several different soil organisms doing the same thing: killing seedlings at or just below the soil line, either before they emerge or in the first fortnight after. Most of the damage in cold wet conditions comes from Pythium and Phytophthora, which are oomycetes rather than true fungi, but Rhizoctonia and Fusarium do the same job in warmer soil. The organisms are present in almost every garden soil and most reused potting mix; what turns them from harmless to lethal is cold, wet, airless conditions and a seedling that takes too long to get up. Utah State names it as one of the three problems on cilantro, which is a slow germinator sown into cool spring soil.

Visual signs

Seedlings that were upright yesterday lying flat this morning, still green · A dark water-soaked pinch in the stem right at soil level, thinner than the stem above and below it · Seedlings that never emerge at all, with the seed soft and brown when you dig one up · A bare patch in an otherwise even row, often roughly circular and spreading outward day by day · Sometimes a fine grey or white web on the soil surface in the affected patch · Roots brown and stringy, with the outer layer sliding off the core when pulled between finger and thumb

Damage pattern

Spreads outward from a centre, so an expanding bare patch rather than scattered single losses. It is a seedling disease and stops mattering once the stem has woody tissue at the base, usually two to three weeks after emergence. A sowing that fails patchily and never recovers, where digging finds soft brown seed, is usually this rather than poor seed.