Fusarium basal rot

older leaves turn yellow from the tip downward and start dying back while the plant is bulbing, and the whole plant looks stunted and wilts in the heat of the day even when the soil is moist. A few plants collapse here and there across the bed.

The roots come away easily — they are brown, few, and rotted instead of white and firm. The basal plate (the hard disc at the bottom of the bulb where roots attach) is brown and soft, watery at first then firm and dry, and the rot creeps up into the bulb.

Infected scales are discoloured and softening, often browning from the base up, and you may see a pinkish-white, cottony mould on the rotted plate or between the scales.

White rot shows fluffy white mould with tiny black dots (like poppy seeds) on the base — Fusarium's mould is pinkish-white with no black dots. Neck rot starts at the top/neck of the bulb in store, not the base. Simple drought or maggot damage will not give you a soft, rotting basal plate with easily-detached roots — that rotting base is the Fusarium fingerprint.

from bulbing right through to maturity and harvest (the mid- and late-season stages) — this is when the disease shows, especially in the warm months when the soil is hot (around 25-32°C, warm enough that the surface soil is uncomfortable to hold your hand on).

every week or two, walk a big "W" across the plot so you cover all four corners and the middle, not just the row by the path. Stop at about 10 spots and check the plants around each stop.

look for plants that are yellowing, dying back or wilting while healthy neighbours stay green. These are your suspects.

At each stop, pull one or two suspect plants and cut across the basal plate. If you find a brown, soft rot and easily-detached roots (with or without pinkish-white mould), that is Fusarium.

even ONE plant that is yellowing and dying back with a soft, rotting basal plate on lifting means the disease is in your soil. There is no spray to save a rotted plant — so the moment you confirm it, switch to protecting the rest of the crop and next season's field (see control). Mark and remove those plants; don't wait for more.

Plant only clean, healthy sets and seed from a trusted source (never save planting material from a field that had basal rot), and choose tolerant/resistant onion and garlic varieties where you can get them — these are your two strongest tools.

Do not grow onion or garlic on the same ground for at least 3-4 years — rotate to a non-allium crop so the fungus has nothing to feed on. Plant on well-drained ground or raised beds; avoid warm, waterlogged soil that stresses the roots during bulbing and lets the fungus in.

The fungus gets in through wounds, so control onion maggot and root-feeding nematodes (tiny soil worms) that chew the roots and basal plate. Remove and destroy rotted plants and crop trash — do not leave infected bulbs in the field or throw them on the compost.

Cure the bulbs well (dry the necks and skins thoroughly after lifting) and store them cool and dry with good airflow. Fusarium keeps rotting bulbs after harvest, so a poorly-dried, warm store will turn a few sick bulbs into a spoiled heap.

There is no spray that cures an infected plant. The only chemical option is a seed treatment: treat seed or dip the sets in a registered fungicide before planting (a nurseryman's or shop's recommended, registered product for onions — never an unlabelled mix), and where available drench the planting furrow with a biocontrol such as Trichoderma (a friendly fungus that crowds out Fusarium). Follow the label rate exactly. Safety: cover your skin, eyes, nose and mouth when handling the treatment, mix and dip in the cool of the day with the wind behind you, keep it away from food, drinking water, children and animals, and wash your hands and clothes after. Because you use it so rarely, rotate to a different fungicide group the next time so the fungus doesn't build resistance.

if rotted plants keep appearing despite clean sets and drainage, treat that field as infected soil — stop growing onions and garlic there and put it under a long rotation (4 years or more) of unrelated crops. If a crop is already collapsing at bulbing, lift the healthy bulbs early, cure them hard and sell or use them quickly — don't hold badly-affected bulbs in store, as they will rot and spoil the rest. Cut your losses on that field and protect the clean ground you have left.