Garlic Problems: White Rot, Rust, Yellow Leaves and Pests

Start by asking whether the trouble is above or below ground

Garlic problems split into three groups, and they need completely different responses. Soil-borne diseases such as white rot and basal rot attack the plate and roots, cannot be cured, and contaminate the ground for years. Leaf diseases such as rust and downy mildew reduce the bulb size but do not persist in the soil in the same way. Everything else is usually water, nitrogen or a pest.

So when a plant looks wrong, pull one up. If the roots are gone and the base is soft, you are dealing with a soil disease and the response is removal and rotation. If the roots are white and firm, the problem is on the leaves or in your management, and the response is a change of practice.

White rot: the one you must never let onto your plot

White rot is the most serious disease of garlic and onions. Leaves yellow and collapse, and at the base you find a white fluffy fungal growth studded with black specks the size of poppy seeds. Those specks are resting bodies that survive in the soil for 20 years or more, waiting for the next allium root to come near, and there is no practical cure or treatment for a home garden.

It arrives on infected planting stock or on soil carried in on boots and tools, so buy certified planting garlic rather than supermarket bulbs and clean your tools between beds. If you find it, lift the affected plants along with the surrounding soil, bag them and dispose of them off the plot. Never compost them. Then keep every allium off that ground indefinitely, and treat it as a permanent boundary in your rotation.

1. Confirm by looking at the base Pull a collapsed plant and inspect the basal plate. White cottony growth with black specks is white rot. Brown mush without the specks is more likely basal rot.
2. Remove the plant and the soil around it Dig out the plant with a spadeful of surrounding soil, bag it and put it in the bin. Do not compost it and do not shake the soil off in the bed.
3. Clean everything that touched it Wash tools, boots and gloves before working elsewhere. The disease moves on soil more than anything else.
4. Retire that ground for alliums Grow anything else there, but never garlic, onions, shallots or leeks again. If your plot is small, switch to growing garlic in containers of fresh compost.

A four year rotation handles most allium problems but not white rot, which outlasts any realistic rotation. Prevention through clean stock and clean tools is the entire defense.

Rust and downy mildew: manage the air, accept the loss

Rust shows as small raised orange pustules on the leaves in mild, damp weather and spreads quickly through a crowded row. It rarely kills a plant, but each affected leaf stops feeding the bulb, so a bad attack in spring cuts your bulb size noticeably. Downy mildew produces pale streaks with a grayish furry growth in humid conditions and behaves similarly.

Neither is worth spraying for in a home garden once it has taken hold. Remove the worst affected leaves, take them off the plot, and improve airflow by weeding thoroughly and not crowding the plants. If it appears late in the season with the bulb nearly full, do nothing at all and harvest on schedule. Next autumn, space wider, avoid overhead watering and clear all allium debris in the winter.

1. Take off the worst leaves only Remove heavily pustuled leaves and bag them. Do not strip the plant, because every leaf you remove costs a wrapper layer on the bulb.
2. Open the row up Weed thoroughly and clear anything crowding the plants. Rust spreads fastest where leaves stay wet into the afternoon.
3. Water at the base only Never water over the tops. Overhead watering in spring is the most common way rust gets going in a home garden.
4. Harvest early if it is severe If most leaves are affected and the bottom third has browned, lift now. A slightly small bulb is better than one whose wrappers are failing.

Yellow leaves are usually water or nitrogen, not disease

Yellowing garlic sends most gardeners looking for a disease when the cause is normally simpler. Yellowing from the leaf tips downwards across the whole plant, with firm white roots when you lift one, is either waterlogging or a nitrogen shortage. The lowest one or two leaves yellowing in early summer while the rest stay green is not a problem at all, it is the normal approach to maturity.

Check the soil before you do anything. If it is wet and heavy, stop watering and cut a channel to drain the bed. If it is dry and the plant is pale and thin, and you are still more than two months from harvest, give a nitrogen feed. If you are inside that final two months, do nothing, because feeding late gives you thick necks and bulbs that will not store.

Pests worth knowing and the records that catch them

The pests that matter are onion fly, whose maggots tunnel into the base and cause a plant to yellow and topple, thrips, which rasp silvery streaks into the leaves in hot dry weather, and stem and bulb nematode, which twists and swells the leaves and makes the bulb soft and cracked. Nematode is the serious one: it persists in soil for years and is spread by planting infested cloves, so it is another argument for certified planting stock.

Keep a note each season of what appeared, roughly when, and where in the bed. Garlic gives you only one observation a year, so problems that look random in isolation become obvious patterns across three seasons. Record the date you first saw rust, the row position of any collapsed plants, and how many bulbs were affected at harvest.

1. Cover the row against onion fly Fine insect mesh laid over the bed from the restart of spring growth stops the adults laying at the base. It has to be sealed at the edges to work.
2. Pull and bin collapsed plants A plant that yellows and falls over usually has maggots or rot at the base. Remove it whole so the problem does not build up in the bed.
3. Never replant a suspect bulb Cloves from a crop that showed twisted leaves or soft cracked bulbs carry nematode. Do not save them for planting, whatever they look like.
4. Log first-sighting dates Note the date and location of the first sign of every problem each year. Three seasons of those dates tell you when to put the mesh on and when to check.