Garlic Problems: Yellow Leaves, Rust and Small Bulbs
Match what you see on your garlic to its likely cause, and what to do this season and next.
Garlic problems in Canadian gardens usually show up as one of three things: yellowing leaves, orange spots on the leaves, or bulbs that stay small. Some causes are harmless or easy to fix. A few are soil-borne diseases that can stay in a bed for years.
This guide is built from Ontario’s garlic factsheet, Saskatchewan’s Ministry of Agriculture and the University of California’s integrated pest management guidelines on garlic rust.
Garlic problems at a glance: Yellow leaf tips in July are usually normal ripening. Yellowing in spring points to dry soil, low nitrogen, basal rot or nematodes. Orange-red pustules on the leaves are rust. Small bulbs come from scapes left on, late planting, dry spells, weeds or small planting cloves. For soil-borne disease, move garlic and onions to a new bed for at least 3 years.
Want to prevent problems from the start? See Growing Garlic in Canada for clean planting stock, timing and curing.
Quick Diagnosis
| What You See | Likely Cause | What to Do |
|---|---|---|
| Lower leaves browning from the tip in July | Normal ripening | Start checking for harvest; lift at a third to a half of leaves brown |
| Leaf tips yellow in spring, plants weak | Dry soil or low nitrogen; fusarium basal plate rot | Water about 2.5 cm a week; check roots and the base of the bulb |
| Stunted, yellow, swollen or twisted leaves | Bulb and stem nematode | Remove plants; clean stock, clean soil, 3-year rotation |
| Older leaves yellow, wilt and fall over; white fluff and black specks on the bulb | White rot | Remove and bin plants; do not compost; clean tools |
| Small orange-red pustules on the leaves | Rust | Rotate 2–3 years; remove volunteer alliums |
| Tunnels and damage inside the leaves | Leek moth (Ontario) | Remove damaged leaves; cover with fine mesh in early summer |
| Bulbs small at harvest | Scapes left on, late planting, drought, weeds, small cloves | See the small-bulb section below |
| Blue-green mould on stored bulbs | Penicillium mould | Store cool and dry; never plant mouldy cloves |
Yellow Leaves
Garlic leaves dying back from the tips in July is the normal harvest signal, not a problem. Yellowing in spring or early summer is different.
Garlic is sensitive to dry soil. Ontario’s factsheet says dry spells, especially during bulbing, cut yields, and that most soils need about 2.5 cm of water a week. It also lists fusarium basal plate rot, a soil-borne fungus that makes the roots tan, empty and useless and turns the leaf tips yellow with dieback in spring. Warm, wet soil favours it, so rotate with non-allium crops.
Bulb and stem nematode is a microscopic pest. Ontario’s factsheet describes stunted young plants with yellowing, bloated leaves, and later twisting, soft bulbs and lost roots, usually showing from mid-July. Prevention is the only real control: clean planting stock, clean soil and a 3-year rotation.
White rot yellows, wilts and topples the older leaves, and leaves fluffy white growth and pinhead-sized black specks on a rotted bulb. Ontario’s factsheet says it can survive in soil for decades and is worst in cool, wet seasons. Remove infected plants, keep them out of the compost, and clean tools and boots before working other beds.
Orange Spots: Garlic Rust
Rust starts as tiny flecks that grow into oval, reddish to dull-orange pustules, first on the older leaves and then the younger ones, according to the University of California’s IPM guideline. Later in the season the pustules can turn dark. Badly infected leaves yellow and die, and severe infection reduces bulb size and quality.
The same guideline says the fungus is most active in cool weather, with infection best around 15°C and very high humidity, and that it can overwinter on garlic and volunteer alliums. Its first advice is to rotate away from alliums for 2–3 years and destroy volunteer allium plants. It notes no garlic variety is fully resistant.
Small Bulbs
Most small-bulb years have an ordinary cause. Scapes left on can reduce bulb yield by as much as 30%, according to Ontario’s factsheet. Late planting is another: Saskatchewan’s ministry notes that late-planted garlic bulbs poorly.
Saskatchewan’s page also notes temperatures above 30°C hold back bulb formation and make the foliage die back early, and that garlic is a poor competitor with weeds. Heavy soil can make rough, irregular bulbs, Ontario’s factsheet adds. Plant your largest cloves, keep the bed weeded and watered through June, and cut the scapes.
Problems in Storage
Penicillium mould is the main cause of garlic decay in storage, says Ontario’s factsheet: blue-green growth that usually starts at the base of the bulb. Infected cloves planted in fall spread it to the bed. Store eating garlic cool with moderate humidity (the factsheet gives 0–4°C at 60–70%), and never plant a clove that shows mould.
Planting Stock and Storage Rots
Perennia, Nova Scotia’s agriculture extension agency, says that once a rot shows up in stored garlic there is little to be done, so prevention starts at harvest: loosen the soil before lifting, avoid knocking bulbs together, and cure them well.
For blue mould (penicillium), it notes the spores spread to healthy cloves through wounds when bulbs are cracked apart for planting, so plant cloves soon after cracking. Storage below 4.5°C with low humidity and good airflow stops it growing. For Fusarium basal rot, if one clove in a bulb shows symptoms, do not plant any of that bulb, and keep garlic, onions and leeks out of that bed for at least four years.
The University of Saskatchewan adds two common reasons for small “rounds” at harvest: planting small cloves, and dry soil while the bulbs are swelling. It lists onion maggot as an occasional pest.
Frequently Asked Questions
Why are my garlic leaves turning yellow?
If the lower leaves brown from the tips in July, that is normal ripening and a sign harvest is near. Yellowing in spring or early summer points to dry soil, low nitrogen, fusarium basal plate rot or bulb and stem nematode, all described in Ontario's garlic factsheet.
What are the orange spots on my garlic leaves?
Most likely garlic rust. The University of California IPM guideline describes small flecks that grow into oval, reddish to dull-orange pustules, first on older leaves. It favours cool, very humid weather.
How do I get rid of garlic rust?
The University of California IPM guideline's first steps are to rotate away from garlic, onions and other alliums for 2 to 3 years and to destroy volunteer allium plants, since the fungus can overwinter on them. No garlic variety is fully resistant.
Why are my garlic bulbs so small?
Common causes are scapes left on the plant, late planting, dry soil while the bulbs are forming, weeds and small planting cloves. Ontario's factsheet says leaving scapes on can reduce bulb yield by as much as 30 percent.
What is white rot in garlic?
A soil-borne fungal disease that yellows and wilts the older leaves and leaves fluffy white growth and small black specks on a rotted bulb. Ontario's factsheet says it can survive in soil for decades and is worst in cool, wet seasons.
Why did my stored garlic go mouldy?
Ontario's factsheet names penicillium mould as the main cause of decay in storage: blue-green growth usually starting at the base of the bulb. Store garlic cool with moderate humidity and never plant a mouldy clove.
Sources
- Ontario Ministry of Agriculture, Food and Agribusiness — Garlic production factsheet
- Government of Saskatchewan, Ministry of Agriculture — Garlic
- University of California Statewide IPM Program — Onion and Garlic: Rust
- Perennia (Nova Scotia), Gillis-Madden, Rehmen and Hildebrand (2020) — Garlic Storage, Post-Harvest Diseases, and Planting Stock Considerations
- University of Saskatchewan, Gardening at USask — Garlic