Penicillium rot (blue mold) on garlic — symptoms and treatment

Severity: medium

What is Penicillium rot (blue mold)

Penicillium rot (blue mold) is a blue-green mold caused by Penicillium species. It is a common post-harvest disease affecting stored garlic bulbs: the crop looks fine coming out of the ground and then breaks down in the store, or planting stock rots in the ground after a mouldy clove goes in. The fungus is a wound specialist — it rarely troubles a sound, well-cured bulb, but it moves fast through damaged, bruised or damp ones. That makes it a problem almost entirely about how you handle and store the crop.

Symptoms

The first thing most growers notice is a clove that feels wrong — soft where it should be firm, sometimes with a sunken, water-soaked patch. Peel the bulb and the rot is soft and watery; the tissue breaks down rather than drying up. Then comes the sign that names the disease: blue-green powdery spores on the affected cloves, dusty to the touch, often starting at a wound, a bruise or the cut basal plate. A strong musty odour accompanies it and frequently draws attention to a store before anyone looks closely. The disease spreads during storage, so one infected bulb turns into a pocket of them.

• Early: a soft, giving clove; water-soaked or sunken patches; a musty smell in the store
• Advanced: soft watery rot through the clove; blue-green powdery spores across the surface; neighbouring bulbs affected

Planted cloves carrying the fungus may simply rot instead of sprouting, or emerge weak.

Causes and conditions

Penicillium spores are everywhere — in the air, on crop debris, on store surfaces, on the bulbs themselves. They are not the limiting factor; opportunity is. The fungus needs a way in and moisture to work with. Wounds are the way in: nicks from harvest and trimming, bruises from rough handling, damage from splitting bulbs for planting stock, and the exposed basal plate.

Moisture is the other half. Bulbs that were not cured thoroughly, or that go into a damp, poorly ventilated store, hold enough moisture in their necks and wrappers for the fungus to establish. Cool, dry, airy storage does not kill the spores — it denies them the conditions they need.

Treatment

There is no rescuing a rotted clove, and no spray to fall back on. Control is entirely cultural, and all of it happens around harvest and storage.

Proper curing and storage — cultural

Timing: post-harvest. Cure garlic thoroughly before storage. Store in cool, dry, well-ventilated conditions. Handle carefully to avoid wounding.

Thorough curing dries the necks and outer wrappers so the bulb goes into store as a sealed, dry package. Cool, dry, well-ventilated storage keeps it that way. Careful handling — no tipping, no dropping, no rough trimming — denies the fungus the wounds it needs to get in.

Prevention

• Cure the crop thoroughly before it goes into store; do not rush bulbs in damp.
• Store cool, dry and well ventilated, never in sealed bags or plastic.
• Handle gently at every stage — most infections start at a bruise or nick.
• Sort before storing and discard damaged or soft bulbs rather than keeping them "to use first".
• Inspect the store periodically and remove any bulb showing blue-green mold.

Frequently asked questions

Can I still eat a garlic bulb with blue mold on it? Discard affected bulbs. Do not try to cut around blue-green mold and musty soft rot in garlic.

Will it spread to my other bulbs in storage? Yes — this is a disease that spreads during storage. Remove affected bulbs as soon as you find them and check the rest.

Can I plant cloves that have a little blue mold? No. Infected cloves rot instead of establishing, and they carry the fungus into your next crop. Plant firm, clean cloves only.