Blue Mold Explained: How to Spot It and Save Your Crops?

That sinking feeling when you spot a fuzzy, bluish-green patch on your apples or onions is all too familiar for many gardeners and farmers. Blue mold, a common and fast-spreading disease, can quickly turn a promising harvest into a mushy, spoiled mess. It feels defeating, but you’re not powerless against it. 

By learning to recognize the first signs of blue mold and understanding what causes it, you can take smart, effective steps to protect your crops and save your harvest from this pervasive fungus.

What Is Blue Mold?

Blue mold is one of the most significant post-harvest diseases affecting fruits and vegetables worldwide. It’s primarily caused by fungi from the Penicillium genus, with Penicillium expansum being the most common culprit for apples and pears, and Penicillium italicum often affecting citrus fruits.

The disease is also known as soft rot or bin rot because it causes a soft, watery decay that can be easily scooped away from healthy tissue. The fungus typically invades produce through injuries like bruises, punctures, or splits sustained during picking and handling.

While it’s famous for attacking apples, blue mold has a wide range of hosts. Main host plants:

• Apples and pears
• Citrus fruits
• Onions and garlic
• Strawberries, cherries, and tomatoes
• Corn and rice

Causes of Blue Mold & How It Spreads

The fungus that causes blue mold is practically everywhere in our environment, just waiting for the right conditions to grow.

Primary causes:

• Fruit injuries: Bruises, cuts, and punctures from harvesting and handling are the main entry points for the fungus.
• Contaminated surfaces: Spores are widespread and can contaminate orchard soil, storage bins, packing equipment, and water used for washing produce.
• High humidity: Damp conditions are ideal for spore germination and fungal growth.
• Infected debris: The fungus can survive on decaying organic matter on the orchard floor.

The spread of blue mold often begins long before the fuzzy spots appear. Initially, spores land on the surface of a fruit, often in the orchard or during transport. Then, if there is a tiny wound or bruise, the spore germinates and the fungus begins to grow into the fruit’s flesh.

Furthermore, once an infection is established, it produces millions of new spores that appear as a characteristic blue or blue-green fuzzy patch. Finally, these spores are easily spread by air currents, water splashes, or on contaminated equipment, leading to new infections on other fruits, especially in storage where they are packed closely together.

(Left) Blue mold infection caused by Penicillium expansum on a Golden Delicious apple. (Top right) Close-up of the fungus’s coremia structure visible on the rotted surface under a stereo microscope.

Symptoms to Watch For

Catching blue mold early is key to minimizing losses. Look for these signs:

• Watery, soft spots: The first sign is often a light brown, soft, circular lesion on the fruit’s surface.
• Musty odor: Infected fruit gives off a distinct, earthy, musty smell.
• Blue-green spores: As the rot develops, fuzzy blue-green masses of spores will appear on the lesion, which gives the disease its name.
• Easy separation of rot: The decayed tissue is very soft and watery and can be easily separated from the healthy part of the fruit, leaving a bowl-like cavity.

‘Zaosu’ pears showing symptoms of blue mold (A) and Alternaria rot (B) during postharvest storage.

How to Control Blue Mold?

Control Method	Host Plan	Key Strategies
Cultural Control	Apples, Pears, Citrus, Onions	Practice orchard sanitation by removing and destroying fallen, rotted fruit; avoid injuring produce during harvest and transport.
Sanitation	All susceptible crops	Disinfect storage bins, field crates, and packing equipment with fungicides or steam; ensure water in dump tanks is sanitized.
Storage Control	All susceptible crops	Cool fruit rapidly after harvest; store produce at low temperatures (near 41°F) and low humidity to slow fungal growth.
Chemical Control	Apples, Pears, Citrus	Apply post-harvest fungicide treatments via dips or drenchers; rotate fungicides like pyrimethanil and fludioxonil to prevent resistance.
Organic Control	All susceptible crops	For organic operations, options are limited, but careful sanitation and temperature management are critical; some biological controls are under investigation.

Prevention Tips for Gardeners and Farmers

• Handle carefully: Avoid bruises or cuts on produce when picking and packing.
• Sanitize tools and bins: Clean and disinfect equipment and containers regularly.
• Cool quickly: Move harvested crops to cold storage right away.
• Keep storage right: Maintain low temperature and proper humidity.
• Remove rotting fruit fast: Clear out decayed produce to stop spores spreading.
• Use fungicides carefully: Apply post-harvest treatments as directed when needed.

Final Thoughts

In the end, blue mold can be managed if you stay proactive, and tools like Planteyes make it easier. The app excels at detecting plant diseases early, and you can even chat in-app to find practical solutions as if you were talking to an expert who knows your crops. Download Planteyes today and start protecting your harvest with confidence.

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