Black fuzzy growth: Rhizopus and black pin mold

Tall, loose grey fuzz with tiny black pinheads on the ends of upright stalks. It grows fast — noticeably taller each day — and it looks unlike anything else in a grow.

This is Rhizopus, the same organism that goes furry on forgotten bread. It is not treatable and it is one of the two contaminants on this site worth taking respiratory precautions with.

What you’re looking at

The structure is the identifier. Rhizopus sends up upright stalks — sporangiophores — each capped with a spherical black sporangium full of spores. Under a hand lens they look like tiny lollipops. To the naked eye it reads as grey fuzz with black dots, and the dots are what distinguish it from everything else.

It starts white or pale grey and blackens within 24–48 hours as the sporangia mature. Growth is fast and three-dimensional; unlike Trichoderma it stands well off the surface.

Telling it apart

From Trichoderma: Trichoderma is flat, dense and green. Rhizopus is tall, loose and black-dotted. They don’t really look alike once you’ve seen both.

From Aspergillus niger: genuinely similar, and the practical answer is the same for both — bin it, safely. A. niger forms a more compact, powdery black layer without the tall stalks. If you want to be precise, look for the upright structures; if you just want to know what to do, it doesn’t matter.

From cobweb mold: cobweb is grey and stays grey with no black points, and it’s treatable. Check for the black pinheads before reaching for the peroxide.

Can it be saved?

No. On grain, on bulk, at any stage.

Rhizopus grows faster than any cultivated species and produces enormous spore counts. By the time it’s visible the substrate is compromised throughout, and removing the visible growth removes almost none of the organism.

Getting it out safely

This one deserves more care than most.

Rhizopus and Aspergillus are both capable of causing serious respiratory infection in people who are immunocompromised, and both are potent allergens even for people who aren’t. Heavy spore exposure in an enclosed room is worth genuinely avoiding.

1. Wear an N95 or better. Not optional for this one.
2. Seal the container before moving it. Lid on, or into a contractor bag tied shut. Do not carry an open tub of sporulating Rhizopus through your house.
3. Open it outdoors, downwind, well away from your grow space.
4. Bag the substrate for trash. Not compost.
5. Wipe the container with 70% isopropyl, then wash with soap and hot water.
6. Change clothes before returning to your grow area.

If you are immunocompromised, pregnant, or have a respiratory condition, hand this job to someone else.

What caused it

Substrate too wet. The dominant cause. Rhizopus thrives in free water and poor gas exchange — the same anaerobic conditions that favour bacteria. Water pooling in the bottom of a tub is the classic setup.

Sterilisation failure. Rhizopus spores are heat-sensitive and die readily at proper sterilisation temperatures. Finding it in a grain jar means the cook was inadequate. See wet spot for the usual pressure cooker failures — they’re the same failures.

Airborne contamination at inoculation. Rhizopus is ubiquitous, especially anywhere food is stored. Working in a kitchen with fruit on the counter is working in a room full of it.

Substrate sitting too long before use. Hydrated, untreated substrate left overnight grows Rhizopus readily. Treat it the same day you hydrate it.

High temperatures. It runs fast above 80°F, faster than most cultivated species can respond to.

Preventing the next one

Get the water right. Rhizopus is more strongly associated with over-hydration than any other common contaminant, and field capacity is the specific target: a firm squeeze produces a few drops, not a stream, and never a puddle in the bottom of the container.

Then check the pressure cooker, for the same reasons as everything else on this site.

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Tall grey fuzz with black pinheads. Mask on, seal it, take it outside. This is not one to inspect closely on the kitchen counter.