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Moulds: What to cut and what to throw​

How it gets in // Types of mould // Cut it or throw it out // do's and don'ts // the good moulds // sources

The tip of the iceberg

One day a strawberry looks fine, the next it is covered in a soft grey fuzz. Mould.
​
Not a pretty sight to behold, unless you can turn your scientific glasses on and look at it as a wonder of nature.
Either way, it is not what you want in your fruit bowl. But does one fuzzy strawberry mean the whole punnet goes?

That depends on something you cannot see. The fuzz is not the fungus; it is only the part that makes spores.
Think of a mushroom. What you pick is the fruiting body, and the fungus itself is a network of threads running through the wood or the soil, older and much larger than anything above ground.
Your strawberry has that network too, running through the flesh.
​It was there days before anything showed on the surface.
​
Which is why cutting off what you can see so rarely helps, and why the answer to the punnet question depends on what the fruit is.

How it gets in

The question is not whether they are present but when they got in, and the answer is usually: long before you did.
Which does not mean nothing can be done.
What you cannot undo is an infection that started in the field or on the packing line. What you can change is how fast it takes hold.

And mostly, they got in long before you did.
The mould arrives with the shopping: not because your kitchen contaminated the fruit, but because the fruit came in already carrying it.

​There are two ways it gets there:

1. Through a wound, after harvest.
​Intact skin is a good barrier against anything that lands on it: the moulds that spoil citrus can only infect through damage in the rind, and the bacteria that cause soft rot work the same way, entering through wounds and natural openings.
The wound in that orange was made at harvest, in the packing house or on the way to the shop, and the fungus went in with it. (source)

​And a wound is a wound wherever it happens.
​A bruise from the checkout belt or the bottom of your bag is the same open door as one made by a packing line, which is the real reason a bruised fruit keeps less well than a sound one. (here you'll find all about bruising)

2. before there was a skin to stop it.
Grey mould settles in the strawberry flower and waits, dormant in the green fruit, until the berry ripens.
Blue mould can colonise the open calyx of an apple during blossom, which is how a perfectly sound-looking apple turns out to be rotten at the core. (source)
Brown rot in peaches and grey mould in grapes work the same way. By the time you buy the fruit, the infection may be weeks old.

It also travels fruit to fruit.
Grey mould spreads to neighbouring berries on contact, which happens at picking, in the punnet and on the shelf. Which is why one fuzzy strawberry is a message about the rest of the box, not an isolated accident.
​
​It does not mean the whole box is lost.
​It does mean the ones that were touching it have had days of exposure, so take those out with it, check the rest, and eat them today rather than tomorrow.

Different moulds and what to do with them

Botrytis cinerea (grey mould)

How to recognize it

Looks grey and velvety, sometimes white and cottony. 
The spot starts pale to light brown, and can appear white and cottony first in humid conditions. It takes on a velvety grey coat once the fungus begins making spores. (source)
The fruit stays fairly firm and does not leak, which is how you tell it apart from other rots.

Where it comes from

Can start in the flower, long before harvest, and sit dormant in the green fruit until it ripens. (source) 
By the time you see fuzz, the fungus may have been inside that berry for weeks.
It also settles on dying or damaged tissue: a bruise, a torn leaf, or the green calyx of a tomato (source)

How it spreads

direct contact: it grows straight from an infected berry into the one beside it, without needing a wound to get in, which sets it apart from most of the moulds here. High humidity triggers the sporulation, which is why a closed punnet turns a single spot into a grey patch overnight. (source)

Mostly found on

strawberry and other soft berries, grapes, lettuce, tomatoes, cabbage, broccoli and beans​

Is it dangerous?

No. It makes toxins, but they are aimed at the plant, not at you. The problem is spoilage (source)

What to do with it

On soft fruit or beans the whole infected fruit goes; the threads run well beyond the patch you can see and there is nothing to trim. Check the rest for pale brown soft spots, which is what it looks like before the grey appears. The remainder is usually fine, but eat it today.
On a tomato it usually starts on the calyx rather than in the fruit, so remove the crown and check the stem end: still firm, still good.

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Sclerotinia Sclerotiorum (white mould)

How to recognize it

White mould or cottony rot, carrying black grainlike bodies.
It starts as a soft, water-soaked patch and then covers the vegetable in dense white fluff. What identifies it are the black bodies that form in that fluff: hard, seed-like grains, white on the inside. (source)
Has no smell of its own, until secondary bacteria arrive.

Where it comes from

​The soil. Those black grains are the fungus's resting structures, and they survive there for years before germinating. The spores that follow cannot infect healthy tissue on their own: they need dying tissue to start on, an old leaf or a spent flower, and free water on the surface. (source)

How it spreads

direct contact: In a crate it moves from one vegetable to the next until the whole box is gone, which is why it is also called nesting in beans.

Mostly found on

Vegetables: lettuce, beans, celery, cucumber, cabbage, and above all carrots in storage.​

Is it dangerous?

No. It produces no mycotoxin that regulators track. The problem is spoilage, and how completely it works through a stored crop.

What to do with it

Throw out the affected vegetable and check its neighbours; in a closed box, assume it has moved.
NB The black grains belong in the bin rather than the compost heap, since they survive in soil for years.

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Penicillium expansum

How to recognize it

Blue-green powdery mould.
It starts as a soft, watery brown patch, round and with a sharp border between the decayed and the sound flesh. The spore mass appears white before turning blue-green. (source)

Where it comes from

​Not from the orchard, mostly, but from everything the fruit passed through afterwards. The spores live on in storage bins, on walls and in wash water, and stay viable on dry surfaces for years, so the fungus recycles from one season to the next. Cold storage slows it down without stopping it, which is why apples can sit for months and still turn out rotten. (source)

How it spreads

Spreads via spores in the air and in water. It can only get in through a wound or a natural opening, so a sound skin keeps it out, and it does not grow from fruit to fruit the way grey mould does.​

Mostly found on

Apples and pears, where it is the main cause of postharvest decay, but also on quince, medlar, stone fruit, grapes, strawberries and kiwi. (source)​

Is it dangerous?

Yes. Take this one seriously: it produces patulin, which survives cooking and reaches a centimetre into flesh that still looks sound. (source)
It is not an acute poison; the worry is repeated exposure, and above all in juice and sauce, where you cannot see or taste it and where children take in far more per kilo of body weight than adults. (source)

What to do with it

Throw it out whole; trimming does not remove the toxin. The apples around it are only at risk if they are damaged themselves, so check them for bruises rather than discarding them.

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Rhizopus stolonifer

How to recognize it

Long hairs with black pinheads.
The stalks lift the spore heads clear of the surface. Unlike grey mould, the fruit goes wet and collapses completely, often leaking juice, and it smells distinctly yeasty and fermented. (source)​

Where it comes from

​A common soil fungus that lives on dead plant material, so its spores are simply everywhere: in soil, in the air, in any kitchen. It is picked up early in the chain, at harvest and during handling in the packing house, and it needs a wound or a bruise to get into the fruit. (source)

How it spreads

Spreads through the air; the raised stalks exist to launch spores away from the surface. It likes warmth, growing best around twenty-three degrees and slowing right down below five. (source)

Mostly found on

Soft and stone fruit above all: strawberries, raspberries, grapes, peaches, nectarines, plums, cherries, tomatoes. Among vegetables, sweet potato, carrot and parsnip. Also the classic bread mould. (source)​

Is it dangerous?

No. Not a toxin producer; it wrecks the fruit rather than poisoning it.

What to do with it

Throw it out, and move the rest somewhere cooler. Once it is sporulating, everything nearby has been dusted, and warmth is what decides whether those spores get anywhere.

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Penicillium digitatum (green mould)

How to recognize it

Green powdery mould.
It starts as a soft, water-soaked patch with no clear edge, growing into a white rot several centimetres across; the green colour arrives last, as spores form, usually with a thick white rim of mycelium still around it. Eventually the fruit dries out to a hollow shell (source)

Where it comes from

​A soil fungus of citrus-growing regions, carried into the packing house and the storage room, where the spores persist in the air and on surfaces.
​Infection almost always happens after picking, through damage made during harvesting and handling. (source)

How it spreads

Airborne spores, and from fruit to fruit in contact during storage. It can only infect through damage in the rind, so an undamaged peel keeps it out.

Mostly found on

All citrus. This is the more aggressive of the two citrus moulds and the main cause of citrus decay worldwide. (source)

Is it dangerous?

No known mycotoxin. The fruit is inedible long before anything else matters.

What to do with it

Throw out the mouldy fruit and check the others for dents and soft spots. Keep citrus loose rather than bagged, so the fruit are not pressed against each other.​

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Penicillium italicum (blue mould)

How to recognize it

Blueish green powdery mould. It starts as a soft, water-soaked patch with no clear edge, growing into a white rot several centimetres across, after which the blue-green powder appears and the fruit eventually turns slimy. 

Where it comes from

A soil fungus carried into the packing house and the storage room, where the spores persist in the air and on surfaces. Infection almost always happens after picking, through damage made during harvesting and handling. (source)

How it spreads

Airborne spores, and from fruit to fruit in contact during storage. It can only infect through damage in the rind. Its trick is temperature: below ten degrees it grows faster than the green mould, so in cold storage, and in your fridge, this is the one more likely to take over. (source)

Mostly found on

All citrus, lemons in particular. Less common than the green mould, but harder to avoid in the cold.​

Is it dangerous?

No. It makes a minor compound, but the fruit is inedible long before that matters.

What to do with it

Throw it out, and check the rest for dents and soft spots.
​This is the one the fridge does not slow down, so do not count on cold to buy you time.

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Alternaria alternata (black rot)

How to recognize it

Dark, sunken spots. It starts as small water-soaked circles that darken from the centre out, then sink into the flesh, often in concentric rings, and in humid conditions get a dense dark green to black velvety coat. Less fuzzy than the others; more a dark blemish that pulls the surface inwards. (source)​

Where it comes from

​It is already there. Alternaria lives on the surface of many plants, tomatoes included, as part of their ordinary flora, and it overwinters in soil, seed and crop debris. The spores travel easily in dry, warm, windy weather. So the question is never whether it has arrived, only whether it finds a way in. (source)

How it spreads

Spreads through air, an opportunist rather than an aggressor: it does not force its way in but takes whatever opening it finds, from bruises and cracks to tissue already damaged by sunscald or chilling. It also gets in through the stomata and lenticels, and through the open calyx of an apple. (source)​

Mostly found on

Tomatoes above all, where it is the most common cause of postharvest decay, plus peppers, cucumbers, melons, citrus, mango and apples. On brassicas it is a related species, A. brassicicola, which gives the same dark spots and sooty black patches on broccoli and cauliflower heads. (source)​

Is it dangerous?

Still being worked out. It makes alternariol and tenuazonic acid, among others, which EFSA treats as emerging mycotoxins: potentially hazardous, with the risk assessment incomplete. Its spores are also a well-documented airway allergen. (source)​

What to do with it

Discard rather than trim, and look at the neighbours rather than binning them. Its spores are on them anyway; what matters is whether any of them have a crack or a bruise for it to get into. Use those first, keep the sound ones.
In apples it has one trick the others do not: entering through the calyx, it can rot the core from the inside with no external sign at all. (source)​

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Bacterial soft rot (Pectobacterium and relatives)

How to recognize it

Soft, slimy and smelly, with no fuzz at all.
Not a mould. This is bacterial soft rot. It hollows out a carrot from the inside while leaving the skin intact, and on a potato it makes a sharply outlined patch with a dark border.
​Early on it has no smell; the stench comes once other bacteria move in. (source)

Where it comes from

​The soil. It is a common soilborne bacterium with a very wide host range, and it comes in on the vegetable itself, picked up in the field or in storage. Warmth and standing water are what set it off, which is why it does most of its damage in the weeks after harvest rather than before. (source)

How it spreads

Carried by moisture. It needs wounds or natural openings to get in, and free water to move, which is why it thrives in a sealed bag. (source)​

Mostly found on

Carrots, potatoes, onions, brassicas, peppers and leafy greens. It causes more loss than any other bacterial disease of stored produce. (source)​

Is it dangerous?

No. No mycotoxins here, since it is not a fungus at all. But the tissue is being dissolved from the inside and you would not want to eat it.​

What to do with it

Throw it out and dry off whatever was next to it. Moisture is what carries this one, so store the rest loose and unwrapped rather than bagged, and check them: a carrot can be hollow inside while the skin still looks fine.​

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Cut it out, or throw it out?

The question is not how big the patch is but how far the threads reach beyond it, and that depends on the tissue. Wet, soft flesh lets the network run well past what you can see. Dense, dry tissue holds it close to the surface.

Throw it out whole

all soft fruit and berries, tomatoes, cucumbers, courgettes, peppers, soft stone fruit, beans.
​
The threads reach well beyond the patch you can see, and there is nothing to cut back to.

Cut it out generously

hard cabbage, carrots, parsnips, celeriac, pumpkin and winter squash.

​Take a wide margin, two or three centimetres, and keep the knife out of the mouldy part so you do not carry spores into the clean flesh. (source)

It depends which part

broccoli and cauliflower, where the stem can be cut around but the florets cannot, so mould on the head means the head goes.

Lettuce and cabbage, where you strip the affected outer leaves.

Tomatoes, where mould on the calyx (the crown) is not the same as mould in the fruit.

Firm, but still goes whole

One exception to all of it: apples and pears go whole, however small the patch, because patulin spreads about a centimetre into flesh that still looks sound.
​The rest of the bowl is a different matter: this mould travels as airborne spores and needs a wound to get in, so check the others for bruises and cuts rather than discarding them. Use the damaged ones first.

Keeping mould at bay

A few things to keep in mind if you want to keep mould from taking hold.

Do buy undamaged

Most moulds need a wound or opening to get in.
​The most effective thing you can do happens in the shop.

Don't wash until eating

Surface water is what spores need to germinate.
Wash only just before eating.

Do let it breathe

A closed punnet holds humidity, and humidity is what turns white mycelium into grey fuzz. Open the lid, or move berries to a plate.

Do cool produce ​(but not all)

Cold slows moulds down, but it does not stop them. 
Rhizopus barely moves below five degrees. 
​Penicillium italicum however, grows faster below ten degrees.

Do take out the bad ones

One mouldy fruit sheds spores onto everything around it.
Take it out as soon as you see it, and check whatever was touching it.

Don't jostle the mouldy one

That powder on a mouldy peach is spores, and shedding them is the whole point of the fuzzy layer.
Move it gently, or slide a bag over it first, rather than carrying it across the kitchen. And don't sniff it to check: dry spores are a known allergen.

Do wash the bowl

Spores stay viable on dry surfaces for years, and your fruit bowl, with dust and sticky residue left behind is a perfect host.
​Wash well with hot water and soap, dry it properly before using again.
​The same goes for the fridge drawer.

The good moulds

The moulds on this page are not a separate category of bad fungi.
​They do what all moulds do: break things down.
​Aimed at the right thing, that same habit gave us penicillin and the best cheeses.

​The mould that saved more lives than any other

In September 1928 Alexander Fleming returned from holiday to a London laboratory he had left in some disorder.
​On a dish of Staphylococcus bacteria a mould had settled, and around it the bacteria had gone clear. He named the substance penicillin, after the genus.

That was the easy part. His strain made so little, and so unstably, that he gave up on it in 1931. Howard Florey and Ernst Chain worked out how to isolate and purify it a decade later, and the strain that finally made mass production possible was found in 1943 in a market in Peoria, Illinois, on a mouldy cantaloupe melon. Every industrial penicillin strain in use today descends from it.
​So the mould on your fruit and the mould in your medicine cabinet are not just relatives: one particular piece of mouldy fruit is where the whole industry started.

Which is not an argument for eating the apple. Fleming's mould was Penicillium rubens; the one on your apple is Penicillium expansum and the one on your orange Penicillium digitatum, and neither produces penicillin. They produce patulin.
​Even the right species yields only a trace, which is why Fleming abandoned his own strain, and the hard part was never finding the mould but isolating the compound inside it. A mouldy apple gives you none of the benefit and some of the toxin.

​The moulds we tamed

Two of the moulds on your cheeseboard are relatives of the ones on your fruit, and both have been reshaped by people until they stopped behaving like wild fungi at all.

Penicillium roqueforti makes the blue veins in Roquefort, Stilton and Gorgonzola.
Penicillium camemberti makes the white rind on camembert and brie, and it is the more remarkable of the two: a domesticated form of P. commune, an ordinary cheese spoiler, selected over generations the way dogs were selected from wolves. It is whiter than its wild relatives, grows faster in cellar conditions, and makes far less toxin.

And the selection went too far. The industry wanted a rind that was purely white and silky, so it bet everything on a single albino strain. That strain could already not reproduce sexually, and it has gradually been losing the ability to make asexual spores as well. There is no genetic diversity left to work with, which is why researchers warn that camembert as we know it is on borrowed time. Blue cheese is in better shape: a wild population in Bleu de Termignon can still reproduce sexually, and may yet restore some variety to P. roqueforti.
​
Safe to eat? P. roqueforti can make roquefortine and PR toxin in pure culture, but PR toxin is unstable in cheese and breaks down, and roquefortine turns up only at low levels. Domestication did part of that work: the selected strains make far less of these compounds than their wild cousins.

Which is the whole page in one genus.
​Penicillium gave us an antibiotic, a cheese and a toxin.
Same family, different species, and the difference between them is the difference between dinner and the compost heap.

That difference is what this page was for.
​Not to make you throw more away, but to let you tell the grey fuzz on a strawberry from the white rind on a brie, and the apple that has to go from the cabbage that only needs a wide cut.

Sources

Williamson, B., Tudzynski, B., Tudzynski, P. and Van Kan, J.A.L. (2007), Botrytis cinerea: the cause of grey mould disease. Molecular Plant Pathology, 8: 561-580. - ​visit

Bolton MD, Thomma BP, Nelson BD. Sclerotinia sclerotiorum (Lib.) de Bary: biology and molecular traits of a cosmopolitan pathogen. Mol Plant Pathol. 2006 Jan 1;7(1):1-16. doi: 10.1111/j.1364-3703.2005.00316.x. PMID: 20507424- visit

UC Statewide IPM Program. Botrytis Rot (Gray Mold) on Strawberries. University of California. - visit

O'Neill T. Grey Mould (Botrytis cinerea) of Tomato. ADAS / AHDB. - pdf

Bolton MD, Thomma BP, Nelson BD. Sclerotinia sclerotiorum (Lib.) de Bary: biology and molecular traits of a cosmopolitan pathogen. Mol Plant Pathol. 2006 Jan 1;7(1):1-16. doi: 10.1111/j.1364-3703.2005.00316.x. PMID: 20507424.- visit

Business Queensland. Sclerotinia Rot. Queensland Government. - visit

Hort Innovation. Sclerotinia Rot of Vegetable Crops. Fact sheet VG15010; 2018. - pdf

Palou, Lluís. (2014). Penicillium digitatum, Penicillium italicum (Green Mold, Blue Mold). 10.1016/B978-0-12-411552-1.00002-8.  - visit

Luciano-Rosario D, Keller NP, Jurick WM 2nd. Penicillium expansum: biology, omics, and management tools for a global postharvest pathogen causing blue mould of pome fruit. Mol Plant Pathol. 2020;21(11):1391-1404. doi:10.1111/mpp.12990. - visit

Taniwaki MH, Hoenderboom CJOM, DE Almeida Vitali A, Eiroa MNU. Migration of Patulin in Apples. J Food Prot. 1992 Nov;55(11):902-904. doi: 10.4315/0362-028X-55.11.902. PMID: 31084053. -  visit

Arnau Vidal, Salma Ouhibi, Ridha Ghali, Abderrazek Hedhili, Sarah De Saeger, Marthe De Boevre, The mycotoxin patulin: An updated short review on occurrence, toxicity and analytical challenges, Food and Chemical Toxicology, Volume 129, 2019, Pages 249-256, ISSN 0278-6915, https://doi.org/10.1016/j.fct.2019.04.048 - visit

Cornell University Plant Pathology, Hudson Valley Laboratory. Blue Mold - visit

Bhatta UK. Alternative Management Approaches of Citrus Diseases Caused by Penicillium digitatum (Green Mold) and Penicillium italicum (Blue Mold). Front Plant Sci. 2022;12:833328. Published 2022 Feb 22. doi:10.3389/fpls.2021.833328 - visit

Konstantinos Papoutsis, Matthaios M. Mathioudakis, Joaquín H. Hasperué, Vasileios Ziogas, Non-chemical treatments for preventing the postharvest fungal rotting of citrus caused by Penicillium digitatum (green mold) and Penicillium italicum (blue mold), Trends in Food Science & Technology, Volume 86, 2019, Pages 479-491, ISSN 0924-2244, https://doi.org/10.1016/j.tifs.2019.02.053 - visit

Pacific Pests, Pathogens and Weeds. Citrus Storage Moulds. - visit

Qianqian Liu, Qingmin Chen, Hu Liu, Yamin Du, Wenxiao Jiao, Fei Sun, Maorun Fu, Rhizopus stolonifer and related control strategies in postharvest fruit: A review, Heliyon, Volume 10, Issue 8, 2024, e29522, ISSN 2405-8440, https://doi.org/10.1016/j.heliyon.2024.e29522 - visit

Wu G, Fan C, Zang X, Wang B, Chen Y, Kou J, Zhu G. The Control of Postharvest Soft Rot Caused by Rhizopus stolonifer on Kokei No. 14 Organic Sweet Potato Roots by Carvacrol, Thymol, and Thyme Oil. Foods. 2025; 14(7):1273. https://doi.org/10.3390/foods14071273 - visit

Mersha, Zelalem, Shouan Zhang, and Jerry A. Bartz. 2013. “A Postharvest Fruit Rot Caused by Alternaria Sp. On Imported Plum Tomatoes in South Florida: PP303, 1 2013”. EDIS 2013 (2). Gainesville, FL. https://doi.org/10.32473/edis-pp303-2013 - visit

​Troncoso-Rojas, Rosalba & Tiznado-Hernández, Martin. (2014). Alternaria alternata (Black Rot, Black Spot). 10.1016/B978-0-12-411552-1.00005-3. - visit

Fraeyman S, Croubels S, Devreese M, Antonissen G. Emerging Fusarium and Alternaria Mycotoxins: Occurrence, Toxicity and Toxicokinetics. Toxins (Basel). 2017;9(7):228. Published 2017 Jul 18. doi:10.3390/toxins9070228 - visit

Pacific Northwest Plant Disease Management Handbook. Broccoli: Alternaria Diseases. - visit

Niem J, Miyara I, Ettedgui Y, Reuveni M, Flaishman M, Prusky D. Core rot development in Red Delicious apples is affected by susceptibility of the seed locule to Alternaria alternata colonization. Phytopathology. 2007;97(11):1415-1421. doi:10.1094/PHYTO-97-11-1415 -  visit 

UC Statewide IPM Program. Bacterial Soft Rot: Carrot. University of California. - visit

Utah State University Extension. Bacterial Soft Rot. - visit

Houbraken J, Frisvad JC, Samson RA. Fleming's penicillin producing strain is not Penicillium chrysogenum but P. rubens. IMA Fungus. 2011 Jun;2(1):87-95. doi: 10.5598/imafungus.2011.02.01.12. Epub 2011 Jun 7. PMID: 22679592; PMCID: PMC3317369 - visit

Hoff B Pöggeler S, Kück U 2008. Eighty Years after Its Discovery, Fleming's Penicillium Strain Discloses the Secret of Its Sex. Eukaryot Cell7:.https://doi.org/10.1128/ec.00430-07 - visit

Gaynes R. The Discovery of Penicillin - New Insights After More Than 75 Years of Clinical Use. Emerg Infect Dis. 2017;23(5):849-853. doi:10.3201/eid2305.161556 - visit

Ropars J, Didiot E, Rodríguez de la Vega RC, Bennetot B, Coton M, Poirier E, Coton E, Snirc A, Le Prieur S, Giraud T. Domestication of the Emblematic White Cheese-Making Fungus Penicillium camemberti and Its Diversification into Two Varieties. Curr Biol. 2020 Nov 16;30(22):4441-4453.e4. doi: 10.1016/j.cub.2020.08.082. Epub 2020 Sep 24. PMID: 32976806 - visit

CNRS. French Cheese Under Threat. CNRS News. — visit

US Environmental Protection Agency. Attachment I: Final Risk Assessment of Penicillium roqueforti. Office of Pollution Prevention and Toxics; February 1997 - pdf