What Is a Sclerotium? A Fungal Survival Structure

A sclerotium is a compact, hardened mass of fungal tissue that allows a fungus to survive harsh conditions like drought, extreme cold, or nutrient scarcity. Think of it as the fungal equivalent of a seed’s tough coat or a bear’s hibernation: a way to ride out bad times and spring back to life when conditions improve. Sclerotia are found across a surprisingly wide range of fungi, from devastating crop pathogens to species prized in traditional medicine, and their toughness has made them both a persistent headache for farmers and a subject of genuine fascination for biologists.

What a Sclerotium Looks Like Inside

From the outside, most sclerotia look like small, dark pellets or irregular lumps, anywhere from a fraction of a millimeter to several centimeters across depending on the species. Some resemble peppercorns; others look like tiny potatoes. But cut one open and you find a surprisingly organized structure. Studies of sclerotia from ectomycorrhizal fungi have revealed distinct internal layers: an outer pigmented rind, inner and outer cortex regions, and a large central medulla made up of tightly packed cells filled with stored nutrients including glycogen, protein, and lipids.¹ The rind acts as a protective shell, while the interior serves as a pantry stocked for long-term dormancy.

The cell walls of sclerotia contain chitin and glucans, the same structural molecules found in regular fungal cell walls but typically deposited more thickly. The extracellular spaces between cells are also packed with glucans, and the main energy reserves inside the cells are glycogen, protein, polyphosphate, and lipid.² This dense stockpile of fuel is what lets the sclerotium sit dormant in the soil for months or years and still have enough energy to germinate when the time comes. During the formation process, the fungal cells themselves undergo visible changes: the cytoplasm pulls away from the cell wall, the wall thickens, and in some species crystalline structures begin forming inside the cells at the tips of the medullary hyphae.³

Why the Dark Color Matters

Most sclerotia are dark brown or black, and that pigmentation is not accidental. The color comes from melanin, the same broad class of pigments that darkens human skin. In fungi, melanin serves as a shield against ultraviolet radiation and other environmental stressors. When researchers knocked out genes responsible for melanin production in the crop pathogen Sclerotinia sclerotiorum, the resulting sclerotia developed abnormally and were far more vulnerable to UV light.⁴ The melanin-deficient mutants also grew more slowly and produced less overall biomass, suggesting that melanin contributes to more than just UV protection. Interestingly, losing melanin did not make the fungus any less able to infect plants. That is a useful detail: the pigment protects the resting sclerotium in soil, but once the fungus is actively parasitizing a host, it relies on other tools.

Melanin also plays a role in the sclerotium’s resistance to microbial attack and enzymatic breakdown, which helps explain why sclerotia can persist in soil for remarkably long periods. In the case of Sclerotinia sclerotiorum, sclerotia can remain viable for years under unfavorable conditions.⁵ That kind of longevity in soil is exactly what makes sclerotium-forming pathogens so difficult to eradicate from agricultural fields.

What Triggers a Fungus to Form Sclerotia

Sclerotia are not produced continuously. They form in response to stress, and the signals that trigger their development are more complex than simply “things got bad.” Research on the morel fungus Morchella importuna found that reactive oxygen species, particularly hydrogen peroxide, play a central role. In areas where hydrogen peroxide accumulated heavily, sclerotia did not form. Instead, the fungus initiated sclerotial development in adjacent zones with lower oxidative stress. The signaling pathway involved is MAPK, a well-known cellular communication route, and the fungus appeared to pass signals from high-stress areas to lower-stress areas where sclerotia could safely develop. A difference in hydrogen peroxide concentration of about 20 millimoles was enough to kick off the process.⁶

As sclerotia mature, the expression of genes for superoxide dismutases, enzymes that neutralize reactive oxygen, ramps up. So the developing sclerotium is actively managing its own internal chemistry to protect itself during formation. Other triggers documented across various fungal species include nutrient depletion, changes in light exposure, and temperature shifts. The common thread is that sclerotia form when the fungus “decides” that continued active growth is no longer viable and pivots to a survival strategy instead.

Two Ways to Wake Up

When conditions improve, a sclerotium can germinate, but not all germination looks the same. In Sclerotinia sclerotiorum, one of the best-studied sclerotium formers, there are two distinct modes. In carpogenic germination, the sclerotium produces small mushroom-like structures called apothecia, which release airborne spores that can infect plants from above. In myceliogenic germination, the sclerotium simply sprouts new fungal filaments (hyphae) that grow through the soil and infect nearby roots or stems directly.⁷

Which mode a sclerotium uses depends partly on what it has experienced. Researchers found that when sclerotia of a Canadian isolate of S. sclerotiorum were exposed to sub-freezing temperatures (around −10 or −20°C) for four weeks, their germination switched from carpogenic to myceliogenic. Sclerotia kept at above-freezing temperatures retained their ability to germinate carpogenically.⁸ This has real consequences for disease management: the type of infection a farmer faces the following season can depend on how cold the previous winter was.

A Major Problem in Agriculture

Sclerotinia sclerotiorum alone can infect over 400 plant species, including soybeans, sunflowers, canola, and lettuce. The disease it causes, white mold, is a persistent economic headache in temperate agriculture worldwide. Because sclerotia can survive in soil for years, simply rotating crops for a season or two often is not enough to break the cycle. The pathogen’s life cycle is relatively simple: it causes disease either through direct soil infection from myceliogenically germinated sclerotia or through airborne spores released from carpogenically germinated ones.⁷ That simplicity is part of what makes it so effective: the sclerotium is both the waiting stage and the launch pad.

Another sclerotium-forming pathogen worth knowing about is Verticillium dahliae, which produces microsclerotia: tiny versions, often just clusters of a few dozen melanized cells. These microsclerotia are the primary survival structures for Verticillium wilt, a devastating disease of tomatoes, strawberries, cotton, and many other crops. Studies of cauliflower fields found that microsclerotia tend to be distributed in aggregated patterns in the soil, influenced partly by bed preparation and tillage practices.⁹ Knowing where microsclerotia concentrate in a field can help target management, but because they are so small and so numerous, eradication is practically impossible.

Ergot and Its Infamous Sclerotia

Perhaps the most historically notorious sclerotium belongs to Claviceps purpurea, the ergot fungus. Ergot sclerotia replace the grain kernels in cereal crops like rye, producing dark, elongated structures packed with alkaloids that are toxic to humans and livestock. Ergot poisoning, or ergotism, caused devastating outbreaks in medieval Europe, producing symptoms ranging from convulsions to gangrene. The alkaloids in ergot sclerotia vary depending on the host grass. Research on grass seed production areas in the U.S. Pacific Northwest found that ergocornine and its related form were the dominant alkaloids in sclerotia from Kentucky bluegrass, while ergotamine dominated in sclerotia from perennial ryegrass.¹⁰ These different alkaloid profiles matter because the toxicity and physiological effects differ between compounds, and grain contamination thresholds have to account for which specific alkaloids are present.

Fighting Sclerotia With Biology and Heat

Because sclerotia are so durable, conventional approaches like fungicide sprays often fail to reach them in the soil. Two alternative strategies have shown genuine promise: biological control and soil solarization.

The fungus Coniothyrium minitans is a mycoparasite, meaning it parasitizes other fungi. It specifically targets the sclerotia of Sclerotinia sclerotiorum, colonizing and destroying them in the soil. Its spores remain dormant until they detect sclerotia nearby, then germinate and invade, deploying a broad arsenal of cell-wall-degrading enzymes to break into the host.¹¹ Genomic studies have identified at least 17 genes for fungal-cell-wall-degrading enzymes that are turned up during parasitism, alongside genes for nutrient uptake, detoxification, and stress response.¹² In soybean field trials in Indiana, treatment with C. minitans confirmed that mycoparasitism is the primary way it reduces sclerotia populations.¹³

Soil solarization, the practice of covering moist soil with clear plastic sheeting during hot weather, takes a different approach. The trapped solar heat raises soil temperatures enough to weaken sclerotia, especially in the top five centimeters. But the killing blow often comes not from the heat itself but from soil microbes that attack the weakened sclerotia afterward. Solarization reduced both the number of viable sclerotia and their ability to produce the spore-releasing apothecia, with significant effects detectable down to 15 centimeters deep.¹⁴ The combination of heat stress and microbial opportunism makes solarization more effective than either factor alone.

Far More Common Than You Might Think

Sclerotia are not a quirk of a few pest species. A comprehensive survey found sclerotium-forming fungi scattered across 85 genera in 20 orders of higher fungi, and the evidence suggests that the ability to form sclerotia evolved independently at least 14 separate times.¹⁵ That level of convergent evolution says something important: when multiple distantly related lineages independently arrive at the same solution, the solution must confer a powerful survival advantage. Sclerotia are found in saprophytes that decompose dead matter, in mycorrhizal fungi that partner with tree roots, and in aggressive plant pathogens. The trait has been reinvented again and again across the fungal tree of life.

Sclerotium-like structures also show up outside the true fungi. The acellular slime mold Fuligo septica, which is not a fungus but a protist, forms sclerotia when its feeding stage (the plasmodium) is starved and slowly dried. These slime mold sclerotia also contain melanin-like pigments, and the darker the sclerotium, the stronger the melanin signal detectable by electron paramagnetic resonance. However, only the lighter yellow sclerotia were able to regenerate into viable plasmodia, suggesting that heavy melanization may come at a cost to revival ability.¹⁶ The parallel between fungal and slime mold sclerotia is a striking case of convergence across entirely different kingdoms of life.

Sclerotia in Traditional Medicine

Not all sclerotia are villains. The sclerotium of Wolfiporia cocos (commonly called Poria cocos or fu ling) has been used in traditional Chinese medicine for centuries. This large, underground sclerotium grows in association with pine tree roots and can weigh several kilograms. Its two main classes of active compounds are triterpenes and polysaccharides, which have been linked to a range of pharmacological effects including anti-tumor, anti-inflammatory, antioxidant, and immunomodulatory activity.¹⁷ More recent research has expanded the list of potential benefits to include kidney protection, improved digestive function, better sleep quality, and modulation of blood sugar and lipid levels.¹⁸

Another medicinally valued sclerotium comes from Polyporus umbellatus (also known as Grifola umbellata), called zhu ling, which has a long history in East Asian herbal traditions for promoting urination and reducing edema. The tiger milk mushroom (Lignosus rhinocerus) of Southeast Asia is yet another example: its sclerotium is the part traditionally used, not the fleeting fruiting body. These examples illustrate that the same biological strategy fungi evolved for survival, packing dense reserves into a tough resting structure, also makes sclerotia rich sources of bioactive compounds.

A Hidden Pool of Soil Carbon

The durability that makes sclerotia such a headache for farmers also makes them relevant to soil science and carbon storage. Cenococcum geophilum is one of the most widespread ectomycorrhizal fungi on the planet, partnering with tree roots in forests across the globe. Unlike most mycorrhizal fungi, it is a prolific producer of sclerotia. Because these sclerotia have a high carbon content, are heavily melanized, and persist for long periods, they represent a substantial pool of belowground carbon that is rarely measured in standard soil surveys.

A recent study found that chronic nitrogen deposition, the kind caused by agricultural runoff and air pollution, significantly reduces the standing biomass of Cenococcum sclerotia in forest soils. The observed losses were substantial, on the order of 84 to 127 grams per square meter, representing meaningful reductions in a recalcitrant carbon pool.¹⁹ The implication is that nitrogen pollution may be quietly undermining a form of long-term carbon storage that most climate models do not account for. Sclerotia, in other words, are not just interesting biology; they may be a small but overlooked piece of the global carbon puzzle.

Why Sclerotia Are Easy to Overlook and Hard to Ignore

One reason sclerotia do not get more public attention is that they are not visually dramatic. They lack the appeal of a colorful mushroom cap or a sprawling mycelial network under a rotting log. They sit in the soil, dark and inert, doing nothing visible until conditions trigger them into action. But that very inconspicuousness is the point. A structure that draws no attention, resists UV light, shrugs off microbial attack, and carries enough stored energy to regenerate an entire fungal colony is an extraordinary piece of evolutionary engineering. Farmers, forest ecologists, pharmacologists, and carbon cycle scientists all have reasons to pay sclerotia more attention than they currently receive. These small, dense packages of fungal persistence quietly shape the ecology of soils, the economics of agriculture, and the potential of fungal medicine in ways that are only beginning to be fully appreciated.