What Causes Fungi Growth and Where Does It Thrive?

Fungi grow wherever moisture, organic matter, and a tolerable temperature range converge. Of these three requirements, water is the single most powerful driver: deny a fungus adequate moisture and it stalls no matter how warm or nutrient-rich its surroundings. But “tolerable” is a flexible word for organisms whose kingdom has colonized everything from Arctic soil to the walls of a damaged nuclear reactor. Understanding what launches fungal growth, what sustains it, and where fungi find conditions most favorable reveals a picture far more varied than the moldy bread on a kitchen counter might suggest.

Water Activity Is the Master Switch

Fungi do not need standing water to grow, but they do need water molecules available at the surface of whatever material they are colonizing. Scientists express this availability as “water activity,” a scale from zero (bone dry) to one (pure water). Most bacteria need water activity above about 0.90 to grow, while many fungi can push forward at levels as low as 0.61 to 0.85. Fungi dominate spoilage at those intermediate moisture levels not because they grow faster than bacteria under dry conditions, but because most bacteria simply cannot compete there at all.¹ That competitive vacuum is why dried fruits, cured meats, and improperly stored grains often go moldy rather than slimy.

Relative humidity in the surrounding air translates directly into water activity on a surface. Laboratory tests on the germination of fungal spores show the effect clearly: at 100% relative humidity, spore germination can exceed 95%, while dropping humidity substantially cuts germination rates and slows the extension of the germ tubes that allow hyphae to invade a substrate.² This is why bathrooms, basements, and any area with poor ventilation tend to be fungal hotspots indoors: relative humidity stays persistently high, keeping surface moisture available.

One subtlety often missed is that fungi generally need higher water activity to produce spores than to germinate them. A fungus can germinate and start growing at a lower moisture level than the one it needs to reproduce and spread further. This means a colony can establish itself during a brief damp spell and then go dormant, waiting for the next wet period to sporulate and colonize new territory.¹

Temperature and Its Interaction with Moisture

Most fungi thrive in a broad temperature band somewhere between about 15°C and 35°C, with the sweet spot for many common species sitting around 25–30°C. But temperature alone tells only part of the story, because its effect on fungal growth changes depending on how much moisture is available. Optimum water availability allows spores to germinate across the widest possible temperature window. Better nutrition does the same. In other words, when conditions are otherwise ideal, a fungus can tolerate cooler or warmer extremes than it otherwise would.¹

This interaction between temperature and moisture has real consequences for food storage. Low temperatures slow many molds, but the assumption that “cold equals safe” has limits. Species of Penicillium and Fusarium are generally more capable of growth at low temperatures than toxic species of Aspergillus. That means a refrigerated product stored at slightly elevated humidity might grow cold-tolerant molds that produce toxins such as patulin or zearalenone, even while Aspergillus species and their associated aflatoxins remain dormant.³

Oxygen, Carbon Dioxide, and pH

Fungi are generally aerobic: they need oxygen. But “need” is relative. Even when oxygen in the headspace drops to fractions of a percent, some heat-resistant molds can still grow, though their colonies are smaller and take much longer to appear. Research on heat-resistant molds found visible growth even at 0.03% oxygen, although those colonies were markedly smaller than those grown at higher oxygen levels.⁴ This matters for vacuum-packed or controlled-atmosphere food: reducing oxygen to around 2% depresses the production of certain mycotoxins, but complete inhibition of both growth and toxin production requires pushing oxygen below about 0.2% for some species.³

Elevated carbon dioxide also slows fungal growth, but the levels required are high. A 20% carbon dioxide atmosphere markedly depresses mold growth and aflatoxin production, yet for patulin- and sterigmatocystin-producing molds, concentrations of 40% CO₂ only partially suppress growth, and roughly 90% is needed to completely stop toxin production.³ In practical terms, modified-atmosphere packaging helps but is not foolproof on its own.

The pH of a substrate, meanwhile, has surprisingly little effect on whether a mold can grow, though it can strongly influence which toxins that mold produces. Most fungi tolerate a wide range of acidity, which is one reason they colonize such diverse materials.

What Fungi Feed On

Fungi cannot photosynthesize. They acquire nutrients by secreting enzymes that break down organic matter externally and then absorbing the resulting small molecules. The range of things fungi can digest is staggering: sugars, starches, proteins, fats, and in many species, the structural polymers that make up wood and plant cell walls.

Wood-decay fungi are a striking example. These organisms deploy specialized enzyme families to attack cellulose, hemicellulose, and lignin, the tough polymers that give wood its strength. Among the most prolific wood-decay genera is Armillaria, which produces a relatively high abundance of the enzymes responsible for breaking down these materials.⁵ The fungi commonly known as white-rot and brown-rot species divide the labor differently. White-rot fungi attack lignin and cellulose together, leaving pale, spongy residues, while brown-rot fungi preferentially break down cellulose and hemicellulose, leaving a brown, crumbly lignin skeleton behind. Machine-learning analysis of gene families has identified specific enzyme groups, including certain polysaccharide-digesting enzymes and peroxidases, that reliably distinguish the two strategies.⁶

This enzymatic versatility means fungi show up wherever organic material accumulates. Kitchens, compost heaps, fallen logs, damp drywall, leather shoes left in a closet: all of these offer digestible carbon sources. Even human skin and mucous membranes can serve as substrates. Candida species, for instance, are part of the normal oral flora in many healthy people and only cause infections when host defenses are compromised.⁷

How Fungi Spread

Growth at a single site is only half the story. Fungi are extraordinarily effective at getting their spores to new locations. Many disperse passively, carried by wind currents or splashed by raindrops. Others have evolved active launch mechanisms that are among the fastest movements in biology: pressurized squirt-gun structures, explosive stalks, and catapults driven by surface tension.⁸

Once airborne, spore survival and travel distance depend partly on how well the spore handles dehydration. Research on fungal spore folding shows that when conidia are exposed to low humidity, they undergo shape changes that help separate them from the parent colony and launch them into the air. As they lose water, the spores become lighter and more compact, slowing their settling speed and allowing them to travel farther on air currents.⁹ Dry, windy conditions that seem hostile to fungal growth are actually excellent for spore dispersal, setting the stage for colonization once moisture returns.

Where Fungi Thrive Outdoors

In natural ecosystems, the largest fungal communities tend to be in soil and decaying plant matter. The decomposition of leaf litter is a relay race among fungal groups. Fresh and recently fallen leaves are dominated by Ascomycota, the largest phylum of fungi, which accounted for about 71% of fungal sequences in one study of decomposing litter. But by 24 months of decomposition, Basidiomycota had taken over, representing roughly 60% of the community as they attacked the tougher structural compounds remaining in heavily decayed material.¹⁰

Fungi in forest soils do not just decompose dead material. Many form intimate partnerships with living tree roots. Ectomycorrhizal fungi extend networks of fine threads called hyphae into the soil around tree roots, dramatically expanding the root system’s effective reach for water and mineral nutrients. Research on temperate forests has found that trees associated with ectomycorrhizal fungi and trees associated with a different partnership type (arbuscular mycorrhizal fungi) use distinct foraging strategies: one type produces more roots in nutrient-rich patches, while the other produces more fungal hyphae.¹¹ The balance between decomposer fungi and these root-associated fungi also shifts with the amount of available leaf litter: saprotrophic fungi dominate where fresh organic material is plentiful, while ectomycorrhizal fungi dominate where those labile substrates are scarce.¹²

Indoor Environments and Building Materials

Indoors, fungi exploit many of the same principles, just on different substrates. Drywall (gypsum board) is one of the most vulnerable building materials. The paper facing on standard drywall provides a digestible carbon source, and the gypsum itself absorbs and holds moisture. Lab experiments have shown that drywall can support critical mold development in as little as one week at 95% relative humidity, while at 75% relative humidity the timeline extends to about 12 weeks. Other substrates, like mineral wool insulation, require slightly higher humidity (around 85%) before mold can establish.¹³

The practical lesson is that the substrate matters as much as the ambient humidity. A concrete wall at 80% relative humidity may stay clean while the drywall next to it is already colonized. This is why water damage is so destructive: it raises the moisture content of the most susceptible materials above their critical threshold. Fixing a leak but leaving damp drywall in place does not fix the mold problem, because the material itself holds enough water to keep the growth cycle going for weeks.

Crop Canopies and Agricultural Fields

Farmers deal with fungal disease on a massive scale, and the microclimate within a crop canopy turns out to be a powerful driver. Dense canopies trap moisture and extend the duration of leaf wetness, sometimes by many hours. In pea fields, researchers found that leaf wetness duration during rainfall periods averaged about 15 hours, and the wetness period inside the canopy lasted 3 to 10 hours longer than outside it. Only those prolonged wet periods inside the canopy were favorable for infection by the fungal pathogen studied.¹⁴

Irrigation amplifies this effect. In dry bean fields, higher irrigation levels led to elevated soil moisture in the top layer of soil, increased leaf wetness, and correspondingly higher incidence of white mold disease.¹⁵ Even mixing different cultivar heights within a field can alter the outcome. Heterogeneous mixtures of tall and short plants open up the canopy, reduce spore fluxes, and lower disease compared to dense, uniform stands.¹⁶ In other words, the physical architecture of the crop is itself a tool for managing fungal disease.

Fungi Inside Healthy Plants

Not all fungi living in plants are enemies. Endophytic fungi reside asymptomatically inside the tissues of healthy plants, and they are remarkably common. A modern working definition describes them as “asymptomatic microbial partners that are intimately associated and co-inhabit within healthy internal plant tissues with the ability to confer benefits, co-evolve and alter their lifestyle depending upon plant life stages and adverse conditions.”¹⁷ Some endophytes help their host plants resist drought, ward off insect pests, or tolerate toxic soils. Others can shift from a benign lifestyle to a pathogenic one if the plant becomes stressed. The relationship is dynamic, not fixed, and the same fungal species can behave differently depending on the plant’s health and environment.

Competition Between Fungi

Where multiple fungal species colonize the same substrate, competition for space and resources shapes which ones dominate. Ecological theory describes three main strategies. A fungus can claim unoccupied territory first, simply by arriving and germinating before competitors. It can outcompete neighbors by exploiting nutrients more efficiently. Or it can engage in direct combat: physically overgrowing a rival’s mycelium, or producing volatile or diffusible chemicals that suppress nearby species at a distance. Which strategy wins depends on a suite of environmental factors: temperature, pH, water availability, the gaseous atmosphere, the quality of the substrate, and even grazing by tiny soil invertebrates.¹⁸

This competitive landscape is one reason a piece of fruit left on a counter often develops a succession of different molds rather than one uniform blanket. Early colonizers grab the easy sugars; later arrivals bring enzymes suited to tougher compounds. The presence of other microorganisms, including bacteria, can also suppress fungal growth and toxin production on a substrate, a principle exploited in some biocontrol strategies for agriculture.³

Fungi in Extreme Environments

The fungal kingdom is ancient. Phylogenomic evidence and molecular dating place fungal origins hundreds of millions of years ago, with early diverging lineages evolving in aquatic environments before colonizing land alongside the first terrestrial plants. Some researchers have proposed that icy environments served as a transitional niche between water and emerged land during this colonization process.¹⁹ That long evolutionary history has given fungi time to adapt to niches that seem almost uninhabitable.

Perhaps the most striking example involves melanized fungi and ionizing radiation. Darkly pigmented fungi have been found thriving in the cooling water and walls of the destroyed Chernobyl reactor, on the exterior of space stations, and in the high-radiation highlands of Antarctica. Rather than merely tolerating radiation, these organisms appear to respond to it with enhanced growth, migrating toward radioactive sources. Their melanin pigments absorb a broad spectrum of electromagnetic radiation, and researchers have raised the possibility that this melanin functions analogously to chlorophyll, harvesting radiation energy for biological use.²⁰²¹ The idea remains speculative, but the phenomenon itself, fungal growth stimulated by radiation levels that would kill most organisms, is well documented.

Urban Heat Islands and Fungal Adaptation

Fungi are not just responding to extreme natural environments. Human-altered landscapes are reshaping fungal communities in real time. Cities create “heat islands” where pavement and buildings raise soil and air temperatures several degrees above surrounding rural areas. Decades of elevated temperatures in these urban cores appear to be driving measurable evolutionary shifts in common soil fungi.

Studies comparing urban and rural isolates of the same fungal species have found that urban strains of some species grow faster at warmer temperatures but slower at cooler ones, a pattern consistent with thermal specialization under sustained warmth.²² More recent work in Baltimore found that fungal isolates from warmer neighborhoods show greater heat tolerance and lighter pigmentation than isolates of the same species from cooler parts of the city. Lighter pigmentation may help cells stay cooler, consistent with known mechanisms of pigment regulation that modulate cell temperature. One opportunistic human pathogen, Rhodotorula mucilaginosa, from a warmer neighborhood survived more intense heat-ramp stress than isolates from a cooler area.²³

The concern here is medical as much as ecological. Most fungi cannot grow at 37°C, the internal temperature of the human body, and that thermal barrier is one of our key natural defenses against fungal infection. If urban warming selects for strains with higher heat tolerance, cities could become incubators for fungi better equipped to infect people. None of the Baltimore isolates could grow at a sustained 37°C, but their increased resistance to heat stress suggests the gap between environmental and body temperature is narrowing for some urban populations.²⁴

1
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ISME Communications. Environmental fungi from cool and warm neighborhoods in the heat island of Baltimore City show differences in thermal susceptibility and pigmentation