Fungi fall into two broad camps when it comes to what they feed on: parasitic species that attack living plants, and saprophytic species that colonize non-living materials like wood, drywall, fabric, and food. Some fungi, like certain Fusarium species, can do both. Understanding which fungi belong to each group helps whether you’re dealing with a wilting tomato plant or a moldy basement wall.
Fungi That Attack Living Plants
Plant-parasitic fungi are responsible for some of the most devastating crop losses worldwide. They penetrate living tissue using specialized structures and enzymes, then feed on the plant from the inside. A molecular plant pathology survey ranked the most economically destructive species, and the list reads like a global threat assessment for food security.
Rice blast, caused by Magnaporthe oryzae, is the single most destructive disease of rice and also strikes barley, wheat, and millet. Fusarium oxysporum causes vascular wilt across a remarkably diverse range of crops, from tomatoes and melons to cotton and bananas. Botrytis cinerea, the gray mold familiar to gardeners and winemakers alike, can infect more than 200 plant species and is especially destructive on mature or aging tissue. Rust fungi in the Puccinia group cause three distinct diseases on wheat alone: stem rust, stripe rust, and leaf rust. And virtually every crop grown anywhere in the world is susceptible to at least one species of Colletotrichum, which causes major losses in fruits, vegetables, ornamentals, and staple crops like bananas, cassava, and sorghum.
Powdery mildew, caused by Blumeria graminis, coats wheat and barley leaves in a white powdery film. Corn smut, caused by Ustilago maydis, produces grotesque gray galls on ears of corn (though in Mexican cuisine, those galls are a delicacy called huitlacoche). Fusarium graminearum is highly destructive to all cereal species and contaminates grain with toxins that make it unsafe for humans and livestock.
How Plant Fungi Invade Their Hosts
Plant-parasitic fungi don’t just land on a leaf and start eating. Many of them build a specialized infection structure called a haustorium, a tiny feeding organ that pushes through the plant’s cell wall without killing the cell. The haustorium invaginates the plant’s own membrane, creating an intimate interface where the fungus siphons off sugars and nutrients while the plant stays alive, at least temporarily. Rust fungi and powdery mildews both rely on this strategy.
The haustorium does more than steal food. It also delivers proteins called effectors into the host cell, which suppress the plant’s immune defenses and redirect its metabolism to benefit the fungus. This is why infected plants often look wilted, stunted, or discolored long before they actually die. The fungus is quietly hijacking the plant’s internal chemistry. Other fungi take a more aggressive approach, secreting enzymes that dissolve cell walls outright and killing tissue as they advance.
Fungi That Grow on Inanimate Objects
The molds you find on walls, furniture, food, and fabrics are saprophytes. They feed on dead organic matter by secreting enzymes externally, breaking down complex materials into simple sugars they can absorb. Several genera dominate indoor environments.
Penicillium is one of the most common indoor molds, growing on spoiled food, walls, insulation, carpet, and mattresses. Aspergillus thrives in oxygen-rich environments and turns up on food, air conditioning systems, and damp walls. Cladosporium favors old fabrics, wood, floorboards, and painted surfaces. Chaetomium targets damp or water-damaged buildings, especially drywall. Aureobasidium colonizes painted surfaces, window frames, and caulk. Acremonium appears on window sealants, humidifiers, drain pans, and wallpaper. Serpula, the dry rot fungus, destroys wooden structures in damp conditions.
Then there’s Stachybotrys chartarum, commonly called black mold. It specifically requires high-cellulose materials to grow: fiberboard, the paper facing on gypsum board, straw, hay, and plant debris. It needs sustained moisture and temperature fluctuations. Prolonged exposure to heavy black mold growth has been associated with respiratory symptoms (nasal irritation, coughing, wheezing, chest tightness), headaches, concentration problems, sleep difficulties, skin rashes, and eye irritation.
How Fungi Break Down Non-Living Materials
Wood, paper, cotton fabric, and drywall all contain cellulose, the most abundant organic polymer on Earth. Lignin, the second most abundant, gives wood its rigidity. Both are complex molecules that resist breakdown, but fungi have evolved powerful tools to dismantle them.
White-rot fungi produce a cocktail of enzymes that attack lignin directly, including laccases and several types of peroxidases. They also generate hydrogen peroxide, which fuels a chemical reaction that produces highly reactive free radicals. These radicals attack cellulose and lignin indiscriminately, creating cracks and openings that allow the fungi’s larger enzymes to penetrate deeper into the material. Brown-rot fungi use a similar radical-based strategy. The result is that a piece of untreated wood or a damp cotton curtain gradually softens, discolors, and loses structural integrity as the fungus digests it from the surface inward.
Natural fibers like cotton (mostly cellulose) and wool are well-established targets for fungal degradation. Synthetic materials like polyester and nylon are more resistant, but not immune. Laboratory studies have documented initial steps of nylon degradation by white-rot fungi, and microscopy has confirmed fungal growth on all fiber types in mixed textiles, including synthetics. Whether fungi can fully metabolize synthetic polymers remains an open question, but they clearly colonize them.
What Fungi Need to Grow Indoors
Moisture is the single most important factor controlling mold growth on building materials. At typical room temperatures of 20 to 25°C, fungal growth on wood and starch-containing materials begins at around 78% relative humidity. Gypsum board requires about 86% relative humidity. Ceramic materials need above 90%. Some mold species can initiate growth at relative humidity as low as 75%, though this is near the absolute lower limit.
Cold temperatures raise the moisture threshold. At 5°C, wood needs 90% relative humidity before mold will grow, compared to 78% at room temperature. This is why mold problems tend to concentrate in warm, humid areas: bathrooms, basements, around leaky windows, and inside walls where condensation collects. Without sustained moisture, even the most aggressive mold species cannot establish themselves on any surface.
Protecting Plants and Indoor Spaces
For crops, fungicides remain the primary defense. The most widely used class, triazoles, works by blocking a key step in the production of ergosterol, a molecule fungi need to maintain their cell membranes. Triazoles are effective against leaf rust, leaf spots, and powdery mildew. Other fungicide classes target different parts of fungal cell biology, and farmers typically rotate between classes to slow the development of resistance.
For indoor environments, the strategy is simpler: control moisture. Keeping relative humidity below 75%, fixing leaks promptly, ensuring good ventilation in bathrooms and basements, and drying water-damaged materials within 24 to 48 hours will prevent nearly all indoor mold problems. Materials that have been saturated for extended periods, particularly cellulose-rich ones like drywall and carpet padding, are often better replaced than cleaned, since fungal growth can penetrate well below the visible surface.