Understanding the Differences: Mold, Mildew, Lichens, and Algae

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Understanding the Differences: Mold, Mildew, Lichens, and Algae

June 11, 2024 Fred Hueston Comments Off

Understanding the Differences: Mold, Mildew, Lichens, and Algae

Frederick M Hueston  stoneforensics.com

When discussing infestations in homes, gardens, or on buildings, terms like mold, mildew, lichens, and algae often come up. While these organisms can appear similar and are often confused for one another, they each represent different types of growth with unique characteristics and implications. Understanding these differences is crucial for effective identification and management.


Mold refers to various types of fungi that grow in filaments and reproduce by forming spores that can travel through the air. Molds are an essential part of the environment, aiding in the decomposition of organic matter. However, indoors, molds can be problematic, growing on virtually any surface where moisture and oxygen are present. They appear in a range of colors, including black, white, orange, green, or purple.

Mold growth can lead to significant property damage and can exacerbate health issues, particularly respiratory problems. It thrives in damp, poorly ventilated areas such as basements, bathrooms, and kitchens. Eliminating moisture through proper ventilation and using dehumidifiers is key to controlling mold growth.


Mildew is a specific type of mold, typically forming flat growth that remains on the surface of a moist area. It is usually white or gray and appears powdery or fluffy. Mildew is most commonly found on damp surfaces, organic materials like paper, leather, and fabrics, and on agricultural plants.

While mildew can cause damage to the materials it grows on, its health risks are generally less severe than those of mold. Cleaning mildew is relatively straightforward and involves using a mildew-specific cleaner or a solution of bleach and water.


Lichens are complex life forms that arise from the symbiotic relationship between fungi and a photosynthetic partner, usually algae or cyanobacteria. This partnership allows lichens to survive in harsh environmental conditions, from arctic tundras to rocky deserts. Lichens are not harmful to what they grow on, but they can often be mistaken as a sign of decay.

Lichens appear as crusty patches or bushy growths on rocks, walls, trees, and roofs and can be many different colors, including greens, oranges, yellows, and more. While they do not damage the structures or plants they inhabit, their presence can be indicative of poor air quality, as lichens are sensitive to air pollution.


Algae are a diverse group of photosynthetic organisms that can be found in a variety of water bodies, from oceans to freshwater ponds. Algae can also grow on surfaces where moisture is present, such as on walls, roofs, or driveways. These growths are typically green or blue-green and can create slippery, hazardous surfaces.

Algae growth on buildings is primarily an aesthetic issue, but it can also lead to moisture retention and surface degradation over time. Control measures for algae include regular cleaning of affected areas and reducing moisture accumulation.

While mold, mildew, lichens, and algae might be lumped together as unwanted growths, they are distinct in their characteristics and the ways they interact with their environments. Mold and mildew are primarily concerns indoors due to health risks and property damage, while lichens and algae typically affect outdoor surfaces and are more of an aesthetic or structural concern. Proper identification and understanding of each can help in effectively managing their presence and mitigating their potential negative impacts.