Occasionally molds appear on the surface of growing media in containers. These are saprophytic fungi which live on dead plant material such as bark, peat, coir or compost. They are more likely to occur when the growing media remains wet for prolonged periods of time, especially during stretches of frequent overcast weather like we are experiencing the current time. They also occur due to overwatering or poor drainage. Excessive moisture creates conditions favorable for the mold fungi to grow on the surface of the media. These mold fungi will not hurt the plants or roots. They are not harmful to plants or people, but they can be unsightly. Although they are not harmful to the plants, in severe cases these molds can form a layer on the surface of the growing media and limit water penetration.
To control these molds, reduce irrigation and increase airflow to lower the relative humidity in the greenhouse. Growers that are concerned about appearance can scrape off the fungi. Loosening the compressed media will also help to inhibit the fungal development by drying it out.
These mold fungi should not be confused with algae (green slime), which are a more significant problem. Algae creates an ideal environment for fungus gnats and shore flies. It is a food source for fungus gnats and shore flies. Fungus gnats not only feed on algae and other fungal growth in the growing medium, but also on plant roots, and can transfer plant pathogens through their mouthparts. Algae can form an impermeable layer on the media surface making it difficult for water to penetrate into the growing media.
Geoffrey Njue, UMass Extension