Fungus in soil is a common issue that gardeners face, encompassing organisms from simple molds and mildews to larger mushrooms. While most fungi are beneficial decomposers that support soil health, a subset are pathogenic or nuisance species that can severely damage or kill plants. Addressing problematic fungi involves recognizing the signs of infection and implementing specific environmental and chemical strategies to remove the threat and prevent its return.
Recognizing Fungal Infections in Soil
Accurately diagnosing the infection is the first step in controlling a fungal problem, often done by observing both the soil and the affected plants. Direct visual signs on the soil surface include white, cottony, or gray-colored molds, which are often the fungus’s mycelium. Clusters of small mushrooms, particularly in container plants or mulched garden beds, also indicate active fruiting in the soil environment.
Plant symptoms provide a secondary indication of a root-based fungal issue. Young seedlings may exhibit “damping-off,” where the stem becomes water-soaked and constricted at the soil line, causing the plant to collapse and die. Established plants may show general wilting or chlorosis (a yellowing of the leaves), even when soil moisture appears adequate. These symptoms suggest the fungus is attacking the root system, leading to root rot and preventing the uptake of water and nutrients.
Environmental Conditions That Encourage Fungi
Problematic fungi thrive in specific environments, and understanding these conditions is necessary for effective prevention. The primary factor is excessive moisture, often resulting from overwatering or poor soil drainage. When soil pores are saturated with water, oxygen is pushed out, creating hypoxic conditions that favor the growth of pathogens like some Fusarium species.
Waterlogged soil encourages the rapid spread of water molds like Pythium and Phytophthora, which produce motile zoospores that swim through water films to infect new roots. Soil temperature is also a factor, as different pathogens have different optimal ranges. For example, some Pythium species are active in cool soils below 15°C, while others and Rhizoctonia prefer warmer, moist conditions between 25°C and 30°C. High ambient humidity and poor air circulation around plants further contribute by keeping the top layer of soil persistently damp.
Cultural Practices for Fungal Control
Non-chemical cultural practices are the preferred first line of defense for eliminating soil fungus by altering the environment to be inhospitable to pathogens. The most impactful adjustment is modifying watering habits and timing. Watering in the early morning (ideally between 6 AM and 10 AM) allows the soil surface and foliage to dry quickly as the sun rises, preventing fungal spores from germinating and spreading.
Improving soil aeration and drainage is crucial, especially in compacted or heavy soils. For lawns, core aeration (which removes small plugs of soil) is more effective than spike aeration because it reduces compaction and opens channels for oxygen and water. For containers, using a skewer to poke holes in the soil can temporarily improve aeration and reduce the risk of root rot. In cases of severe infection, physically removing the top inch of affected soil and replacing it with fresh, sterile potting mix can eliminate a significant portion of the fungal mass.
Targeted Treatments and Soil Sterilization
When cultural adjustments are insufficient, more direct treatments are necessary to eliminate the fungal infection. Common household remedies can treat mild to moderate cases. A diluted solution of 3% hydrogen peroxide (one part peroxide to four parts water) can be poured directly into the soil to oxygenate the root zone and kill fungal cells on contact. Baking soda (sodium bicarbonate) can also be used as a soil drench or spray, typically mixed at one teaspoon per quart of water, as its alkalinity disrupts fungal growth.
For aggressive intervention or to prepare soil for susceptible plants like seedlings, sterilization is an option. For small batches of container soil, heat sterilization can be achieved by baking moist soil in an oven at 180°F (82°C) for 30 minutes, which kills most pathogens. For larger garden beds, solarization involves covering the moist soil with clear plastic sheeting during the hottest months, trapping solar energy to raise the soil temperature to lethal levels (above 140°F or 60°C) over several weeks. Chemical fungicides should be used as a last resort, following all label instructions, as they directly target and kill fungal organisms.