Soil fungus includes a diverse group of organisms; many are beneficial, aiding in nutrient cycling and decomposition. However, certain fungi are pathogenic, causing diseases that can severely harm or kill plants, particularly vulnerable seedlings. Pathogenic fungi, such as Fusarium, Pythium, and Rhizoctonia, cause common issues like root rot and “damping-off.” Proactively treating the soil before planting reduces the population of these harmful organisms, ensuring a healthy environment for new seeds or transplants. The goal of pre-planting treatment is to suppress pathogens without harming the overall soil structure or beneficial microbial life.
Assessing Soil for Pathogenic Fungi
Before treatment, determine if a pathogenic fungal problem exists. The most obvious indicator is a history of poor performance in the planting area, such as repeated crop failures, wilting, or the death of young plants. Pathogens often survive for years by forming hardened structures like sclerotia or thick-walled spores.
Visual inspection can also offer clues, as fungi like Phytophthora thrive in saturated, poorly drained soil. Excessive moisture or clay-heavy soil is a diagnostic indicator of high-risk conditions. Sending a soil sample to a diagnostic lab, often through a local university extension office, is the most accurate method. Advanced testing, like DNA-based quantification, can measure the level of specific fungal pathogens present, informing the necessity of treatment.
Physical Methods for Soil Sterilization
Physical methods use environmental forces to reduce the pathogenic fungal load without introducing chemical agents. Soil solarization is the most effective and widely practiced physical method, relying on the sun’s heat to pasteurize the top layer of soil. This process works by trapping solar energy under a clear plastic sheet, raising the soil temperature to levels lethal to many fungal pathogens and weed seeds.
To begin solarization, the planting area must first be cleared of all plant debris and tilled to a depth of six to twelve inches. Tilling breaks up large soil clumps, which allows for better heat penetration throughout the soil profile. Next, the soil needs to be thoroughly moistened to at least a one-foot depth, as wet soil conducts and retains heat more efficiently than dry soil.
A sheet of clear, UV-resistant plastic, typically one to four mils thick, is then laid flat over the prepared area, ensuring it makes close contact with the soil surface. The edges of the plastic must be completely buried in a trench or secured with heavy objects to seal the area, trapping the moisture and solar heat. During the hottest part of the year, usually a four to six-week period, the soil temperature under the plastic can reach up to 140°F in the top six inches, with temperatures between 102°F and 104°F being sufficient to kill many soil-borne organisms.
Biological and Organic Fungus Control
Biological control leverages the natural competition within the soil microbiome by introducing beneficial organisms that suppress or outcompete the harmful fungi. These biofungicides are live products containing specific strains of fungi or bacteria. The fungus Trichoderma species, for example, is a widely used beneficial agent that works through direct parasitism, where it attacks and consumes the hyphae of pathogenic fungi.
Certain beneficial bacteria, such as species of Bacillus, are also effective biofungicides that colonize plant roots and create a protective barrier. Bacillus bacteria suppress pathogens like Fusarium and Pythium through competitive exclusion, rapidly occupying the ecological niche the pathogens need to establish. They also produce antibiotic-like compounds that directly inhibit the growth of the disease-causing organisms.
Incorporating high-quality compost or compost tea into the soil before planting boosts the overall beneficial microbial life. Compost acts as a prebiotic, supplying the organic matter and diverse microbial community that naturally suppresses pathogen populations. This strategy helps create a suppressive soil environment where the sheer number of good microbes makes it difficult for pathogenic fungi to gain a foothold and cause disease.
Applying Chemical Fungicides Before Planting
Chemical fungicides are an option for pre-planting treatment, especially in cases of severe, confirmed soil infestation, but they require careful handling and application. These chemicals are designed to eradicate or suppress existing fungal spores and mycelium in the soil before the crop is introduced. Fungicides are broadly categorized as either contact, which remain on the soil surface to kill on contact, or systemic, which are absorbed by the plant’s roots after planting to offer internal protection.
For pre-plant applications, the product label will specify whether the chemical is a soil drench, a granular application, or a fumigant. It is necessary to follow the label instructions exactly, particularly regarding the application rate and the required pre-planting interval, which is the waiting period before seeds or transplants can be safely introduced. This interval allows the chemical residue to break down to a safe level, preventing phytotoxicity or harm to the new plants.
Personal protective equipment, such as gloves, protective eyewear, and sometimes a respirator, must be worn during mixing and application to minimize personal exposure. To prevent the development of fungicide-resistant fungal strains, professional growers often rotate between products with different modes of action. While chemical treatment offers a fast and potent solution, the emphasis must remain on caution and adherence to all safety and regulatory guidelines.