Fungi produce spores that spread in soil when it's tilled or cultivated; some propel themselves through water in the soil using special zoospores.
Nematodes are microscopic round-bodied worms. Most are beneficial, but the plant-parasitic types cause wilting and stunting.
Bacteria enter plants through openings in roots or tissue.
Viruses must have a host (like nematodes) in order to multiply. Aboveground symptoms are most often caused by these bad actors assaulting the root system, but in certain cases infected soil splashes onto foliage, where the disease then takes hold. Tomato early blight, a disease that causes spots on leaves, is a familiar example of this.
"Plant diseases develop when three conditions align: There must be a susceptible host plant, a pathogen must be present, and the environment must be conducive. This is called the disease triangle," explains Andreas Westphal, Ph.D., a plant pathologist at Purdue University. The key to overcoming plant diseases in your garden is to make these conditions as unfavorable for disease as possible. "Certain soilborne diseases, such as those caused by Pythium and Phytopthora, favor saturated soil conditions, while a few (like potato scab and charcoal root rot) thrive in dry soil," says Brenna Aegerter, Ph.D., of the University of California Cooperative Extension in San Joaquin County.
Compost for competition.
Pathogens exist in soil?there's no getting rid of them. But you can keep the harmful microbes from multiplying and causing severe damage by encouraging a diverse population of good microbes, which suppress disease by outcompeting pathogens for nutrients and by feeding on bad guys themselves. Some beneficials even secrete antibiotics or chemicals that are harmful to pathogens. Add organic matter to your soil to increase the pathogen-fighting microbe populations up to 1,000-fold. Compost, the most valuable kind of organic matter, also acts as a source of food and shelter for the good microbes, which help prevent the germination of fungal spores. In addition, by promoting good drainage, compost eliminates the overly wet soil conditions many pathogens need. As it matures, compost becomes more suppressive, so use only your best black-gold compost. Apply organic matter that has not fully decomposed at least four months before planting.
Go with the resistance.
Choose varieties bred to resist diseases common in your area. You can tell which are disease-resistant by looking carefully at catalog descriptions, seed packs, and plant tags for initials such as "VFN," which mean the plant is resistant to Verticillium wilt, Fusarium wilt, and root-knot nematodes.
All in the family.
Pathogens build up steadily in soil if their favorite host plants are available year after year, notes Kenneth Johnson, Ph.D., a professor of plant pathology at Oregon State University. The solution, Johnson says, is to rotate each season what you plant in each section of your garden. Certain pathogens attack plants in the same family, so rotate to a different crop outside of that family. The longer your rotation cycle, the more protection you'll get. It takes five years or longer without peas or a susceptible host for the Fusarium fungus that causes wilt to die off, for example, and two years without pumpkins for the black rot pathogen to die.
A healthy diet.
Stressed plants with nutrient deficiencies or excesses don't withstand attacks as well as healthy, well-nourished plants. Get a soil test three to six months before planting season and follow the recommendations carefully. You may need to alter the soil's pH along with its nutrient levels. The pH directly affects the survival of both the pathogen and the plant. For instance, clubroot, a fungal disease that preys on cabbage, broccoli, and their kin, is more severe in acidic soil (a pH of 5.7 or less). Adding calcium to your soil controls soilborne diseases caused by Pythium, such as damping off. Crops for which this has proved effective include wheat, peas, beans, peppers, tomatoes, onions, and snapdragons. Recently, researchers in Ontario found that incorporating bone meal, soy meal, and poultry manure into the top 6 inches of soil significantly reduced the incidence of Verticillium wilt and common scab of potato.
Mulch as a shield.
A layer of organic mulch, such as straw or shredded leaves, prevents diseased soil from splashing onto foliage and keeps fruit off the bare ground, where pathogens might invade it. It also stops weeds, which stress plants by competing for nutrients and water. Weeds may also host plant diseases.
The sun cure.
So what can you do when despite all the preventive techniques, diseases just don't go away- Try solarization, suggests Aegerter. This technique uses the sun's energy to heat moist soil to a degree where plant pathogens cannot survive. Cover the affected area with a sheet of clear polyethylene during the warmest, sunniest part of the year, Aegerter says, for periods of four to six weeks. "The main limitation of solarization is that the land must be out of production for most of the summer," she points out. After this is done, incorporate organic matter to replace beneficial organisms that are killed in the process.
You can buy beneficial agents (sometimes found in high-quality compost) that you apply to your soil to fight specific pathogens. The fungus Trichoderma harzianum (sold as RootShield) kills the pathogen Rhizoctonia (one of the many fungi that cause damping-off). Trichoderma locates Rhizoctonia by a chemical the pathogen releases, then attacks the damaging fungi and destroys it. In a study testing biological fungicides on vinca plants, researchers at Clemson University found that greenhouse plants treated with SoilGard (Gliocladium virens) had excellent shoot and root growth and were the equal of those treated with chemical fungicides.
Among the most beneficial root-inhabiting organisms, antibiotic-producing mycorrhizal fungi (BioVam) cover plant roots to protect against pathogens, forming a "fungal mat," which also increases nutrient-uptake ability.