For years, gardeners have known of bacterial and fungal agents such as Bacillus thuringiensis (Bt) that attack insects. The next wave in microbial pesticides is biofungicides, which use naturally occurring fungi and bacteria to protect plants from harmful fungi.
Biofungicides work in several ways. They exude chemicals that inhibit the growth of fungal pathogens, they can out-compete fungi for nutrients, and some can attack and colonize invading fungi. Depending on the disease, they're most effective used as a preventive foliar spray, or to drench the soil. They can't stop a disease that's well established. Although they affect several plant pathogens, they are not broad-spectrum fungicides and don't appear to upset the soil's natural microbial balance. Four products are currently on the market.
AQ-10 (Ampelomyces quisqualis) is effective against powdery mildew. Use it on edibles such as grapes and strawberries as well as on ornamentals such as roses and crape myrtles. AQ-10 prevents powdery mildew from getting established; it will also parasitize newly established mildew. Apply it in late afternoon when humidity is high; it can last one to three weeks.
Use Mycostop (Streptomyces griseo-viridis), Rootshield (Trichoderma harzianum), and SoilGard (Gliocladium virens) to prevent a variety of seed, stem, and root rots, including fusarium wilt, pythium (damping off), rhizoctonia, and phytophthora. They are most effective applied as a preventive drench at sowing or planting on vegetables such as cucumbers and tomatoes and ornamentals such as daisies and geraniums.
Indoors, incorporate these fungi and bacteria into the potting mix. Moisten the mix, wait one to two days for them to get established, and then sow. They will protect seedlings for two to three weeks. Outdoors, reapply every five to six weeks while weather conditions favor fungal diseases.
SoilGard is available from Gardens Alive, (812) 537-8650, www.gardensalive.com. AQ-10 and Mycostop are available from Peaceful Valley Farm Supply, (888) 784-1722, www.groworganic.com. All four are available from Harmony Farm Supply, (707) 823-9125, www.harmonyfarm.com.