In organic gardening, preventing plant stress and environmental imbalances are important in controlling disease. The only plant diseases you can control effectively after the plants become infected are those caused by fungi. Some of the organic products and techniques listed here must be used with care:
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Solarization: Solarization captures the sun's heat under a sheet of clear plastic and literally bakes the soil, killing fungi and bacteria as well as weeds. Unfortunately, it also kills both good and bad microbes.
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Particle films: Particle films are made from fine clay particles that are mixed with water and sprayed directly on plants. Treated plants look like they've been misted with white spray paint. In addition to reducing disease infection, the material deters insect feeding, and helps protect plants against heat stress and sunburn. One brand is Surround.
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Botanical sprays: Some organic fungicides contain citric acid and mint oil. They are broad-spectrum products, killing a range of fungi and bacteria. One brand is Fungastop.
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Antitranspirants: These waxy or oily materials are designed to help evergreens maintain leaf moisture during winter months. Look for Wilt-Pruf and similar products, and follow the instructions on the label.
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Potassium bicarbonate: This natural chemical controls powdery mildew and some other diseases in roses, grapes, cucumbers, strawberries, and other plants. It also supplies some potassium fertilizer when sprayed on foliage. Follow label directions carefully.
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Fungal fungicides: Apply these fungicides to the soil before planting or water them into lawns and gardens. You can also apply these fungicides to foliage. These products contain viable fungi, so you must store them properly and use them according to label.
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Bacterial fungicides: Two fungicides derived from naturally occurring bacteria are Serenade (derived from Bacillus subtilis) and Sonata (derived from Bacillus pumilus). They boost the plants' natural immune systems and inhibit fungal germination and growth.
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Neem oil: This multipurpose pesticide thwarts black spot on roses; it also prevents powdery mildew and rust fungi, as well as insects and mites. Read the label to be sure that the product is labeled for the plant you want to treat.
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Sulfur: Useful for controlling nearly all fungus diseases on leaves and stems, you can dust sulfur powder directly on leaves or mix finely ground dust with water and a soapy wetting agent that helps it adhere to leaf surfaces.
Sulfur can cause leaf damage if it's applied within a month of horticultural oil, however, or when temperatures exceed 80 degrees Fahrenheit. It also lowers soil pH and harms many beneficial insects. Inhaled dust can cause lung damage.
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Copper: Copper sulfate controls many leaf diseases, including fungal and bacterial blights and leaf spots, but it is toxic to humans, mammals, fish, and other water creatures. It also can build up in the soil and harm plants and microorganisms. Use copper-containing products only as a last resort, and take full precautions to avoid poisoning yourself and others.