Commercial products containing potassium bicarbonate (such as GreenCure) and homemade baking soda (sodium bicarbonate) sprays prevent fungal spores from establishing themselves on plants and may even prevent established fungi from continuing to develop. Dissolve 1 teaspoon of baking soda and 1 drop liquid soap in 2 quarts of water, mix well, and spray on plants. A higher concentration isn't better: If these sprays are too concentrated, they may burn plant foliage.
An oil extracted from the seeds of the neem tree is an ingredient in many products formulated to kill a wide variety of pest insects. Neem oil also is effective at killing some types of fungal disease spores.
Direct contact with sulfur prevents the development of disease organisms. However, it also damages important soil microorganisms and beneficial insects and is moderately toxic to mammals, including humans. Apply sulfur sparingly, and always take appropriate safety precautions.
Both a plain spray mix of elemental sulfur and mixtures of sulfur and other substances are effective preventive fungicides. Powdered sulfur is almost insoluble in water. Wettable sulfur has been finely ground with a wetting agent and is easier to use. Liquid sulfur is the easiest to dissolve. Sulfur also can be applied as a dust or as a fumigant.
Adding lime to sulfur increases its effectiveness as a fungicide. Lime allows the sulfur to penetrate leaves and kill recently germinated disease spores. However, lime sulfur sprays are more likely to damage plant tissue than are plain sulfur sprays. Certified organic growers must follow strict guidelines when using sulfur for disease control.
At temperatures above 85°F, sulfur can injure plant tissues. Combining sulfur and oil also causes damage to growing plants. A combination of oil and lime sulfur can be applied to dormant trees.
Copper is a powerful, nonspecific fungicide that kills disease organisms. It damages beneficial soil microorganisms and beneficial insects and is more toxic than sulfur to plants. Repeated applications of any copper product will stunt plants. Copper sulfate is classified as very toxic to humans. Organic gardeners often choose to avoid copper fungicides when possible because of their negative effects on nonpest species.
Copper is available as a powder or liquid. Fixed-copper fungicides are available as dusts or sprayable solutions.
Continue reading about Common Plant Diseases and Disorders.