Least-Toxic Controls of Plant Diseases
The best way to control plant diseases is to make sure they don't get a foothold in the garden in the first place. (Click here for Nine Steps to Disease Prevention) However, if they are already established, you may feel it's necessary to resort to one of the controls described below. Most of these mentioned here have low acute toxicity to mammals, including humans, and are not toxic to beneficial insects. Some, such as baking soda, are practically non-toxic, while others, including bordeaux mixture, lime-sulfur, concentrated silicate salts, and streptomycin should be used with caution. Always follow application instructions carefully and apply only at the appropriate time in the plant's growth cycle and at the proper time of day. Be sure to protect yourself using the proper precautions when applying these controls. It is wise to avoid inhaling any kind of pesticide spray.
Controlling Fungal Diseases
Most garden diseases are caused by fungi: more than 8,000 species are known plant pathogens and either inhabit the above ground portions of plants or are denizens of the soil. Most of the fungicides described below are essentially preventive measures, acting as barriers between pathogenic agents and plant tissues, and must be applied before new leaves or other susceptible plant parts appear, at the first sign of disease, or when weather conditions are favorable for disease.
Copper and Sulfur
Sulfur, lime-sulfur, and bordeaux mixture—a combination of copper sulfate and lime—have been used as fungicides for a hundred years or more. These are all toxic to mammals, so avoid ingesting them and wear protective clothing when applying them. bordeaux mixture is both fungicidal and bactericidal, and can be useful against diseases such as leaf spot and apple scab, among others. It contains copper sulfate, which is acidic, and lime, which is alkaline and helps neutralize the acidic salt. This mixture is a potent eye, skin, and gastrointestinal irritant and highly corrosive, and can cause nausea and vomiting if ingested. bordeaux mixture can be applied as a dust or purchased in a liquid formulation, which is easier to use, especially if larger areas are involved.
Sulfur can be used as a preventive fungicide against apple scab, brown rot, powdery mildew, rose black spot, rusts, and other diseases. You can apply sulfur as a dust or purchase it in liquid form; it is acidic and can irritate eyes and lungs. But do not use sulfur if you have applied an oil spray within the last month. Sulfur can injure plants if used when temperatures exceed 80 degrees Fahrenheit. Lime is sometimes added to sulfur to help it penetrate plant tissue, but this mixture is more caustic than sulfur on its own and can cause severe eye and skin irritation. Do not inhale or ingest and wear protective clothing and eye-wear when applying.
Sprays Containing Salts
Baking Soda: Baking soda (sodium bicarbonate) is non-toxic, readily available, and very inexpensive. It can be effective against powdery mildew and somewhat useful against black spot. If you repeatedly spray leaves with bicarbonate, though, it will eventually reach the soil below, where it can accumulate and lead to slower plant growth. Bicarbonate can form insoluble particles with calcium and magnesium ions when it concentrates in the soil, making these important nutrients unavailable to plants. High levels can also prevent plants from absorbing iron and can lead to chlorosis.
Bicarbonate is most likely to build to damaging levels in drought-stressed areas where there is little rain to flush it away. It is also likely to build up when applied in a small area, and when used in conjunction with drip-type irrigation. Garden situations are so complex that it is hard to predict the point at which you will see adverse effects. Stop applying bicarbonate sprays, however, at the first sign of plant damage or lower quality blooms.
Phosphate Salts: Foliar sprays containing potassium phosphate salts, unlike most of the non-toxic sprays, can not only prevent powdery mildew but in some instances even cure it. These salts seem to stimulate a systemic effect that builds up plant resistance to other diseases, including some forms of rust and northern leaf blight. Phosphate salts are ideal as foliar sprays because plants quickly absorb and circulate them. Other advantages are their low cost, low toxicity, and environmental safety (phosphate buffers, salt mixtures that protect against rapid changes in pH, are constructed from these salts and often used in soft drinks). Phosphate salts can even improve plant growth, as they may increase plant nutrition. Effects of added phosphate are most pronounced on plants deficient in phosphorus. Like any salt, though, these can damage plants when applied as foliar sprays. Test small areas before applying to whole plants. You can purchase phosphate salts from chemical supply houses and some horticultural nurseries. (Dibasic potassium phosphate is slightly less effective as a fungicide than the monobasic salt.)
Silica and Silicate Salts: Organic gardeners have long used sprays containing extracts of the common plant horsetail (Equisetum arvense) (which contains 15 to 40 percent natural silica) to protect against fungal diseases. Scientific experiments have recently verified this garden folklore, and show that potassium silicate solutions can protect cucumbers against damping-off, and cucumbers, grapes, and squash against powdery mildew. Sodium silicate can also protect plants against disease. How these salts act isn't clear yet, but like the phosphate salts above, they may be absorbed by plants and act systemically; such was the case with the cucumbers in the experiments above. Spray small areas to check for phytotoxicity before general use. Sodium or potassium silicate solutions can be obtained at drugstores. Caution: be extremely careful when using silicate salts, as the concentrated solution is strongly alkaline.
Petroleum-based horticultural oils (mineral oils), essential plant oils, neem oil, vegetable seed oils, and even fatty acids can be used effectively not just to fight insect pests, but to control pathogenic fungi as well. Oil sprays protect against fungi probably by helping to repel the water that is needed for fungal growth. The best approach to protection and control may be to rotate different classes of oil. Rotation of oils minimizes possible environmental accumulation of one kind. Petroleum is the most persistent; vegetable and neem oils are more easily biodegradable.
Petroleum-based oils: These oils have a long history of use in horticulture. Before the 1970s when lighter formulations were developed, orchardists sprayed their trees in the spring, while the trees were still dormant, with heavy ("dormant") oils to protect against insect pests. You can now purchase refined horticultural oils such as a product named SunSpray, which is effective against powdery mildew and sometimes against black spot. You can also purchase mineral oil at a drugstore and use it to make your own, less expensive, spray. Horticultural oils should not be used on drought-stressed plants or those weakened by disease, and they should not be used when temperatures exceed 85°F. With repeated use petroleum-based sprays can also build up in your soil.
Vegetable oil sprays: Cooking and salad oils are more readily available than most other oils and are probably less disruptive to the environment. Vegetable oils are biodegradable and shouldn't cause any long-term problems in the garden. Emulsified vegetable oil sprays of sunflower, olive, canola, peanut, soybean, corn, grapeseed, or safflower can control powdery mildew on apple trees, roses, and possibly other plants, and cottonseed oil has considerable protective value against powdery mildew. However, emulsified vegetable oil can leave a greasy film on leaves, which you might find objectionable. Check for plant damage before general use, and be especially careful of blooms.
Herbal oil sprays: Essential oils such as those made from basil, fenugreek, cumin, mint, clove, and eucalyptus may be effective against a number of fungal pathogens. For instance, solutions of cumin or clove oil completely inhibit sugarcane rot, and basil oil can inhibit growth of soilborne pathogens. A commercial formulation of mint oil (Funga-Stop) is available to help control soilborne pathogens. However, these essential oils need to be researched further before they become prevalent in horticulture.
Neem oil: Neem is derived from the neem tree, a native of Myanmar (the former Burma) and India. Extracts of neem seeds are used as insecticides; they kill insects as they molt or hatch. Recently, fungicides made with neem oil have become available commercially. Neem oil appears to have better fungicidal properties than many of the oils described above, perhaps because neem contains sulfur compounds, which have their own fungicidal properties, as well as other natural pesticides. A neem-oil formulation called Trilogy has been approved by the EPA for use on foods, while Rose Defense and Triact (for control of powdery mildew, rust, black spot, Botrytis, downy mildew, and other common diseases) are designed for use on ornamentals. Make sure you buy neem with fungicidal rather than insecticidal properties.
Like many other methods outlined here, soaps have been used for many years by organic gardeners, particularly as insecticides. Commercial formulations now include soap solutions with fungicidal properties, which show some control of powdery mildew, black spot, canker, leaf spot, and rust. You can also make your own version. All soaps can damage plants when applied improperly. Test before you spray widely.
Plant preparations have been used for centuries in medicine and pest control. For example, opium from the opium poppy was one of the first pain killers. Farmers in India use neem leaves to protect their stored grain from insects. Herbs and spices, such as basil and clove, have been used by many cultures to protect food from spoilage, as both have antimicrobial properties.
Milsana: The German corporation BASF capitalized on this concept in 1993 by screening a large number of plant extracts for their fungicidal properties. The most promising result was a dried extract of the giant knotweed, Reynoutria sachalinensis, which is now sold as a fungicide under the brandname Milsana. Knotweed extract has only recently become commercially available in the United States, so feedback from U.S. gardeners is sparse. Italian researchers have found that Milsana reduced powdery mildew infection on cucumber by 50 percent, and similar sprays protected roses, but these were less effective than oils, soaps, and other non-toxic products. Repeated sprays of Milsana induced a greener and glossier coloration of the leaves, but they became brittle to the touch.
Garlic: Sprays made from aqueous garlic extracts have antibiotic and antifungal properties and will suppress a number of plant diseases, including powdery mildew on cucumbers and, to some extent, black spot on roses. Activity may be due to sulfur-containing compounds such as ajoene or allicin.
Antitranspirant coatings made from very dilute mixtures of polymers and water are sometimes sprayed onto foliage to prevent water loss. Growers also use them to protect a number of different ornamental plants against diseases caused by fungi. Antitranspirants are just as effective as some chemical fungicides against downy mildew on zinnia, hydrangea, and crapemyrtle, and against powdery mildew and black spot on roses. They are available commercially under the names Wilt-Pruf and Vapor Gard, among others.
Antitranspirant coatings are non-toxic, biodegradable, and inexpensive, and are readily available at local nurseries in liquid form. Unlike fungicides, their action against pathogens is non-specific and so they are not likely to cause a buildup of resistance. Antitranspirants do not protect new growth, though, so the coatings have to be reapplied on a regular basis. Since leaf coatings reduce the rate of photosynthesis by about 5 percent, antitranspirants are probably better suited for sunny climates.
Controlling Bacterial Diseases
There are very few effective chemical controls for bacterial diseases. bordeaux mixture is one possible treatment for bacterial diseases occurring on stems and leaves. The active component in this mixture is copper ion, which is both fungicidal and bactericidal; it is commonly used for bacterial leaf spot. Some bacterial diseases, such as fire blight, walnut blight, and bacterial spot of tomato, can be treated with antibiotics, including streptomycin (sold as Agrimycin). Caution: Agrimycin can be extremely toxic to mammals, aquatic invertebrates, fish, bees, and beneficial insects, and can cause plant damage.
The most effective approach to viral diseases is to control the insect vectors that often transmit them from plant to plant. For instance, successful management of thrips can help prevent the spread of tomato spotted wilt virus. Non-toxic treatments are ineffective against viruses, and in response to chemical controls, viruses easily mutate. (See Natural Insect Control: The Ecological Gardener's Guide to Foiling Pests, 1994, Brooklyn Botanic Garden Handbook #139, for garden-safe methods for controlling insect pests.)
Parasitic nematodes are most often soilborne. Their effects can be minimized by rotating crops, increasing organic matter in the soil, and planting nematode-free material. Applying compost helps control nematodes because compost teems with bacteria and fungi that attack pathogens. Adding organic matter to the soil also helps create a large microbial soil population, and as these microbes feed near plant roots they form a barrier that makes nematode penetration less likely. If the problem is severe, you can combine compost and inoculations of plant roots with mycorrhizae, symbiotic fungi that help feed the plant by enhancing mineral uptake. Since mycorrhizae colonize roots, they make nematode penetration more difficult. You can also add beneficial nematodes, which can displace parasitic species.
You can add chitin—a polysaccharide complex found in both shellfish shells and nematode eggs—to the soil to help control nematodes; a commercial form called ClandoSan is available. This soil amendment stimulates the growth of soil microbes that produce chitinase, an enzyme that breaks down the chitin in nematode eggs, destroying the eggs and larvae. As chitin is metabolized, ammonia is released, which is toxic to nematodes.
Using Beneficial Organisms
Beneficial fungi or bacteria can control garden diseases by competing with disease-causing organisms for nutrients and space, by producing antibiotics, by preying on pathogens (a process called hyperparasitism), or by inducing resistance in the host plant. Antagonists do not persist in the environment, are non-toxic, and in some cases are as effective as chemical fungicides.
Beneficial fungi are effective only when humidity is high (usually 60 to 80 percent), so their usefulness is restricted to greenhouses or to regions, such as Louisiana and other Gulf states, where humidity is always very high during the summer growing season. Beneficial bacteria are less sensitive to moisture, and so have a wider range of use.
A Beneficial Bacterium
Preliminary research shows that the beneficial bacterium Bacillus laterosporus (sold as Rose Flora) is as effective at protecting black spot-susceptible rose cultivars as some chemical fungicides. It probably protects against black spot through competition, but this agent is still relatively new and experiments detailing its mode of action have not been completed. As a ground spray, it can help control new sources of black spot infection. As a foliar spray, it seems to be more effective when mixed with the antitranspirant sold commercially as Wilt-Pruf. The powdered formulation can cause eye irritation, so use eye protection when mixing solutions and applying.
Ampelomyces quisqualis is a powdery mildew hyperparasite first described in the mid-nineteenth century. The fungus attacks a wide range of powdery mildew species and genera; it spreads naturally through the air, and acts quickly. The commercially available strain, AQ-10, can provide some control of powdery mildew on cucumber, grapes, roses, and possibly other plants. Research has shown that better results are obtained when AQ-10 is mixed with a horticultural-oil solution before spraying.
Water extracts of fermented compost, or "compost teas," are full of antibiotics, microbial products, and beneficial microbes that compete with pathogens, such as those that are responsible for powdery mildew, Botrytis gray mold, and leaf blight. The "tea" can be used as a foliar spray to help suppress plant disease. Undiluted compost may also benefit a plant's roots, and stem, as well as the soil when applied as a thin layer of mulch around the plant.
Using Combination Treatments
You may have greater success combating plant diseases when you combine control treatments instead of employing a single control strategy. For example, baking soda is usually more effective when used with oil because both have antifungal properties. The oil may also help provide an even distribution of baking soda on plant leaves. Pathogens can also develop resistance to ingredients that are applied frequently, so a rotation of active ingredients can reduce the likelihood of pathogen resistance. Combination treatments can be applied simultaneously or sequentially, in a planned rotation. There are so many complex interactions taking place in each home garden that you will have to come up with the most effective treatment rotations through trial and error. A good rule might be to start experimenting with the least expensive and most readily available products first. To control fungal diseases, for example, it's likely that you will have to spray weekly, especially in areas of high rainfall; so this time interval might be a good starting point to set a rotation schedule. Adjust the schedule through trial and error. You might get away with fewer applications, or you might need more frequent treatments.
Some examples of treatment rotations:
- To prevent powdery mildew, rotate vinegar, baking soda, and vegetable oil (the least expensive and most easily available remedies). Spray plants once a week, alternating substances.
- Black spot and rust require preventive sprays. Neem oil sprays in rotation with antitranspirants or garlic might prevent these diseases, especially on resistant species. The systemic effect of phosphate salts could be useful as well.
- Antitranspirants can protect against mildew for up to 30 days. Foliage put out in that interval, however, is not protected. You can protect it by spraying with baking soda or another foliar spray between applications of antitranspirants.
- As diseases and severity vary with location, you will have to experiment to find the most effective combination. With all these options, there is no longer any need to use toxic sprays, especially when resistant species and good cultural practices can help prevent the problem in the first place.
Controlling Soilborne Pathogens
Conventionally, soilborne pathogens are controlled by soil fumigation or by addition of chemical fungicides to the soil. The most commonly used soil fumigant is methyl bromide, a toxic and dangerous gas that also depletes the stratospheric ozone layer. Another common soil fungicide is Dazomet (sold under the brand name Basamid), a granular material that releases a toxic gas when it comes in contact with the water in the soil. Among the alternatives to these poisons are plants such as garlic that release fungicidal chemicals into the soil. Rotation of garlic with tomatoes, for instance, can reduce the likelihood of soilborne tomato diseases. Incorporating broccoli residues into the soil can help disinfect your soil of the fungus that causes Verticillium wilt. In areas with abundant sunlight, you can solarize the soil to disinfect it of fungi, bacteria, nematodes, and even weed seeds.
Seeds and soil can also be treated with biocontrol agents that prevent soilborne diseases. These beneficial bacteria and fungi work by competing with disease-causing organisms for nutrients and space, by producing antibiotics, by preying on pathogens, or by inducing resistance in the host plants. Biocontrol agents can help control damping-off, wilt, and a number of soilborne diseases caused by Pythium, Fusarium, Rhizoctonia, Verticillium, and other pathogens. Commercially available agents include the beneficial fungi Trichoderma harzianum (sold as Root Shield) and Gliocladium virens (SoilGard), and bacteria such as Bacillus subtilis (Kodiak), Streptomyces griseoviridis (Mycostop), and Burkholderia cepacia (Deny). These agents are non-toxic and some occur naturally in compost—but like compost, these agents are not always 100-percent effective at disease control.