A Who's Who of Plant Pathogens
Microorganisms that can cause diseases include fungi, bacteria, viruses, and pathogenic nematodes.
Fungi: Fungi are primitive plants. They don't have chlorophyll, the pigment that allows green plants to convert sunlight and air into food. Instead, fungi obtain nutrients by inserting special rootlike structures (called haustoria) into host plants or dead organic matter. Many fungi live on and decompose dead organic materials. These beneficial fungi are an important ally in the garden. Parasitic fungi, on the other hand, are a leading cause of plant disease. Some attack only one species of plants, while others attack a wide array of plants.
Fungi produce tiny spores that are spread by wind, water, insects, and gardeners. Spores germinate to form mycelia—the body of the fungus. Mycelia rarely survive winter, but spores easily survive from season to season.
It's often easy to spot signs of fungi—mushrooms are the most common example. Often the structures that produce fungal spores look like dots or discolored areas on leaves, stems, or fruit. These structures are one of the best ways to distinguish fungal diseases from other plant problems.
Bacteria: Most bacteria are beneficial—they help to break down dead organic matter. Some, however, cause plant diseases. Bacteria usually reproduce asexually—the cells simply split in half. Bacteria spread via wind, water, insects, garden tools, and gardeners' hands. Bacterial diseases are usually more difficult to control than fungal diseases, and they spread more quickly than other types of diseases. You need a microscope to see actual bacterial cells, but some of the disease symptoms they cause, such as dark streaks on leaves or stems and bacterial slime are easy to see with the naked eye. (The slime often smells bad, too.)
Nematodes: Nematodes swim freely in the film of moisture surrounding soil particles and plant roots. A few types are barely visible to the naked eye. Many nematodes do not cause plant diseases and are important members of the soil community. In fact, beneficial nematodes often prey on the nematodes that attack plants.
Parasitic nematodes can be very destructive. They lay eggs that hatch into tiny larvae. Larvae molt several times before maturing to adults. Nematodes puncture plant cell walls, inject saliva, and suck out the cell's contents. Some species move from plant to plant to feed; others attach themselves permanently to one root. They can travel short distances on their own but are spread through the garden by water and on tools or gardeners' hands.
Viruses: Viruses are so small, they're difficult to see even with a microscope. Viruses are not complete cells and must be inside living cells of a host in order to reproduce. The symptoms they cause vary widely from distorted growth to mottling of leaves to stunting. Viruses are transmitted by vegetative propagation, in seeds and pollen, and on tools and gardeners' hands. Viruses are also transmitted by aphids and other insects, mites, nematodes, and parasitic plants.
Natural Defenses Against Disease
There may be hundreds of species of disease organisms in your garden soil or living in weeds in and around your yard. However, even though disease-causing organisms are nearly always present, that doesn't mean that your garden crops and ornamentals will develop a disease. Untold millions of potential diseases never amount to anything, because conditions aren't favorable for their development.
Plants have intriguing defenses that help protect them from infection. Leaves have a waxy coating called the cuticle that prevents them from staying wet, making it hard for disease-causing organisms to survive. The leaf cuticle may also prevent spore germination and slow the penetration of disease-causing organisms. Leaf hairs trap spores and hold them away from the surface of the leaf. Some leaf hairs actually secrete chemicals that prevent spores from germinating or sticky substances that help catch pathogens and/or the insects that transmit them. Leaves also exude substances that promote the growth of beneficial microorganisms that compete for space with, or are antagonistic to, pathogens.
Plants can also spring to their own defense when pathogens try to invade. For example, some plants can form a corky layer of tissue around the site of attack. Other plants may seal off the diseased part, which then dies along with the disease organisms.
Plants also have natural chemical defenses that repel or damage the pathogens themselves. Some of these defenses are present all the time and some are "turned on" when disease-causing organisms are present.
The discovery of these natural defense mechanisms (and more new discoveries are still being made) has been of great benefit to organic farmers and gardeners, because plant breeders can select for these mechanisms when they're working to develop disease-resistant cultivars. It also appears that products containing harpin and other plant growth promoters help to prevent disease by triggering plants to turn on their natural chemical defenses.
Most of the time, simple preventive controls will stop disease problems from developing or limit their severity. In a few instances, you may have to resort to spraying substances such as neem or sulfur to prevent a disease from ruining a plant or crop. These substances are considered organic because they derive from natural sources—however they are not innocuous. For example, sulfur can burn plant leaves, and spraying sulfur too often can cause soil imbalances that harm beneficial soil organisms.
Smart Gardening Helps
The smartest way to help prevent disease problems in your garden is simply to take good care of your plants. It also pays to take simple steps that avoid the risk of infection. Some of the most important smart gardening practices to prevent diseases are listed here.
- Build up soil organic matter content, and correct nutrient imbalances.
- In the vegetable garden, interplant crops and use crop rotation if you can.
- Some disease problems can be avoided by planting earlier or later than usual.
- Prune your plants at the proper time of year and thin out growth as needed throughout the growing season to promote good air circulation around plants.
- Put up a barrier against insects such as leafhoppers and cucumber beetles that spread disease by covering crops with row covers from seeding until harvest when possible.
Clean Gardening Helps
Fungi, bacteria, and other pathogens have evolved many ways of surviving cold temperatures or other unfavorable conditions. They may spend winter inside the bodies of insects or in the soil—even in bits of soil clinging to tools stored in a garden shed. Some can survive winter in infected plant debris, seeds, or plant tissue. A few widespread diseases can produce "resting spores" that stay dormant in the soil for years until conditions are right or a host plant is planted in the area.
Many good gardening practices help prevent disease because they prevent disease organisms from overwintering and spreading from place to place during the gardening season. You probably already take some of these steps routinely in your garden.
- Choose disease-resistant or tolerant cultivars, and make sure all transplants and seeds you plant are free of disease.
- Stay out of your garden when the leaves are wet because disease organisms spread easily in wet conditions.
- Avoid damaging plants, as every wound is an opening for disease to enter.
- Clean and disinfect tools, hands, and feet regularly, whether you are working with diseased material or not.
- Do an annual fall garden cleanup.
- Dispose of diseased material throughout the season. Pull up plants or prune off infected portions, and get rid of them by burning, putting them in sealed containers for disposal with household trash, burying them deeply, or putting them in the center of a hot compost pile.
Growth Promoters Help
Going beyond the basics of maintaining healthy plants and a clean garden, you can apply homemade and commercially produced sprays that will make your plants even more resistant to disease problems. These products are different from fungicides, which work by killing fungal spores via toxic or caustic chemicals. Instead, these growth-promoting sprays work by stimulating beneficial microorganisms, and seemingly by triggering plants to turn on their own natural defense systems.
Homemade sprays: Compost tea is one of the homemade sprays that help boost plant defenses. Applying sprays of seaweed extract or comfrey tea may have similar beneficial effects. Learn more about making compost tea. Make comfrey tea using a similar method, but dilute comfrey tea half-and-half with water before applying. A homemade garlic spray may work by killing spores on plant surfaces. Mix 5 to 10 cloves with 1 pint of water in a blender, strain, and spray on plants.
Harpin: Harpin is a protein that reportedly stimulates plant defense systems, and some commercial products are available with harpin as the active ingredient. It has no reported adverse side effects on plants or the environment, but it has not been listed for use by certified organic growers.
For organic gardeners, the two primary ways to fight disease are to take steps to kill spores in the soil or on plant surfaces—before the spores infect roots or leaves. Soil solarization kills many types of disease spores, along with some pests and weed seeds. Applying biocontrol agents to soil or plants can kill or outcompete pathogens. And as a last resort, sulfur and copper sprays will kill spores and in some cases even prevent a disease organism from spreading within a plant.
The more you learn about the common disease problems in your area, the less often you'll need to resort to using dusts and sprays. Keep records of the disease problems that occur in your garden, or ask fellow gardeners what diseases to expect in your area and when to expect them. This way, you can limit spraying or dusting to those seasons and weather conditions when your plants are most vulnerable to becoming infected.
It's also important to make sure you know what disease you're trying to control by applying a spray or dust. If you run into a problem that you can't identify, submit a fresh plant sample to a diagnostic laboratory or your local extension office for identification.
Always take appropriate safety precautions when applying sprays and dusts; see the Pests entry for guidelines. The following descriptions of disease-control methods and products are arranged from least to most toxic.
If an area of your vegetable garden has been troubled by disease, or if you plan to start a new planting of any kind and are concerned about soil-borne diseases, consider solarizing the soil before you plant. You'll need to plan ahead, because it's important to solarize soil during the hottest period of the year if you live in the North.
Solarizing is a simple procedure: You tightly cover the soil with clear plastic for 1 to 2 months. This can generate high enough temperatures in the top 6 to 12 inches of soil to kill many disease organisms, nematodes, pest insects, and weed seeds. The beneficial effects seem to last for several seasons. The illustration at right shows how to prepare a bed for solarizing. For even better results, support a second layer of plastic on wire hoops over the covered bed to provide added insulation.
Midsummer is the best time to solarize soil, especially in the North. Cultivate and remove crop residues from the soil, rake it smooth, and water if it is dry. Dig a trench several inches deep around the bed, and spread thin clear plastic film (1 to 4 mils) over the bed. Press the plastic into close contact with the soil, and seal the edges by filling the trench with soil. Leave in place for 1 to 2 months, then remove the plastic.
One of the most exciting areas of plant disease research focuses on using naturally occurring bacteria and fungi to fight against plant pathogens. The bacterium Bacillus subtilis kills or outcompetes the fungus that causes powdery mildew as well as some other plant 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 it 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 (sold as BioVam) cover plant roots to protect against pathogens, forming a "fungal mat," which also increases nutrient-uptake ability.
A thorough spray of vegetable or light horticultural oil coats plant surfaces, acting as a barrier to infection. Oils seem to help prevent fungal rusts and mildews.
Garlic appears to be a fungicide as well as an insecticide. Horsetail (Equisetum arvense) infusion sprayed on plants may help prevent fungal diseases.
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.