Understanding Plant Diseases and Disorders

Keeping plants healthy is at the heart of organic gardening.


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.