A Glossary of Plant Disease

Plant fungus and vegetable disease can decimate an entire garden. Gardeners should be aware of the ways to prevent and fight these ailments that can affect their crops.

  • New  Encyclopedia of Gardening Techniques
    "The New Encyclopedia of Gardening Techniques" by the American Horticultural Society provides an in-depth look into gardening.  This book contains details on getting rid of plant disease in the garden and preventing plant problems.
    Cover Courtesy of Mitchell Beazley
  • Potato with blight
    Potato blight is a hard, reddish brown patch that extends into the tuber. Secondary bacteria often infect these wounds to cause a slimy soft rot.
    Courtesy of Mitchell Beazley

  • New  Encyclopedia of Gardening Techniques
  • Potato with blight

Plant problems can be prevented with appropriate preparation and attention. The American Horticultural Society’s New Encyclopedia of Gardening Techniques (Mitchell Beazley, 2009) gives a detailed look into planting from preparation to harvest. Learn the basics of plant disease and prevention in this excerpt taken from Chapter 4, “Growing Vegetables & Herbs.”     


Most plant diseases affecting crops are fungal or viral. Airborne fungal infections are spread by spores, so good spacing and airflow help to reduce incidence, but some are spread in the soil and are harder to combat. Viruses are mainly spread by handling, or by sap-sucking insects.

White rust

This fungus is common on many brassicas. White, chalky pustules develop on the undersides of leaves, and the upper surfaces are sometimes distorted and discolored. It is unsightly, but not serious. Remove affected leaves, and reduce incidence by spacing plants well and using rotation.

Onion white rot

Roots and basal tissues develop white, fluffy growth and rot, sometimes causing plants to fall over, and leaves to yellow and die. Dig up and destroy affected plants. The fungus produces black spores that can survive in soil for 15 years. Grow onions elsewhere or replace the soil: do not spread it.

Onion downy mildew

Onion leaves wither and collapse. If humid, an off-white mold develops. The bulbs do not store well. Destroy affected plants and avoid growing onions on the site for five years. Control weeds to encourage good airflow.

Leaf spots

Several fungi and bacteria cause brown spots, particularly on older leaves and in wet seasons. Diseased tissue may fall out to leave shot holes. Destroy affected leaves and if necessary remove alternate plants to improve airflow.

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