Plant Immune Systems - August 7, 2002
Jeff Schalau, County Director, Agent, Agriculture & Natural Resources
Arizona Cooperative Extension, Yavapai County

Animals have immune systems that produce antibodies and other cellular compounds that eliminate pathogens and prevent or limit disease development. Similar to animals, plants attacked by insects and/or diseases can trigger complex metabolic responses that lead to the formation of defensive compounds that fight infection or make the plant parts less attractive to insects. At the risk of going overboard with technical terms and scientific processes (which, by the way, I am sometimes accused of), I will share some research information that was recently published on plant "immune systems".

Plant scientists call this immune response within plants induced systemic resistance (ISR). ISR was first observed about 100 years ago and has now been identified in over 30 species of plants. Researchers noticed that disease attacks on plants led to a hypersensitive reaction characterized by lesions at the point of entry. The hypersensitive reaction prevented the spread of the disease within the plant. Not only is the disease organism localized, but the rest of the plant is also made resistant to attack by that disease.

When plants are attacked by an insect or disease, certain chemical compounds become more abundant within the plant. The compounds produced may cause resistance themselves or be chemical messengers that signal the plant to produce other compounds which defend the plant from that disease or make it less palatable to insects. Salicylic acid is one such compound.

Salicylic acid occurs naturally within many plants and may have evolved as a defense against insects. Salicylic acid was first isolated from willow bark in 1828. It is chemically related to aspirin (acetylsalicylic acid), which was first produced by the Bayer Company in 1859. In the 1990's, researchers noticed that plants attacked by a pathogen (disease) displayed a 180-fold increase in salicylic acid concentration. This correlated with an increase in other proteins that promoted disease resistance. Later research also showed that external applications of salicylic acid to the plant also caused increased disease resistance.

The metabolic processes that lead to disease resistance triggered by elevated levels of salicylic acid are well documented in the literature so I'll spare you the details. Suffice it to say that the more researchers examine salicylic acid's relationship to ISR, the more supporting evidence they find.

So, can we simply apply aspirin to our plants to promote insect and disease resistance? The answer is "yes and no". Plant species vary widely in their tolerance to applications of salicylic acid and aspirin at varying concentrations. Often at high concentrations, plant damage occurs. However, relatively large concentrations are needed to induce resistance because much of the salicylic acid become immobilized in the plant tissues that were initially contacted during application.

For example, in one study, barley seedlings treated with salicylic acid showed 97% protection against powdery mildew for at least 12 days. Aspirin gave 93% protection. In this experiment, a concentration of 15mM (millimoles) was used. However, little of the salicylic acid (1.4%) moved into new plant tissues after 24 hours.

Syngenta, a chemical company, has produced a salicylic acid derivative called Actigard that is kinder and gentler to plants than salicylic acid or aspirin. Actigard will induce a systemic response within four days and is registered for prevention of certain diseases on leafy vegetables, tomatoes, and tobacco.

There are several other compounds being explored and researchers are finding results similar to those of salicylic acid. Induced systemic resistance is an encouraging development in the realm of pest and disease control. In the end, these methods are safer and less harmful to non-target organisms and the environment.

The University of Arizona Cooperative Extension has publications and information on gardening and pest control. If you have other gardening questions, call the Master Gardener line in the Cottonwood office at 646-9113 or E-mail us at and be sure to include your address and phone number. Find past Backyard Gardener columns or submit column ideas at the Backyard Gardener web site:

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Arizona Cooperative Extension
Yavapai County
840 Rodeo Dr. #C
Prescott, AZ 86305
(928) 445-6590
Last Updated: August 1, 2002
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