Amazing, But True—Plant Defenses Against Diseases and Pests: Aspirin By Paula Szilard Researchers pursuing ways to protect plants from diseases and insect pests are taking a close look at how plants protect themselves from these invaders naturally. What they are learning is nothing short of astonishing. In the past a whole arsenal of chemicals was developed to kill off insects and disease causing organisms, sometimes at great cost to our environment and human health. In the future, we may simply use chemicals occurring naturally in plants to enhance their resistance to diseases and pests. It’s a major shift in the paradigm. This works because plants do react with a type of immune response when attacked by pathogens or insects. Instead of producing antibodies like animals, plants generate defensive proteins and other substances that hold the attackers at bay. This plant defensive process is called systemic acquired resistance (SAR) or induced systemic resistance (ISR). The response is called systemic because it affects the entire plant. Scientists have been aware of this phenomenon since the 1930’s, but the chemistry behind it is still only partly understood. There are a variety of substances or chemicals that induce this response. The one that has received the most attention of late is salicylic acid (more precisely, acetylsalicylic acid), a compound of considerable medicinal value first isolated in 1828 from willow bark and first marketed as Bayer aspirin in 1859. It has been found in over 30 plants and scientists theorize that it may have evolved as a defensive measure, signaling the plant to mobilize its defenses. Now many gardeners are using dilute solutions of aspirin sprayed on leaves or as a soil drench to activate this immune response. When a plant is attacked, levels of salicylic acid in plant tissues can rise to 180 times normal amounts. These high levels are correlated with high levels of resistance proteins, substances that actually enable the plant to defend itself. These proteins causes changes in the plant that make it difficult for disease organisms to penetrate plant tissues or survive, if they succeed in penetrating. One makes the plant tissue more woody, making it difficult to penetrate. Others function essentially as antibiotics, while still others are enzymes that destroy the cell walls of disease producing fungi. Amazingly, one even breaks apart the ribonucleic acid (RNA) in viruses. When a bacterium, a fungus or a virus attacks a plant, the plant has a mechanism for immediate action. To contain the pathogen, the plant commands the cells immediately surrounding the infected area to kill themselves. This is called the hypersensitive reaction. The plant’s systemic acquired resistance mechanism begins later, when a signal from the wounded area activates the defensive proteins to destroy the invader. Experiments thus far show that salicylic acid protects cucumbers, tobacco, tomatoes, potatoes, beans and cowpeas. Master gardeners in Vermont sprayed selected eggplants, tomatoes, basil and bean plants with aspirin water (1.5 aspirins to 2 gallons of water) in their demonstration garden every three weeks. Yields of all sprayed plants were much greater, but the tomatoes did the best! The sprayed plants produced twice as many tomatoes as the unsprayed plants. Salicylic acid protects plants primarily against disease organisms. It is also known to protect against some insects. For instance, bean beetles seem to find bean leaves sprayed with aspirin water less attractive. However, there are much better substances to defend against insects generally. These will be described in the second part of this article. For now, there is no commercially viable method of combating viruses infecting crops and ornamental plants. Since plants usually contract virus diseases from insects feeding on them, affected plants must be destroyed to prevent the spread of the disease. Researchers at the Agricultural Research Service at the U.S. Department of Agriculture have been working to test salicylic on virus diseases of potatoes, and preliminary results indicate it can be an effective control. So why add additional salicylic acid when the plant already produces it? And why add such large amounts? Most plants apparently don’t produce sufficient amounts, nor do they produce it quickly enough. They also immobilize it and it has to be added at regular intervals. There is a downside. Gardeners applying aspirin water to their plants need to be aware that a cautious approach is advised, because for many plants protective doses are perilously close to levels that could damage plants. Researchers have observed that certain plants in the cabbage family and some of our common cereal crops can tolerate much higher doses than beans, for instance. Protective levels for most tropical plants grown in the home or greenhouses are unknown, as are phytotoxicity levels! So for now, smaller amounts are the safest approach, and testing on an inconspicuous part of the plant is always a good idea. A general guideline is 3 aspirins dissolved in 4 gallons of water. References: Anonymous. “Helping plants defend themselves” Agricultural Research (USDA) December 2003: 8-10 Quarles, W. “Aspirin, composts, talking plants and induced systemic resistance” IPM Practitioner 24 (5/6) May-June 2002: 1-9 Quarles, W. “Protect your garden with aspirin and salicylate” Common Sense Pest Control 12 (2) Spring 1996: 16-19

Amazing, But True—Plant Defenses Against Diseases and Pests: Aspirin

Paula Szilard

Researchers pursuing ways to protect plants from diseases and insect pests are taking a close look at how plants protect themselves from these invaders naturally. What they are learning is nothing short of astonishing. In the past a whole arsenal of chemicals was developed to kill off insects and disease causing organisms, sometimes at great cost to our environment and human health. In the future, we may simply use chemicals occurring naturally in plants to enhance their resistance to diseases and pests. It’s a major shift in the paradigm.

This works because plants do react with a type of immune response when attacked by pathogens or insects. Instead of producing antibodies like animals, plants generate defensive proteins and other substances that hold the attackers at bay. This plant defensive process is called systemic acquired resistance (SAR) or induced systemic resistance (ISR). The response is called systemic because it affects the entire plant. Scientists have been aware of this phenomenon since the 1930’s, but the chemistry behind it is still only partly understood.

There are a variety of substances or chemicals that induce this response. The one that has received the most attention of late is salicylic acid (more precisely, acetylsalicylic acid), a compound of considerable medicinal value first isolated in 1828 from willow bark and first marketed as Bayer aspirin in 1859. It has been found in over 30 plants and scientists theorize that it may have evolved as a defensive measure, signaling the plant to mobilize its defenses. Now many gardeners are using dilute solutions of aspirin sprayed on leaves or as a soil drench to activate this immune response.

When a plant is attacked, levels of salicylic acid in plant tissues can rise to 180 times normal amounts. These high levels are correlated with high levels of resistance proteins, substances that actually enable the plant to defend itself. These proteins causes changes in the plant that make it difficult for disease organisms to penetrate plant tissues or survive, if they succeed in penetrating. One makes the plant tissue more woody, making it difficult to penetrate. Others function essentially as antibiotics, while still others are enzymes that destroy the cell walls of disease producing fungi. Amazingly, one even breaks apart the ribonucleic acid (RNA) in viruses.

When a bacterium, a fungus or a virus attacks a plant, the plant has a mechanism for immediate action. To contain the pathogen, the plant commands the cells immediately surrounding the infected area to kill themselves. This is called the hypersensitive reaction. The plant’s systemic acquired resistance mechanism begins later, when a signal from the wounded area activates the defensive proteins to destroy the invader.

Experiments thus far show that salicylic acid protects cucumbers, tobacco, tomatoes, potatoes, beans and cowpeas. Master gardeners in Vermont sprayed selected eggplants, tomatoes, basil and bean plants with aspirin water (1.5 aspirins to 2 gallons of water) in their demonstration garden every three weeks. Yields of all sprayed plants were much greater, but the tomatoes did the best! The sprayed plants produced twice as many tomatoes as the unsprayed plants. Salicylic acid protects plants primarily against disease organisms. It is also known to protect against some insects. For instance, bean beetles seem to find bean leaves sprayed with aspirin water less attractive. However, there are much better substances to defend against insects generally. These will be described in the second part of this article.

For now, there is no commercially viable method of combating viruses infecting crops and ornamental plants. Since plants usually contract virus diseases from insects feeding on them, affected plants must be destroyed to prevent the spread of the disease. Researchers at the Agricultural Research Service at the U.S. Department of Agriculture have been working to test salicylic on virus diseases of potatoes, and preliminary results indicate it can be an effective control.

So why add additional salicylic acid when the plant already produces it? And why add such large amounts? Most plants apparently don’t produce sufficient amounts, nor do they produce it quickly enough. They also immobilize it and it has to be added at regular intervals.

There is a downside. Gardeners applying aspirin water to their plants need to be aware that a cautious approach is advised, because for many plants protective doses are perilously close to levels that could damage plants. Researchers have observed that certain plants in the cabbage family and some of our common cereal crops can tolerate much higher doses than beans, for instance. Protective levels for most tropical plants grown in the home or greenhouses are unknown, as are phytotoxicity levels! So for now, smaller amounts are the safest approach, and testing on an inconspicuous part of the plant is always a good idea. A general guideline is 3 aspirins dissolved in 4 gallons of water.

References:

Anonymous. “Helping plants defend themselves” Agricultural Research (USDA) December 2003: 8-10

Quarles, W. “Aspirin, composts, talking plants and induced systemic resistance” IPM Practitioner 24 (5/6) May-June 2002: 1-9

Quarles, W. “Protect your garden with aspirin and salicylate” Common Sense Pest Control 12 (2) Spring 1996: 16-19