Article: Amazing, But True—Plant Defenses Against Diseases and Pests. Part II. Jasmonic Acid, Compost and Compost Tea by Paula Szilard Part I of this article appeared in the May 2005 issue of this newsletter and dealt with mobilizing the plant’s own defenses against disease by spraying dilute solutions of salicylic acid (aspirin). Part II deals with other substances which elicit a reaction known as systemic acquired resistance or induced systemic resistance in plants, against insect attack and disease. Among these substances are jasmonic acid, compost and compost tea. Worm castings and rhizobacteria (root bacteria) will be discussed in Part III , to appear in our October issue. Jasmonic acid Jasmonic acid is a plant hormone that has been found in over 160 families of plants. Its presence in plant tissue is probably universal. Plants produce it from two common fatty acids. When a plant is wounded, chewed on by insects or is under drought stress, levels of jasmonic acid increase dramatically. These increased levels of jasmonic acid in turn trigger the production of substances that interfere with the digestion of proteins in the leaf, making the plant less digestible and therefore less attractive to insects. It’s the yuck factor at work. Interestingly, much more jasmonic acid is produced when a plant is chewed by insects and the compounds are qualitatively different from those produced as a result of mechanical wounding. A volatile form of jasmonic acid, methyl jasmonate, actually acts as a chemical messenger. When a plant is under insect attack, the jasmonic acid in the plant causes the release of methyl jasmonate, an essential oil of the jasmine plant, Jasminum grandiflorum, along with other volatile compounds. The volatile compounds released may vary with the attacking insect. Their function is twofold. They are alarm signals that “warn” nearby plants that war is being waged and their meaning is unmistakeable! “Get your defenses up for the attack!” For instance, researchers have found that methyl jasmonate released by a sagebrush (artemisia) could create a defensive reaction in a nearby wild tobacco plant. At the same time, the release of methyl jasmonate sends a chemical message to beneficial insects that a plant is under attack. It is analogous to a human cry for help, or viewed more optimistically, an invitation to all of your friends that there’s a free lunch available here! (OK, don’t believe me, but if you want to see some literature on this, I’d be happy to oblige! Actually If I had not read some pretty reliable sources, I would not believe it either!) So far the release of these volatile compounds has been documented in 23 plant species. In a field experiment, tomato plants were sprayed with jasmonic acid dissolved in acetone. Elevated levels of proteinase inhibitors, the substances that make the protein in the leaf more difficult to digest and other substances that make the leaf less palatable were noticeable within three days. The effects lasted three weeks, at which time the plants were sprayed again. As anticipated, fewer chewing and sucking insects (army worms, flea beetles, peach aphids, and thrips) were found on the sprayed plants. In other experiments, the two-spotted spider mite, leaf miners, corn earworm caterpillars, and hornworm caterpillars were negatively affected by sprays of jasmonic acid. Aphids were not consistently affected by the treatment. Such results have also been seen in soybeans, cotton and birch. Interestingly, a volatile form a salicylic acid, methyl salicylate has also been found to attract predatory mites which then attack the pest mites found on cucumbers, roses, apples, pears, tomatoes and ground ivies. It is important to note that while treatment with jasmonic acid resulted in less insect damage, it did not affect production levels. This could mean that the treatment would be best reserved for flowers and other high value ornamentals where even a little damage might deter a potential customer. If you are thinking of purchasing jasmonic acid, it may not be that easy to find companies that sell to individuals. For instance, the chemical company listed as a source in one of my articles sells only to researchers and research institutes. I found several companies on the internet which were selling it for over $90.00 for 25 mg. I guess none of us should be in a rush to try this! For me, clearly, this must remain in the realm of the possible! Perhaps some enterprising chemical company will offer us a product that’s cheap and easy to use. Compost and Compost Tea It has been known for some time that compost and compost tea protect plants against disease. This discovery was made by accident in the 1960’s when the nursery industry started using composted bark instead of peat in an effort to cut costs. The plants grown in the composted bark not only grew better, but nurserymen also experienced fewer losses due to phytophthera root rot. There are several possible explanations for this result. Microbes in the compost produce antibiotic compounds which protect plants. The microbes in the compost may also outcompete disease producing bacteria and fungi. Some of the protection may also result from triggering defensive responses (systemic acquired resistance or induced systemic resistance) in plants. Compost and compost tea, for instance, will induce resistance to the fungal disease anthracnose in cucumbers. Compost made out of yard waste has been effective in inducing resistance to pythium, a root and stem rot disease. Compost is just as effective as chemical fungicides in controlling phytophthora root rot. Most of us are aware that not all composts are created equal, but it is especially important to purchase composts that use the correct ingredients and that are made under rigorously controlled conditions for protection against plant disease. For a compost to be effective in preventing disease, it must, first of all, be of a consistent quality. It must have a moisture level of 40-50% so it can sustain the growth of beneficial microbes. Next, it is important that the compost be stabilized, in other words, most of the breakdown of organic material has taken place by the time it is used. It is also important that some microbial activity is still sustainable when it is applied. The disease suppressing microbes have to be present, and to do their job they have to have food. Therefore, the compost still has to have material for them to break down. Very saline composts, made chiefly from manures actually enhance the growth of pythium and phytophthera root rot bacteria, so to maximize disease control, composts made chiefly of plant material definitely have the edge. Composts made out of municipal sewage sludge are a no no for two reasons. Just think of all the chemicals people flush down the drain, not to mention the possible heavy metal content! They are also unsuitable for protecting plants because they have a low carbon to nitrogen ratio and their high nitrogen content actually enhances fusarium wilt. The compost must be made under very exacting conditions. It must be allowed to reach temperatures of 55-70 C (131-158 F) for long enough to kill all existing disease organisms and most seeds. During this time the pile must be turned regularly, so that all parts are allowed to come to temperature. Unfortunately, these high temperatures also destroy the very beneficial organisms which are needed to control diseases. This is why curing is such an essential part of creating an effective compost. When the pile cools, it is allowed to sit for a time to allow the disease controlling organisms to recolonize the compost from the periphery, or from outside of the pile. To assure that the correct disease controlling organisms are in place, sometimes piles are deliberately inoculated with microbes. Composts made in open spaces like fields are better than those made in enclosures. More of the disease controlling organisms are available for recolonization. Compost teas have also been shown to control plant diseases. They are made from compost steeped in water. After filtering, this liquid can be sprayed on plants to help control foliar diseases. Its effectiveness depends on the quality of the compost it is made from and varies with the disease. Some of the commercially produced compost teas have actually been inoculated with disease controlling microorganisms. Some researchers have attributed the plant protective qualities of such compost sprays to systemic acquired resistance or induced systemic resistance in plants resulting from the presence of microbes, but this theory has recently been called into question. Some researchers attribute these effects to microbes present in the soil. Should you add compost to your potting mixes? By all means! Do keep in mind the above criteria and choose your compost carefully. Be sure to use a relatively small amount, so as not to interfere with good drainage in your containers. It is not going to be easy to find a good compost! You might have to call the producer and find out how the compost was made. Selected References: Quarles, W. “Aspirin, composts, talking plants and induced systemic resistance.” IPM Practitioner 24 (5/6) May/June 2002:1-9 Hoitink, H.A.J., A.G. Stone and D.Y. Han. ”Suppression of plant diseases by composts.” HortScience 32 (2) April 1997.

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

Part II. Jasmonic Acid, Compost and Compost Tea

Paula Szilard

Part I of this article appeared in the May 2005 issue of this newsletter and dealt with mobilizing the plant’s own defenses against disease by spraying dilute solutions of salicylic acid (aspirin). Part II deals with other substances which elicit a reaction known as systemic acquired resistance or induced systemic resistance in plants, against insect attack and disease. Among these substances are jasmonic acid, compost and compost tea. Worm castings and rhizobacteria (root bacteria) will be discussed in Part III , to appear in our October issue.

Jasmonic acid

Jasmonic acid is a plant hormone that has been found in over 160 families of plants. Its presence in plant tissue is probably universal. Plants produce it from two common fatty acids. When a plant is wounded, chewed on by insects or is under drought stress, levels of jasmonic acid increase dramatically. These increased levels of jasmonic acid in turn trigger the production of substances that interfere with the digestion of proteins in the leaf, making the plant less digestible and therefore less attractive to insects. It’s the yuck factor at work. Interestingly, much more jasmonic acid is produced when a plant is chewed by insects and the compounds are qualitatively different from those produced as a result of mechanical wounding.

A volatile form of jasmonic acid, methyl jasmonate, actually acts as a chemical messenger. When a plant is under insect attack, the jasmonic acid in the plant causes the release of methyl jasmonate, an essential oil of the jasmine plant, Jasminum grandiflorum, along with other volatile compounds. The volatile compounds released may vary with the attacking insect. Their function is twofold. They are alarm signals that “warn” nearby plants that war is being waged and their meaning is unmistakeable! “Get your defenses up for the attack!” For instance, researchers have found that methyl jasmonate released by a sagebrush (artemisia) could create a defensive reaction in a nearby wild tobacco plant. At the same time, the release of methyl jasmonate sends a chemical message to beneficial insects that a plant is under attack. It is analogous to a human cry for help, or viewed more optimistically, an invitation to all of your friends that there’s a free lunch available here! (OK, don’t believe me, but if you want to see some literature on this, I’d be happy to oblige! Actually If I had not read some pretty reliable sources, I would not believe it either!)

So far the release of these volatile compounds has been documented in 23 plant species. In a field experiment, tomato plants were sprayed with jasmonic acid dissolved in acetone. Elevated levels of proteinase inhibitors, the substances that make the protein in the leaf more difficult to digest and other substances that make the leaf less palatable were noticeable within three days. The effects lasted three weeks, at which time the plants were sprayed again. As anticipated, fewer chewing and sucking insects (army worms, flea beetles, peach aphids, and thrips) were found on the sprayed plants. In other experiments, the two-spotted spider mite, leaf miners, corn earworm caterpillars, and hornworm caterpillars were negatively affected by sprays of jasmonic acid. Aphids were not consistently affected by the treatment. Such results have also been seen in soybeans, cotton and birch. Interestingly, a volatile form a salicylic acid, methyl salicylate has also been found to attract predatory mites which then attack the pest mites found on cucumbers, roses, apples, pears, tomatoes and ground ivies.

It is important to note that while treatment with jasmonic acid resulted in less insect damage, it did not affect production levels. This could mean that the treatment would be best reserved for flowers and other high value ornamentals where even a little damage might deter a potential customer. If you are thinking of purchasing jasmonic acid, it may not be that easy to find companies that sell to individuals. For instance, the chemical company listed as a source in one of my articles sells only to researchers and research institutes. I found several companies on the internet which were selling it for over $90.00 for 25 mg. I guess none of us should be in a rush to try this! For me, clearly, this must remain in the realm of the possible! Perhaps some enterprising chemical company will offer us a product that’s cheap and easy to use.

Compost and Compost Tea

It has been known for some time that compost and compost tea protect plants against disease. This discovery was made by accident in the 1960’s when the nursery industry started using composted bark instead of peat in an effort to cut costs. The plants grown in the composted bark not only grew better, but nurserymen also experienced fewer losses due to phytophthera root rot. There are several possible explanations for this result. Microbes in the compost produce antibiotic compounds which protect plants. The microbes in the compost may also outcompete disease producing bacteria and fungi. Some of the protection may also result from triggering defensive responses (systemic acquired resistance or induced systemic resistance) in plants. Compost and compost tea, for instance, will induce resistance to the fungal disease anthracnose in cucumbers. Compost made out of yard waste has been effective in inducing resistance to pythium, a root and stem rot disease. Compost is just as effective as chemical fungicides in controlling phytophthora root rot.

Most of us are aware that not all composts are created equal, but it is especially important to purchase composts that use the correct ingredients and that are made under rigorously controlled conditions for protection against plant disease. For a compost to be effective in preventing disease, it must, first of all, be of a consistent quality. It must have a moisture level of 40-50% so it can sustain the growth of beneficial microbes. Next, it is important that the compost be stabilized, in other words, most of the breakdown of organic material has taken place by the time it is used. It is also important that some microbial activity is still sustainable when it is applied. The disease suppressing microbes have to be present, and to do their job they have to have food. Therefore, the compost still has to have material for them to break down. Very saline composts, made chiefly from manures actually enhance the growth of pythium and phytophthera root rot bacteria, so to maximize disease control, composts made chiefly of plant material definitely have the edge. Composts made out of municipal sewage sludge are a no no for two reasons. Just think of all the chemicals people flush down the drain, not to mention the possible heavy metal content! They are also unsuitable for protecting plants because they have a low carbon to nitrogen ratio and their high nitrogen content actually enhances fusarium wilt.

The compost must be made under very exacting conditions. It must be allowed to reach temperatures of 55-70 C (131-158 F) for long enough to kill all existing disease organisms and most seeds. During this time the pile must be turned regularly, so that all parts are allowed to come to temperature. Unfortunately, these high temperatures also destroy the very beneficial organisms which are needed to control diseases. This is why curing is such an essential part of creating an effective compost. When the pile cools, it is allowed to sit for a time to allow the disease controlling organisms to recolonize the compost from the periphery, or from outside of the pile. To assure that the correct disease controlling organisms are in place, sometimes piles are deliberately inoculated with microbes. Composts made in open spaces like fields are better than those made in enclosures. More of the disease controlling organisms are available for recolonization.

Compost teas have also been shown to control plant diseases. They are made from compost steeped in water. After filtering, this liquid can be sprayed on plants to help control foliar diseases. Its effectiveness depends on the quality of the compost it is made from and varies with the disease. Some of the commercially produced compost teas have actually been inoculated with disease controlling microorganisms. Some researchers have attributed the plant protective qualities of such compost sprays to systemic acquired resistance or induced systemic resistance in plants resulting from the presence of microbes, but this theory has recently been called into question. Some researchers attribute these effects to microbes present in the soil.

Should you add compost to your potting mixes? By all means! Do keep in mind the above criteria and choose your compost carefully. Be sure to use a relatively small amount, so as not to interfere with good drainage in your containers. It is not going to be easy to find a good compost! You might have to call the producer and find out how the compost was made.

Selected References:

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

Hoitink, H.A.J., A.G. Stone and D.Y. Han. ”Suppression of plant diseases by composts.” HortScience 32 (2) April 1997.