Years ago I came upon some original research that, at that time, was not widely publicized. The work concerned the use of acetylsalicylic acid (ASA) to treat plants with viral infections. Intrigued, I decided to give this a try for myself. I obtained some laboratory grade ASA and began an experiment, using it on a specimen of Alocasia that had begun throwing distorted viral-infected leaves. To my surprise and delight, the leaves emerging after treatment were perfect and virus free! The results I observed inspired me to write an article about it for the International Aroid Society newsletter, and started me on a quest to find out what was going on.
A little background history of aspirin is appropriate here. As far back as the fifth century B. C., chewing the bark of the Willow tree was known to relieve pain and fever. The active principle in the Willow turned out to be a compound called salicin, which is closely related to acetylsalicylic acid, or aspirin. In fact, the word "aspirin" comes from joining the "a" from acetyl with "spir" from the name of a plant, Spiraea, from which the Bayer Company got their salicin, and "in", a common ending in drug nomenclature. I share this bit of history as a way of demonstrating that the active principle is related to a naturally derived compound, salicylic acid, that plays a significant role in the response I observed in my experiment.
Resistance is not futile!
More years went by and the only times I thought about this discovery again was whenever one of my plants contracted a virus infection. I still had some of my acetylsalicylic acid left, so I would simply mix up a batch, spray my plant, and all would be well. That is, until about a year ago when, with modern access to information via the internet, I resolved to see what the current state of the art was in regards to this. To my immense surprise, a great deal of progress had been made in this area. A whole field of study had sprung up; the study of systemic acquired resistance to disease in plants, or SAR for short.
Scientists, studying the response of plants to infections by pathogens, observed that if one leaf got infected, other leaves nearby began to exhibit resistance to the infection. Much research has been performed to determine the active agent or mechanism for this heightened resistance, and significant strides have been made. Within the plant, certain biochemicals are produced, activating genes that confer resistance to the spread of the infection to uninfected plant parts. Interestingly, one biochemical that is considered to play a role in this is salicylic acid. Thus, my application of ASA externally acted to trigger the activation of the same genes that the internal, or endogenous, salicylic acid did. That explained why my plant responded to the ASA spray.
But it turns out that even more was going on here, as other biochemicals were involved. Furthermore, the question remained as to what from the pathogen was acting to stimulate this response in the plant. Research using Erwinia amylovora, the pathogen responsible for fire blight, revealed that a certain type of protein from the pathogen, called harpin protein, stimulated the SAR response in plants exposed to it. The next step was demonstrating that the protein alone, without the pathogen, would stimulate the response. This turned out to be the case, and the harpin protein is now available to gardeners as a product that can be applied as a spray to induce SAR.
The bottom line is that both acetylsalicylic acid and harpin protein signal the plant to activate an internal immune system. Applying either of these to uninfected plants causes them to respond as though they have been compromised by a pathogen, but without the damage. This is surprisingly similar to the immune response in animals and humans, whereby exposure to a dead or weakened pathogen results in the production of antibodies that confer protection against fully functional disease organisms.
This is exciting research, to say the least. Instead of trying to kill off the pathogens by the use of toxic pesticides, the plant is stimulated to activate its own defense system to guard against attack before it happens. A bonus is that the plant becomes more vigorous and healthy in the process.
Do try this at home!
So now that you are enthralled with the possibility of protecting your plants this way, how do you do it? If you want to try the acetylsalicylic acid technique, all you need is three (3) aspirin tablets and four (4) gallons of water. This combo will make a 1:10000 solution, which is what was used in the research. Dissolve the aspirin tablets in the four gallons of water, then spray the mixture on the plants you'd like to stimulate. In my own work I've used an ounce of dimethyl sulfoxide (DMSO) as a solvent for the acetylsalicylic acid before adding it to the water. DMSO also acts as a powerful cell penetrating agent, so anyone using it should wear personal protective equipment, just as is recommended when working with any garden chemical.
If you choose to try out the harpin protein, you can obtain a product called Messenger, sold by Eden BioScience and designed for just this purpose. Messenger comes in pre-measured packets that mix with one gallon of water to make a final solution for spraying. Eden BioScience's website has plenty of helpful information about harpin protein and SAR, too, so it is worth your while to visit, even if only for the education.
Welcome to the promising new world of induced plant immunity!
Salix nigra line drawing from Britton and Brown (1913) and in the public domain.