By Gary Pilarchik (The Rusted Garden)
Tomatoes use a hormone similar to salicylic acid to trigger a response called 'systemic acquired resistance' or SAR. This response helps a tomato plant fight off bacteria, fungi and other diseases. This defense response is a naturally occurring internal plant process that can be started manually (so to speak) by spraying your tomato plants with aspirin. The salicylic acid in the aspirin will trigger the tomato's defenses, by mimicking the natural hormone, as if it were being attacked. You can preemptively trigger the SAR response in tomatoes. Why is this a good thing?
The defenses of a tomato are typically triggered when a disease takes hold. Be it bacteria, fungi or a virus, the tomato responds once it is attack. Very often the disease has already taken hold and has spread before the tomato's SAR or defense is triggered and fully utilized. It is often too late by the time the natural response is triggered and in full defensive swing... the tomato is already fully infected. By spraying your tomatoes you can activate your tomatoes systemic acquired resistance response before a threat is present. You set your tomato's defenses on alert and they are looking for a disease.You have beefed up your beefsteaks!
The theory goes like this... the tomato has its SAR response or tomato defenses activated before a disease is present. All the defenses are acting as if the plant is being attacked. This makes it much more difficult for future bacteria, fungi or other diseases to take hold and get established. It also slows the progress of the potential future diseases. Two good things occur. Your tomatoes can prevent diseases from taking hold and you can have a better chance of treating symptoms that may take hold and show up on your tomato plants. The goal is to maintain a mostly healthy tomato plant until you get lots of fruit from it.
This is not a 100% solution to tomato diseases. It is a method that I can says really seems to work. I have not gotten Septoria Leaf Spot in my garden for several years after having it severely attack most of my tomato plants. It doesn't completely prevent Early Blight, another common disease, but I have had fewer tomatoes get the disease. And the aspirin sprayed plants that got the Early Blight seemed to have the disease progress much more slowly than untreated tomatoes from the past.
What else do I do with my tomatoes to managed diseases? I prune out leaves to allow air flow. I remove 8-12 inches of lower leaves to prevent soil splash. I mulch to prevent soil splash. I spray with either a baking soda spray or a wettable sulfur spray as a disease preventative. I have been doing all these things before I started using aspirin spray.
I believe aspirin spray works. It is one significant tool in combating tomato disease. You may notice a thicker tomato leaf. I have had this happen on several plants. You might also get some leaves that curl. This is only cosmetic. I recommend you do an internet search on aspirin, salicylic acid and tomatoes. There are extensive research studies that support this entry. It is very interesting to view the research.
I spray my plants about every 2 weeks with 1 standard strength 325 mg aspirin mixed in a gallon of water. Above is the video that explains the basic process. There is no exact recipe. Just get some aspirin in water and briefly spray the plant down. Good luck!