It is based on the theory that since some plants produce a substance similar to salicylic acid when they are stressed, spraying them with an aspirin solution will help ward off disease. If you really want to see if this works, have a control group of plants of the same variety that you don't spray. Otherwise, it may be the weather or other factors that are responsible for your good results. I could find no scientific studies that back this up.

I subscribe to a dude on You Tube that swears by it. It's his little secret. We are going to do it this year.

The United States Department of Agriculture found that salicylic acid produced an enhanced immune response in plants of the nightshade family. The enhanced response helped prepare the plant for microbial or insect attack.

Gardeners at the University of Rhode Island sprayed a mixture of aspirin water on their vegetable gardens and found that plants grew more quickly and were more fruitful than a control group left untreated. Aspirin in vegetable gardens produced healthier plants than the control group. The team used a rate of three aspirins (250 to 500 milligrams) mixed with 4 gallons of water. They sprayed this every three weeks throughout the growing season. The vegetables were grown in raised beds with drip irrigation and compost-rich soil, which probably aided the effects found from using aspirin for plant growth.

The lady who did the experiment in 2004 was a Master Gardener, as am I, not a scientist, and her findings have not been replicated or published by any university. https://www.gardenmyths.com/aspirin-spray-tomatoes-vegetable-garden/

I watched more You Tube videos on the subject, got my uncoated Aspirin, and I'm fixin' to start spraying tomorrow morning early, Can't hurt and I'm no researcher. If you really want to know if Aspirin works, only do half your plants, but I want to believe it works so I'm doing all of them. But it only works for members of the nightshade family like potatoes and eggplant and of course, tomatoes. You can flood the roots too. The You Tubes I watched also said to put a tsp of soap in the gallon to help it adhere to the leaves. Hubby is pruning the leaves today so the spray can reach everywhere. Haha I had a thought. We could start a rumor. Tomatoes cure headaches!