How to use aspirin on tomato plants to prevent diseases:

“The concept is well researched and documented. The aspirin mimics a hormone in the tomato plant used to trigger tomato stress defenses. This is call the Systemic Acquire Resistance (SAR) response. Your beefsteak is fooled into beefing up its natural defenses before fungal leaf diseases arrive. This makes it harder for future diseases to establish on their leaves.”

All-America Selections Flowers & Vegetables