The phytohormone salicylic acid (SA) is known to regulate plant immunity against pathogens. Plants synthesize SA via the isochorismate synthase (ICS) pathway or the phenylalanine ammonia-lyase (PAL) pathway. The ICS pathway has been fully characterized using Arabidopsis thaliana, a model plant that exhibits pathogen-inducible SA accumulation. Many species including Populus (poplar) depend instead on the partially understood PAL pathway for constitutive as well as pathogen-stimulated SA synthesis. Diversity of SA-mediated defense is also evident in SA accumulation, redox regulation, and interplay with other hormones like jasmonic acid. This review highlights the contrast between Arabidopsis and poplar, discusses potential drivers of SA diversity in plant defenses, and offers future research directions.