Small cysteine-rich (SCR) proteins, including fungal avirulence proteins, play important roles in pathogen-plant interactions. SCR protein-encoding genes have been discovered in the genomes of pathogens but their functions during pathogenesis remain obscure. Here, we report the characterization of one SCR protein (namely, SCR82) with similarity to phytotoxic protein PcF. The gene has 10 allelic sequences in the population. Homologs of SCR82 were not identified in fungi or other organisms but in relative species. Initially, was weakly expressed during the mycelium, sporangium, and zoospore stages but quickly upregulated when the infection initiated. Both ectopic expression of SCR82 and recombinant yeast-expressed protein (rSCR82) caused cell death on tomato leaves. Upon treatment, rSCR82 induced plant defense responses, including the induction of defense gene expression, reactive oxygen species burst, and callose deposition. Knockout of in by CRISPR/Cas9 severely impaired its virulence on host plants and significantly reduced its resistance against oxidative stress. Inversely, its overexpression increased the pathogen's virulence and tolerance to oxidative stress. Our results collectively demonstrate that SCR82 functions as both an important virulence factor and plant defense elicitor, which is conserved across spp.[Formula: see text] Copyright ? 2021 The Author(s). This is an open access article distributed under the CC BY-NC-ND 4.0 International license.