The present invention relates to novel strains of avian reovirus that were isolated from clinical cases of viral arthritis/tenosynovitis in chickens in the southeast United States. The invention is directed to these novel group 1 and group 2 avian reoviruses diagnostic assays using nucleotide or amino acid specific components of such viruses such as the S1 gene encoding the sigma C protein and to vaccines that protect chickens from disease caused by such viruses.