Relationships, host-ranges and symptoms of some isolates of phleum mottle virus

SUMMARY Eleven isolates of phleum mottle virus, each from a different grass species, were grouped into five distinct strains: phleum mottle (PM), holcus transitory mottle (HTM), festuca mottle (FM), cocksfoot mild mosaic (CMM) and brome stem-leaf mottle (BSM). Their interrelationship was expressed three-dimensionally using serological differentiation indices. When 105 species of Gramineae were inoculated with each strain, eighty-one in forty genera became infected. These included festucoid, chloridoid, panicoid and andropogonoid species. Electron microscopy was more reliable than back-inoculation for detecting infection in plants that remained symptomless. Annual species were usually more severely affected than perennials. PM, HTM, FM, CMM and BSM did not appear to differ in host range, but sometimes differed in the symptoms induced in a given host. Each was symptomless in some hosts, caused a mild or severe mottle in others and mottling plus necrosis in a few. HTM was symptomless in most, and BSM in fewest species: BSM caused necrosis in most and HTM in fewest hosts.