Abstract A description is provided for Rhytisma salicinum . Information is included on the disease caused by the organism, its transmission, geographical distribution, and hosts. DISEASE: Tarspot disease of willow. HOSTS: Leaves of Salix acutifolia, S. amygdaloides, S. arbuscula, S. arctica. S. arctica var. brownei, S. arctophila, S. atrocinerea, S. atrocinerea × purpurea, S. aurita, S. barclayi, S. brachycarpa, S. caprea, S. cinerea, S. cordata, S. deprena, S. discolor, S. glauca, S. groenlandica, S. hastata, S. herbacea, S. herbacea × polaris, S. humilis, S. kitaibeliana, S. lanata, S. lapporum, S. lasiandra, S. linearifolia, S. longifolia, S. lucida, S. lutea, S. missouriensis, S. monticola, S. myrsinifolia, S. myrsinites, S. myrtilloides, S. nigra, S. nivalis, S. oleaefolia, S. pallasii, S. petiolaris, S. phlomoides, S. phylicifolia, S. piperi, S. polaris, S. pulchra, S. purpurea, S. repens, S. reticulata, S. richardsonii, S. rosmarinifolia, S. rostrata, S. rotundifolia, S. scouleriana, S. silesiaca, S. sitchensis, S. siuzewii, S. starkeana, S. triandra, S. uva-ursi, S. viminalis, Salix sp. GEOGRAPHICAL DISTRIBUTION: Belarus, Belgium, Canada (Alberta, British Columbia, Manitoba, New Brunswick, Newfoundland, Northwest Territories, Nova Scotia, Ontario, Prince Edward Island, Québec, Saskatchewan, Yukon), China, Denmark, Estonia, Faroes, Finland, France, Republic of Georgia, Germany, Greece, Iceland, India (Himachal Pradesh), Iran, Ireland. Italy, Japan, Kazakhstan, Kirghizia, Latvia, Morocco, Norway, Pakistan, Portugal, Rumania, Russia (Altaïskyi Krai, Irkutskaya Oblast, Kamchatskya Oblast, Karelia, Krasnoyarskyi Krai, Kursk Oblast, Leningrad Oblast, Mari, Murmansk Oblast, Novgorod Oblast, Orenburg Oblast, Primorskyi Krai, Pskov Oblast, Ryazan Oblast, Smolensk Oblast, Stavropolkyi Krai, Tambov Oblast, Tomsk Oblast, Tula Oblast, Vologda Oblast, Voronezh Oblast), Slovakia, Spain, Sweden, Switzerland, UK (England, Scotland, Wales), USA (Alaska, California, Colorado, Idaho, Kansas, Maine, South Dakota, Washington, Wyoming), Ukraine. Altitude records exist up to 3400m (India), 1950m (USA), 1500m (Greece), 1370m (Canada), 1100m (UK), 900m (Finland), 850m (Morocco), 660m (Ireland) and 600m (Ukraine). The distribution of Rhytisma salicinum is predominantly north-temperate to sub-arctic and circum-polar. In addition, the fungus can be encountered at higher altitudes in mediterranean countries as far south as north Africa. In Britain the species occurs predominantly in the north and west, though exceptional records are known. This may also perhaps indicate a preference for a cooler and wetter climate. The fungus is probably very widely distributed, and in some places may be locally abundant. TRANSMISSION: By air-borne ascospores in humid conditions.