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Remarks (public):For a complete description including images see 
Remarks (internal):This species has been recorded as severe on pear and quince with up to 50% of the fruit affected where the alternate host was close. Infection can occur, however, at distances of 8-10 miles from the alternate host. Lime sulphur has been found to give some degree of control (Jackson, 1914). Reported as rare on pear in California (4, 201).
Description type:Non-original description 
Description:Gymnosporangium libocedri (Henn.) Kern, Bull. Torrey Bot. Club 35: 509, 1908.
Phragmidium libocedri Henn., Hedwigia 37: 271, 1898.
Aecidium blasdaleanum Dietel & Holway, 1895.
Gymnosporangium aurantiacum Sydow, 1904.
Gymnosporangium blasdaleanum Kern, 1911.
Pycnia on the upper side of the leaves in small groups. Aecia mostly on the undersides of the leaves in small groups or clusters on slight swellings, cupulate, white, 0,3-0,5 mm diam. Aeciospores ± globose, subangular, 17-22 µm diam.; wall hyaline, irregularly verrucose, 1-1,5 µm thick; no pores visible but with 3 (?) pore plugs on the diagonal sides, easily detached. Peridial cells rhomboid, short, verrucose. Telia foliicolous, scattered, cushion-like, usually without swellings but sometimes causing witches' brooms, 1-2 mm diam., reddish-brown. Teliospores 1-5-celled, globose or almost cuboid in some 1-celled spores to long cylindric in 5-celled spores, constricted at the septa, rounded, 20-95 x 18-30 µm; wall yellow to cinnamon 0,5-l µm thick, up to 3 µm thick at the pores; pores 2 per cell, apical in most cells; pedicels often very thick, up to 35 µm diam.
Hosts: Aecia on apple (Malus sylvestris) and other Malus spp., pear (Pyrus communis) and quince (Cydonia vulgaris). Also on Amelanchier, Chaenomeles, Crataegus and Sorbus. Telia on Calocedrus (Libocedrus) decurrens.
Disease: Aecia on leaves and fruits. Telia on leaves; usually not causing distortions but sometimes witches' brooms present.
Geographical distribution: Northwestern USA (Washington, Oregon to northern California and Nevada).
Literature: Jackson, Phytopathology 4: 261-268, 1914.
Taxon name: