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Remarks (internal):In North America, G. tremelloides is limited to the western provinces and states. It has been recorded from soutbeast Alaska and in the states adjacent to Alberta and British Columbia, and south to Utah and Colorado. It is widely recorded in continental Eurasia from Spain to the U.S.S.R., where hosts additional to those known for North America include apple and quince. However, no economically important hosts are affected in North America.
G. tremelloides occurs on the same hosts, both telial and aecial, as G. cornutum. These two rusts are easily separable on characters usable in the field: on Juniperus, G. cornutum has pulvinate telia appearing mainly on the needles, while on Sorbus its coarsely lacerate aecial peridium retains its cylindrical shape far into the fall season. The two rusts are often found together on the same trees or even on the same leaves, and they mature about the same time. Inoculation tests with G. tremelloides show that aecia require 75-87 days to mature under the cool 15-18°C (61-65°F) greenhouse temperatures of the test.
Description type:Non-original description 
Description:Gymnosporangium tremelloides Hartig, Lehrb. Baumkrankh. 55. 1882. (Transl. by Somerville and Ward - Textb. of tree diseases, London, 1894).
Pycnia epiphyllous, yellow-orange, becoming black, on conspicuously discoloured spots. Aecia hypophyllous or petiolicolous, causing considerable swelling of vascular tissue; spore mass dark brown; peridium finely lacerate to base, and usually slightly recurved. Aeciospores globoid to broadly ellipsoid, 32-46 x 30-37 µm; wall dark yellow-brown, 1.6-4.2 µm thick, densely verrucose; pores 10-14, scattered. Peridial cells regularly seen in side view, individual cells straight in lactophenol (but the chain of cells becomes curved), hyaline, broadly rectangular, 55-95 x 27-48 µm,, coarsely rugose; inner wall rough, 8-16 µm thick; outer wall smooth, 1.6-3.2 µm thick. Telia on globoid galls up to 20 mm. diam., caulicolous, dark brown, applanate over most of gall surface. Teliospores ellipsoid, not or slightly constricted at septum, 37-73 x 18-29 µm; wall pale yellow to dark yellow-brown, smooth, 0.7-1.6 µm thick; pores 1 or 2 per cell at septum, sometimes with an extra pore at apex; pedicel hyaline, cylindrical, 5.5-7.0 µm diam. near attachment to spore.
Hosts: O, I: Sorbus. III: Juniperus communis L. var. depressa Pursh.
Distribution: southern Alta., southern B.C. (N to Prince George), southern Yukon and adjacent N.W.T
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