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On a common ornamental tree in New York City, Crataegus phaenopyrum. Aecia tan with whitish tips. Straight and horn like—splitting midway to release brown finely verrucose spores. Spores 17-20 x 21-27um. Reference: Manual of Rusts of the United States and Canada—which lists G. clavipes as an option as well, but spore color and size don’t match. Spore color and size match Arthur’s description of G. trachysorum, however the spores are distinctly angular rather than globoid, perhaps because they’re still immature?