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Remarks (public):For a complete description including images see 
Remarks (internal):This rust is of some importance on apple in Europe (3, 255; 7, 34; 29, 404; 31, 559; 34, 20; 42, 207) where it can cause severe defoliation and also attack the fruit especially at the blosson end (7, 34; 17, 536; 52, 4139) although aecia are rarely found on fruit. Not recorded on apple in N. America. There are two general accounts (31, 559; 36, 279). Varietal susceptibility and resistance are partially given (3, 255; 17, 535; 31, 559). For control lime sulphur (3, 255) and fermate (31, 559) have been used.
Description type:Non-original description 
Description:Gymnosporangium tremelloides Hartig, Lehrb. Baum-Krankh.: 55, 1882.
Roestelia penicillata Fr., 1849.
Pycnia on the upper side of the leaves in small groups 1-3 mm diam. on brown spots. Aecia on the underside of the leaves, in small groups up to 5 mm diam., on slight swellings, pale, soon finely lacerate to the base, 0,5-l mm diam., contents reddish-brown. Aeciospores globose to broadly ellipsoid, 28-50 µm diam., wall cinnamon, finely verrucose, 2-5 µm thick, pores 9- 13, not or hardly thickened. Peridial cells rhomboid, short rugose. Telia caulicolous on slightly fusiform cankers or on large gall-like swellings, or on hemispheric swellings along the sides of branches, cushion-like when dry but swollen irregular tremelloid when moist, chocolate brown, drying paler. Teliospores ellipsoid to fusoid, 2-celled, slightly constricted, rounded or tapered, 40-60 x 20-30 µm; wall yellowish to golden, 0,5-2 µm thick; pores 2 per cell, by the septum.
Hosts: Aecia chiefly on apple (Malus sylvestris) and Sorbus, also on other Malus spp. and quince (Cydonia vulgaris). Telia on Juniperis communis and other species of the 'oxycedrus' group.
Disease: European apple rust. Aecia on leaves. Juniper gall rust, telia on fusiform or gall-like swellings of the smaller branches of juniper (Juniperus communis) or on hemispheric swellings along the sides of large branches. Geographical distribution: Widely distributed in Europe (but not in Britain) and extending into N.W. Africa. Also in China and in western N. America from Alaska to Colorado and Utah.

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