British distribution: Widespread, locally frequent.
Gymnosporangium is a genus in which virtually all species are heteroecious, alternating between members of the rose family (Rosaceae, subfamily Maloideae - pears, apples, hawthorn, etc.) (spermogonial and aecial stages), and the juniper family (Cupressaceae) (telial stage). With very few exceptions, the uredinial stage is absent. The majority of species are N.American.
The four British species are typical of the genus, with aecia on hawthorn, pear, rowan, medlar, etc., and with telia on juniper. G. clavariiforme is the commonest species in Britain on hawthorn (Crataegus spp.), usually on C. monogyna, as pictured here. It forms characteristic swellings on the stems, on which then appear the rather long, ragged-edged aecia. Aecia may also form on the fruits. The similar but rarer G. confusa has less deeply lacerate aecia.
Photographed on Harting Down, Sussex, 1986.
For an explanation of the terms used in this account, go to the profile of the Pucciniales.