Five plant families support natural sporulation of Cronartium ribicola and C. flaccidum in Finland.
Natural sporulation of Cronartium ribicola and C. flaccidum were investigated on known and potential alternate hosts growing in three botanical gardens representing different geographic regions in Finland and in some natural habitats next to the northern botanical garden. Uredinia and telia of Cronartium were searched for from over 27,000 leaves collected or checked in the field in 66-96 species representing 16 plant families susceptible to Cronartium. Cronartium ribicola was found on 19 species or cultivars, while C. flaccidum occurred on 17 species. Uredinia and telia of Cronartium ribicola lacked in the southern botanical garden, but they occurred on 5-16 Ribes species or cultivars in the other two botanical gardens. Cronartium ribicola sporulated on one to four Ribes rubrum cultivars, one to three R. uva-crispa cultivars, three to seven R. nigrum cultivars, R. aureum and R. glandulosum in the gardens. Cronartium flaccidum sporulated on five to nine species in the botanical gardens including species of Paeonia, Melampyrum, Loasa, Pedicularis and Vincetoxicum. In natural habitats next to the northern botanical garden, C. flaccidum sporulated also on Melampyrum sylvaticum and Euphrasia stricta var. stricta. Among all the infected species, C. flaccidum sporulated most frequently on Paeonia. For the first time, C. flaccidum was found on E. stricta growing in its natural habitat. Although the frequency of infected leaves under natural inoculum was low for several infected species, the large variety of susceptible species serve as an efficient tool for Cronartium to spread. Based on the amount of natural sporulation in the gardens and their surroundings, both Cronartium rusts are widely distributed and sporulate in different geographic areas in Finland.