Cronartium rust sporulation on hemiparasitic plants

Susceptibility of potential alternate host plants to pine stem rusts belonging to Cronartium spp. was artificially tested in Finland during 2012–2013. Forty‐three species representing 11 plant families were inoculated in the laboratory; 34 species (11 families) were inoculated in the greenhouse with aeciospores of Cronartium flaccidum or C. ribicola. Twenty‐one selected species (10 families) were also exposed to natural inoculum of C. flaccidum in the field in two severely affected Pinus sylvestris stands. After 5–8 weeks’ incubation, C. flaccidum sporulated on 17 species (nine families) in the laboratory, 17 species (eight families) in the greenhouse and seven species (five families) in the field. Cronartium ribicola sporulated on three species (three families) in the laboratory or greenhouse. All of the hemiparasitic plants that belong to Orobanchaceae were infected by C. flaccidum, and several species supported rust sporulation when exposed to natural inoculum. Susceptible species belonged to genera Veronica, Euphrasia, Castilleja, Pedicularis, Rhinanthus, Saxifraga, Loasa, Ribes, Tropaeolum, Swertia, Physalis, Nicotiana, Hyoscyamus, Paeonia, Apocynum, Impatiens, Vincetoxicum and Myrica.