Pathogenicity of cherry‐spruce rust, Thekopsora areolata, was investigated by inoculations with aeciospores from seven Norway spruce, Picea abies, seed orchards that had suffered from successive severe rust epidemics in the 2000s in Finland. Detached leaves of Prunus spp. were inoculated in the laboratory using aeciospores from cones of various ages. In the greenhouse, live Prunus padus plants were inoculated, and possible autoecism of the rust was tested by inoculations of Picea abies seedlings. Thirty‐five spore sources from the seed orchards formed uredinia on Prunus spp. in the laboratory 2 weeks after incubation, but no telia developed. In the greenhouse, uredinia developed on live P. padus, but no rust symptoms, cankers or sporulation were detected on Picea abies. Thus, no evidence of autoecism was observed among the T. areolata populations and therefore all populations, suggesting all those tested from the Finnish seed orchards were heteroecious.

A severe epidemic of Chrysomyxa ledi needle rust occurred over large areas in northern Finland in 2015. In late summer, young current-year cones and needles of 7-8 Picea species were sampled in a northern botanical garden, and the frequencies of cone rusts, Thekopsora areolata, Chrysomyxa pirolata and C. ledi, were investigated in cones, and C. ledi was investigated in needles. In addition, the frequency of C. ledi was estimated visually in current-year needles of ca. 400 trees of 17 species and varieties of Picea in the garden. Chrysomyxa pirolata was absent in cones of all Picea. No cone rusts were observed in cones of P. rubens and Picea mariana. Aecia of Thekopsora areolata occurred moderately in cones of P. abies (26% of cones) and rarely in P. engelmannii (3%) and P. omorika (6%). Chrysomyxa ledi was the most frequent rust in cones; aecia occurred abundantly in cones of P. abies (100%), P. engelmannii (69%), P. glauca (45%) and P. omorika (41%), and occasionally on P. pungens (13%). Thekopsora areolata from cones of the three Picea species showed 100% similarity in ITS2 sequence to one another and to samples in the GenBank. Samples of Chrysomyxa from needles and cones of Picea and leaves of Ledum palustre had ITS2 sequences 99.9%-100% similar to one another and 99.8%-100% similar to the closest sequence of C. ledi and C. rhododendri in the GenBank. Thekopsora areolata was recorded for the first time in cones of P. omorika and C. ledi in cones of P. engelmannii and P. pungens. In current-year needles of Picea, C. ledi was abundant on P. abies, P. obovata, P. engelmannii, P. jezoensis, P. glauca and P. sitchensis, the rust was scarce on P. asperata var. nobilis, P. mariana, P. pungens and P. omorika, and the rust was absent on P. glehnii and P. rubens.

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Leaf compounds may contribute to plant defense against Cronartium rusts. Secondary compounds are either natural or induced in leaves. We studied the variation of compounds in leaves of six alternate hosts of Cronartium pini and two of C. ribicola that represented either susceptible or resistant species to these rusts. Extracts from the plant leaves were analyzed using LC-MSMS (liquid chromatography tandem mass spectrometry) and compounds were compared between susceptible and resistant species of the same plant genera to identify significant differences between resistant and susceptible species. Also, LC–MS (liquid chromatography mass spectrometry) with external calibration was used to quantify 12 candidate compounds known from the literature. Among these compounds, the most abundant significant ones in C. pini -resistant Melampyrum pratense were chlorogenic acid and quercitrin, in Veronica chamaedrys ferulic acid, quercitrin and luteolin and in Impatiens glandulifera quercitrin, ferulic acid, kaempferol, rutin and hyperoside. In C. ribicola -resistant Ribes rubrum the most abundant significant compounds were caffeic acid, p-coumaric acid and quercitrin. Among all extracted leaf compounds, concentrations of three compounds were over 1000 times greater in rust-resistant M. pratense, three compounds in V. chamaedrys, eight compounds in I. glandulifera, and one compound in R. rubrum than in rust-susceptible species. Among the compounds, the most promising possibly linked to rust resistance were chlorogenic acid and quercitrin.

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