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The forest disease, Scots pine blister rust, is caused by the rust fungus Cronartium pini. This pathogen was previously known as the host‐alternating type C. flaccidum and non‐host‐alternating type Peridermium pini. Recent epidemics of this disease in Northern European forests, especially young Scots pine forests in Sweden, caused significant economic and ecological losses. Cronartium pini can be identified based on the typical orange blister‐like aecia in Scots pine in summer, but any molecular identification and quantification method has not been available for Cronartium spp. This study developed qPCR primers that are specific to Cronartium spp. and evaluated DNA extraction protocols from pine bark and wood to enable robust qPCR assays. As little as three Cronartium ITS copies can be detected with the protocol. Since only C. pini is known to infect Scots pine in Northern Europe, the protocols were applied to detect C. pini from Scots pine samples without typical symptoms and investigate the C. pini colonization in Scots pine branches from the forest. These results will aid the detection and quantification of C. pini in asymptomatic or symptomatic samples and monitoring Scots pine blister rust in the forest in northern Europe. [ABSTRACT FROM AUTHOR]

Cherry spruce rust causes huge yield losses in Norway spruce seed production in Fennoscandia. The causal agent, Thekopsora areolata, has three types of spores that disperse during spring: basidiospores are produced on basidia that grow out from teliospores in overwintered bird cherry leaf litter to infect new pistillate spruce cones, aeciospores are released from old diseased spruce cones to infect bird cherry leaves, and urediniospores are produced from new bird cherry leaves for reinfection. No study has examined the dispersal of T. areolata spores, including the basidiospores that cause primary infection in spruce cones. In this study, teliospores of T. areolata were germinated in the laboratory and the morphology of basidiospores was described. T. areolata spores were sampled in Ultuna, Sweden and Joutsa, Finland with 21 spore traps at each site. Peaks in aeciospores were observed from 11 to 25 May and from 2 to 8 June at the Finnish site, and from 4 to 18 May at the Swedish site. Urediniospores were first observed 2–3 weeks after the peaks in aeciospores and they were mainly distributed within 10 m from the bird cherry trees. Peaks of 1–2 weeks in basidiospore detection coincided with multiple rain events. The basidiospore peak overlapped with the spruce pollen peak in Finland but not in Sweden. The quantities of basidiospores from different spore traps within 100 m from the spore source had no gradient. Information on spatial and temporal spore release is important for making decisions on disease management strategies. [ABSTRACT FROM AUTHOR]

Cronartium pini causes Scots pine blister rust. This rust fungus has two different forms without differentiation in morphology and internal transcribed spacer: the heteroecious form has a macrocyclic life cycle and infects pine and an alternate host; the autoecious form only infects pine. Epidemics caused by these two forms impose severe risk on the pine forest in Sweden, therefore knowledge of their distribution and diversity is needed for strategic disease management. We designed microsatellite markers with improved resolution based on the C. pini genome, developed a multiplex amplification system, and analyzed the C. pini population diversity and structure in Sweden using 396 isolates. The heteroecious and autoecious populations showed clear differences in diversity, linkage disequilibrium, and structure. The heteroecious isolates had unique multilocus genotypes. Autoecious isolates shared the same genotypes more frequently, especially three autoecious multilocus genotypes that were commonly found over a in northern Sweden. The genetic distances among autoecious isolates are closer than those among the heteroecious isolates. The results confirmed that heteroecious C. pini populations were sexual and autoecious C. pini populations were clonal. We further discussed the hypothesis that autoecious C. pini originated from self-fertilization, and frequent self-fertilization and infrequent mutation generate homozygous but diverse genotypes., Competing Interests: Declaration of competing interest The authors, KZ and BS, declare no conflict of interest., (Copyright © 2024 The Authors. Published by Elsevier Ltd.. All rights reserved.)