Fruiting of Cronartium and Coleosporium rust fungi on Melampyrum spp. in Scots pine stands under various forest management applications in northern Finland

Cronartium pini is the causal agent of Scots pine blister rust in northern Europe. New silvicultural methods have recently been launched to manage Pinus sylvestris stands in Finland. Continuous-cover-forest management (CCF) is an approved alternative method for the traditional rotation-forest management (RF). The aim of this study was to compare fruiting and disease incidence of C. pini in forest stands managed applying the CCF and RF approaches. The incidence of C. pini-caused disease was assessed on P. sylvestris before, and on alternate herbaceous plant hosts Melampyrum pratense and M. sylvaticum one year after the CCF or RF application procedures and compared between several forest management types such as seed-tree cutting (STC), clearcutting (CC), gap cutting (GC), selection cutting (SC) and uncut control in seven study sites. Fruiting of the needle rust fungus, Coleosporium tussilaginis f. sp. melampyri, was also investigated. In 2020–2021, the number of P. sylvestris trees with symptoms of C. pini was low in all management types, and disease incidence varied from 0% to 2.8% among the management types. There was no statistically significant difference in the frequency of symptomatic pines between the management types and study sites at the beginning of the silvicultural management operations. Ten M. sylvaticum and/or M. pratense from five sample plots were collected in each managed stand. The incidence of symptomatic leaves of C. pini on M. sylvaticum varied from 0.6% to 7.7% among the management types and on M. pratense from 0% to 1.2% one year after the management applications. Coleosporium needle rust was more common than C. pini on Melampyrum spp. The percentage of leaves with C. tussilaginis f. sp. melampyri telia varied from 7.9% to 55.4% on M. pratense and from 0.6% to 24.8% on M. sylvaticum among the management types. The frequency of C. pini telia on M. pratense did not differ between the management types. The frequency of C. tussilaginis f. sp. melampyri telia on M. pratense did not differ between study sites, but it was significantly higher in unmanaged control stands compared to CC- or STC-managed stands a year after the management application. This indicated that the rust fungus fruited on Melampyrum more abundantly in forest stands that had the highest tree density prior to its fruiting. The results provide basic information of rust incidence in forest stands shortly after their management. The long-term effect of the different forest management types on rust incidence both on pines and alternate hosts will be assessed periodically years later. Keywords: alternate hosts; Melampyrum pretense; Melampyrum sylvaticum; Pinus sylvestris; rust diseases; Scots pine blister rust