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Afforestation of former agricultural lands is a common practice in several countries. This is beneficial for avoiding diseases carry-over from previous forest generations and to expand forest areas. However, several biotic and abiotic risks have been reported in such stands, including a higher risk of Heterobasidion root rot after thinning. Therefore, this study investigates the spread of Heterobasidion root rot in three Picea abies (L.) Karst. plantations in Latvia established on former pasture and meadow lands and subjected to forest management practices. Initially, to determine average infection rate, we sampled all standing trees (157 in total) along transects within sampling areas. On the transects, the proportion of trees suffering from Heterobasidion root rot varied from 16 to 33% among stands. Based on those data and observed dieback over all the stands, we established circular sample plots in disease centres where all trees and stumps were analysed. The average infection rate in the circular plots varied from 34 to 41%. Obtained Heterobasidion isolates were analysed with species-specific primers and were all determined to be Heterobasidion parviporum Niemelä & Korhonen. Isolates were paired to detect the number and size of genotypes. Of 141 genets examined, 99 were isolated from only one tree or stump, while 42 formed genets including two or more trees or stumps indicating spread of infection through root contacts. The total number of Heterobasidion genets per hectare in studied stands varied from 72 to 484. The following conclusions were reached: (i) stands on former non-forest lands (pastures and meadows) can be highly susceptible to Heterobasidion infection after thinning, (ii) the pathogen may form expanding territorial clones in both former pasture and meadows and (iii) stump treatment with biological or chemical control agents is recommended to prevent Heterobasidion infections.
