Inferences on the susceptibility of wood of different tree species to heterobasidion annosum sensu lato primary infections and on the range of pathogen spores dispersal

Stumps play a pivotal role in the epidemiology of the fungal forest pathogens Heterobasidion spp. because they are the main courts of primary airborne infections. The aims of this study were (i) to determine the susceptibility of seven tree species (i.e., Larix sibirica, Picea abies, Picea sitchensis, Pinus contorta, Pinus strobus, Pinus sylvestris and Pseudotsuga menziesii) to primary infection by H. annosum and H. parviporum through comparative inoculation experiments of conidia on wood discs in controlled conditions; (ii) to compare the susceptibility of wood discs of the same tree species to natural airborne infections in two Latvian Norway spruce forest stands infested either by H. annosum or H. parviporum; (iii) to explore the rates of infection of wood discs at increasing distances from spore sources in these two forests to make inferences on the range of spores dispersal. Results obtained by spraying wood discs with conidial suspensions in controlled conditions are in agreement with those obtained by exposing wood discs to the natural airborne inoculum in the forests, as clearly supported by the significant correlation (r = 0.79; p Pinus species, followed by P. abies and P. sitchensis. Susceptibility was lowest for L. sibirica and P. menziesii. The area colonized by Heterobasidion spp. in the sapwood of wood discs was much greater than that colonized in the heartwood. A sharp decrease in the rate of infection of wood discs with distance from spore sources (i.e., fruiting bodies) was observed, further confirming the importance of local spore sources in the epidemiology of Heterobasidion spp. Taken together, these findings could help designing tactics to manage these fungal forest pathogens

Seasonal development of lesions caused by Hymenoscyphus fraxineus on young Fraxinus excelsior trees in Latvia

The spread of the ascomycete Hymenoscyphus fraxineus, causing dieback of common ash (Fraxinus excelsior) in Europe, is rapid and the damage is pronounced, as young ashes can perish over the course of only a few months following infection. The objective of this study was to investigate the rate and extent of lesion formation on young (5-8-year-old) ashes during a vegetation season in the hemiboreal zone in Latvia. Continuous surveys (with monthly intervals) of the health condition of 30 young ash and measurements of lesion area in three stands were performed during the vegetation season of 2015. From June to September of that year, the number of observed lesions gradually rose from 58 to 87. New lesions emerged on branches (55%, 0.5 per tree), top shoots (28%, 0.3 per tree), and stems (17%, 0.2 per tree), mostly appearing at the beginning of the observation period (45%, 52%, and 3% in June, July, and August, respectively). During the vegetation season, 20% of the existing and 28% of the newly-emerged lesions on branches, as well as 20% and 25% of top shoot lesions, respectively, reached the main stem. Some (< 20% of cases) transitions of lesions from the tops and branches to the stems were observed. The extension of lesions was significant until August, and ceased afterwards in a similar fashion in all stands. The mean extension of area significantly differed between the previously-existing and newly-emerged lesions. During the vegetation season, the new lesions expanded by 25.1 ± 4.8 cm2, whereas the existing ones grew by only 7.3 ± 1.1 cm2. The extension of the new lesions varied according to their location on a tree. The spread of emerging lesions on stems was considerably slower than on branches or top shoots (1.9 ± 0.7, 7.3 ± 1.5, and 14.5 ± 4.1 cm2 per lesion per month, respectively). During the studied vegetation season (summer), the overall health score of trees decreased twice, yet the relationship between heath status and development of lesions lacked significance

A review of studies on heterobasidion and its control in Latvia

Approximately 50% of the total area of Latvia is covered by forest: of which 35% is dominated by Scots pine (Pinus sylvestris) and 18% by Norway spruce (Picea abies)