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