Symptoms of dwarf elm (Ulmus pumila L.) health condition in the Left-bank Ukraine

Ulmus pumila L. is widely spread in arid and semi-arid regions of China as a soil-conservation, sand-fixing, and resistant plant. Due to its rapid growth, winter hardiness, low demand for soil fertility and moisture, tolerance of drought, resistance to gaseous pollution and soil salinization, and ability to spread under favorable conditions, U. pumila has been widely cultivated in forests, urban plantations, and along transportation right-of-ways. In some regions, U. pumila is resistant to Dutch elm disease (DED), and verticillium wilt but it is susceptible to bacterial wetwood disease. The trees’ health is usually assessed according to crown and trunk appearance. We assumed that some symptoms may be associated with specific types of pathologies or indicate the latent development of the disease, which is difficult to diagnose visually. The study aimed to search the relations between the distribution of visually detectable symptoms of U. pumila weakening and tree groups with apparent signs of bacterial wetwood and Dutch elm disease. In June – September 2023, the health condition of Ulmus sp. was examined in forest shelterbelts along the M 03 highway, passing through the territory of Kyiv, Poltava, and Kharkiv regions. In this study, the data on 624 U. pumila trees in 11 sample plots were used, particularly, the diameter at breast height (DBH), the class of the health condition, defoliation, the prevalence of dry branches, epicormic shoots, bark beetle infestation, local bark death, and collar rots. Based on these indicators, four groups were examined – with symptoms of bacterial wetwood, DED, weakening of unknown etiology (symptomatic trees), and without visual symptoms of weakening (asymptomatic trees). DBH of symptomatic trees was significantly less than that in asymptomatic and DED-infected trees. The average HCI was from 2.5 for asymptomatic to 3.3 for wetwood disease-infected trees, and 3.5 for DED-infected trees. DED-infected trees were found to have the highest level of defoliation and the lowest prevalence of epicormic shoots. Among wetwood-infected trees, the bark beetle infestation significantly increased with an increase in HCI and defoliation and decreased as the diameter of the trunks decreased. Principal components analysis confirmed the highest association of bark beetle infestation with DED, epicormic shoots with wetwood disease, and dry branches with other symptomatic trees. Symptomatic trees occupied an intermediate position by HCI. Although they have no direct signs of fungal or bacterial disease, health worsening may indicate a latent infection.