Sooty Bark Disease in Sycamore: Seasonal and Vertical Variation in Spore Release of Cryptostroma corticale

Abstract

Fungal pathogens that benefit from climate change have become more prevalent as a consequence of extended drought periods and higher temperatures. Since the dry and hot years of 2018/2019, the causative agent of sooty bark disease Cryptostroma corticale led to an increasing die back in sycamore forest stands in Germany. Thus, in 2019, spore traps were set up in one forest stand to investigate the dispersal of the pathogen under temporal and spatial aspects. The mortality rate in the study site due to sooty bark disease was quite high: 31% in spring, increasing to 49% in autumn 2019. Quantifying the airborne spores of C. corticale in consecutive periods of the year, a correlation with seasonal aspects was detected. In winter and spring, spore release was relatively low compared with summer months. In summer, high abundances of conidia of C. corticale were released. In July, 399 spores were detected per cm2 per day. From June to August, spore release was 12.6 times higher than in springtime. In autumn 2019, a decrease was visible, although the number of conidia was still higher than in spring. The vertical gradient of spore abundances changed over time: the lowest spore release was found along a gradient at 14 m of height in spring, which shifted down to 2 m in autumn. According to our results of spore release, sooty bark disease is strongly driven by the impact of seasonal factors.