Morphologial variation in colonies of Chalara fraxinea isolated from ash (Fraxinus excelsior L.) stems with symptoms of dieback and effects of temperature on colony growth and structure

Abstract

A b s t r a c tThe present paper presents the results of long-term observations of differences in growth and morphology of colonies of C. fraxinea isolated from ash stems with symptoms of dieback and the results of studies on effects of temperature on growth and morphology of colonies in vitro. Thirty randomly chosen C. fraxinea cultures, originating from six Forest Districts in southern and northern Poland, viz. Andrychów, Gryfice, Kańczuga, Lębork, Miechów and Mircze, were included in the temperature assay. Colony growth and morphology were evaluated in vitro, on malt extract agar. Two replicates of each isolate were incubated at 5, 10, 15, 20, 25 and 30 o C in darkness. Colonies of C. fraxinea isolated from necrotic stem tissues of diseased ash trees differed greatly in colour, structure, growth rate, presence of sectors and stromata as well as the intensity of conidial sporulation. Colony characters were often lost in further sub-cultures grown on the same medium. Colonies of C. fraxinea grew at 5-25 o C in vitro. Three isolates still grew, though slowly, at 30 o C. The optimum temperature for growth was 20 o C. Among 30 isolates tested, five grew most quickly at 25 o C and four at 15 o C. Differences among colony diameters recorded at 5, 10, 15, 20 and 30 o C were statistically significant, while differences at 15 and 25 o C were not. At some temperatures, statistically significant differences in growth rate were related to the origin of the isolates. There were three main colony colour types. Temperature was the main factor affecting colony morphology in vitro. The formation of sectors was observed in 36% of colonies, pseudoparenchymatous stromata in 10.4% of colonies, and white crystalline substances in 53.3% of colonies. The results of in vitro observations were used for discussion of the effects of temperature on growth and activity of C. fraxinea in vivo.

Detection of Airborne Inoculum of Hymenoscyphus fraxineus and H. albidus during Seasonal Fluctuations Associated with Absence of Apothecia

“…However, no biological studies about the causal agent of ash dieback had been carried out in our country so far. Our results demonstrate that H. fraxineus and H. albidus are present in the air of the infected forest Hackerova školka since the beginning of April until the end of October (Figure 3), what means some time before and after the ascospores are known to be released and wind-dispersed in nature [7,[12][13][14]. These results are supported by two independent air sampling technics.…”

“…The pathogen establishment on leaves is terminated before the onset of the leaf senescence [13]. Conidia, which are produced on the petiole in autumn at low temperatures [14], may act as spermatia for sexual reproduction, but most likely do not play any role infecting the host. Next summer (growing season) new apothecia develop and a new infection cycle starts [7].…”

Application of fungicides and urea for control of ash dieback

“…Other fungi isolated from ash tissues showed some antagonistic effect against H. pseudoalbidus (Kowalski & Holdenrieder 2009, Kowalski & Bartnik 2010, Gross et al 2014, and these fungi could possibly act as biocontrol agents in natural conditions as well. Interestingly, during our experiments, fungus gnats from the Sciaridae family were found to be feeding on H. pseudoalbidus apothecia; however, there is little evidence that these insects could significantly affect the reproductive potential of the pathogen in nature.…”

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

“…Previously, the highest activity of H. fraxineus has been observed when the mean temperature is close to 20 °C (Kowalski & Bartnik 2010, Timmermann et al 2011. The sharp decrease in lesion development after August (Fig.…”

“…The opposite was observed by McKinney et al (2011) in Denmark, where susceptibility to the disease appeared to be prevailingly controlled by deterministic factors, such as genetics. Such differences could be related to the milder and more humid climate in Denmark that facilitates development of H. fraxineus and extension of lesions (Kowalski & Bartnik 2010, Timmermann et al 2011, enhancing the tree-vice differences in susceptibility (Schumacher et al 2010). Nevertheless, relationships between morphometrics (diameter) and health grade were observed, suggesting that the largest trees were also the healthiest.…”