A versatile method for assessing pathogenicity of Hymenoscyphus fraxineus to ash foliage

Abstract

We describe a method for inoculating rachises of Fraxinus excelsior (European or common ash) with Hymenoscyphus fraxineus, which is faster than previous methods and allows associated foliar symptoms to be assessed on replicate leaves. A total of ten ash seedlings were inoculated with five isolates of H. fraxineus and lesion development assessed over four weeks. A five‐point disease progress scale of symptom development was developed from no lesion (0), lesion on rachis (1), “pre‐top dead,” with curling of distal leaflets and bending of the rachis (2), top dead, with wilting and death of distal leaflets (3) to leaf abscission (4). The method revealed variation in aggressiveness of H. fraxinus isolates and may be suitable for assessing the resistance of F. excelsior and other Fraxinus species to dieback. The in vitro growth rate of H. fraxineus isolates was highly correlated with both disease progress and the length of rachis lesions on susceptible plants, indicating that it can be used as a preliminary step in selecting isolates with high aggressiveness for use in resistance screening.

Variation in aggressiveness of Hymenoscyphus fraxineus genotypes amid the ash dieback epidemic

“…We found no correlation between growth of the fungus in vitro and disease progress in the stem in our study. This can at first sight seem in contrast to the finding of Orton et al (), who discovered a strong correlation between in vitro growth and the length of rachis lesions. However, the disagreement between the two studies could be due to the inoculation of different tissue types (leaves vs. stem).…”

“…However, the disagreement between the two studies could be due to the inoculation of different tissue types (leaves vs. stem). Orton et al () suggested that in vitro growth can serve as a precise proxy for growth in leaves; this would indicate that isolates in our study, such as Hf 22.1, that cause no stem necroses but grow fast in vitro may indeed be competitive when spreading in leaves. Difference in growth in rachis and stem could be due to either physiological differences or differences in the genetic control of resistance between tissue types.…”

“…Differences in aggressiveness among strains of H . fraxineus have been observed in previous studies (Junker et al , ; Kowalski et al , ; Gross and Sieber, ; Schwanda and Kirisits, ; Lygis et al , ; Orton et al , ), but in all cases, the tested isolates were obtained from lesions, thus omitting potential nonaggressive genotypes. In addition, these studies dealt with isolates of different ages from the time of initial spore germination, where variation in age and cultivation may have generated variation in aggressiveness.…”

Non-invasive genetic monitoring for the threatened valley elderberry longhorn beetle

“…Insect frass has emerged as a non-invasive DNA source for multiple types of genetic studies, including those of beetles. This has included studies of pathogen infection of both beetles and their host plants [38] and dietary analysis [39]. Frass has also been utilized for molecular species identification, which can be indispensable in distinguishing closely related species [30].…”

Life at the edge – the cytology and physiology of the biotroph to necrotroph transition in Hymenoscyphus fraxineus during lesion formation in ash

“…The spread of ash dieback disease across the United Kingdom follows a pattern largely consistent with the distribution of ascospores of Hymenoscyphus fraxineus from the east (Timmermann et al ., ). However, there have been few attempts to follow infection from ascospore inocula (Cleary et al ., ; Fones et al ., ; Mansfield et al ., ; Orton et al ., ). The symptoms of the disease, involving death and browning of leaves and stems, suggest that the pathogen has a distinctive necrotrophic mode of infection (Gross et al ., ; Stenlid et al ., ).…”