Forest Pathology最新文献

Beech leaf disease (BLD) is an emerging threat to forests in North America and primarily affects American beech (Fagus grandifolia), an important forest tree species. In this study, we report the results of three separate experiments to examine (1) if the microbiome of the nematode Litylenchus crenatae ssp. mccannii plays a role in the development of BLD symptoms, (2) if there is variation in response to nematode inoculation between beech species, as well as whether the quantity of nematodes affects symptom development and (3) if there is variation in response to nematode inoculation between American beech genotypes, as a first step toward identifying if some resistance to the disease may be present among American beech genotypes. We conducted artificial inoculations of beech buds with live nematodes in a controlled greenhouse environment, including buds on grafted scion of beech trees that displayed low symptoms of BLD under field conditions. We found that only inoculations with live nematodes initiated BLD symptoms, and that inoculations that included portions of the nematode microbiome alone did not cause symptoms of BLD. We also found apparent resistance or tolerance to BLD symptoms on Japanese beech (Fagus crenata) as compared to American and European beech (Fagus sylvatica), suggesting that variation in response to the nematode inoculations exists between beech species. However, although symptom development was significantly affected by the quantity of nematodes, it did not vary between American and European beech. Lastly, American beech genotypes selected from BLD infested sites as displaying either low or high levels of BLD symptoms were inoculated with nematodes. The inoculation work demonstrated lower levels of BLD symptom development on some genotypes of American beech. Many of these genotypes were sourced from an existing plantation containing beech genotypes with known resistance to beech bark disease from field selection and/or resistance screening by the USDA Forest Service Northern Research Station. This suggests that some potential resistance to Litylenchus crenatae ssp. mccannii may co-exist with beech bark disease resistance in some beech genotypes. Further examination is warranted to determine if this natural variation in BLD symptom development can be improved through breeding, potentially producing trees with increased resistance to BLD. Future research could also explore the potential long-term survival of BLD infestation among American beech genotypes.

The invasive fungal pathogen Colletotrichum salicis, the causal agent of willow anthracnose and shoot blight, was first recorded to cause massive damage to willow in the Czech Republic in 2002 on Salix euxina. Since then, the pathogen has been repeatedly documented also in other willow species (e.g., S. caprea, S. cinerea, S. viminalis) with the most severe damage occurring on Salix matsudana ‘Tortuosa’, which appears to be highly susceptible. Typical symptoms on infected hosts include necrotic lesions on young shoots, shoot dieback, premature defoliation and crown dieback, especially in their lower part. Disease occurrence and severity are strongly associated with inversion-prone sites and with cold, rainy weather during spring, which promotes prolonged tissue wetness and delayed maturation of current-year shoots. Based on currently available records, C. salicis represents an underestimated threat to willow stands in the Czech Republic. This short communication documents its occurrence, host range, impact and basic information about its ecology and disease epidemiology and highlights the need for systematic monitoring and precise diagnosis to better assess its current distribution and potential ecological consequences. The increase in the importance of this species may be associated with recent spread of willow species, especially in riparian vegetation, where ash, elm and alder have retreated due to damage by invasive pathogens.

American elm (Ulmus americana L.), an important floodplain tree species in the Eastern U.S., continues to suffer widespread mortality from the non-native pathogen Ophiostoma novo-ulmi. Reliable and efficient methods for screening potted American elms for resistance to the pathogen are lacking, hindering efforts to restore the species. The objective of this study was to compare various screening methods for efficacy in separating known-resistant from known-susceptible American elm genotypes. Over the course of three experiments, we evaluated symptom development in potted American elms inoculated with O. novo-ulmi, testing tree age (1-, 2-, 3-year-old), spore dose (2.5 K, 12.5 K, 25 K spores) and inoculation method (cambium slit vs. hanging drop). Disease progression was rated weekly for at least 7 weeks and used to calculate area under the disease progress curves (AUDPC). In a fourth experiment, we used the refined methods (1-year-old trees, 25 K spore dose and hanging drop inoculation method), to screen for resistance in clones and progeny from large survivor elms that were previously screened in a field-test. Mean AUDPC values from the potted assay explained 82% of the variability in the field-test screen, indicating strong agreement. Breeders can use the results of this early screen to identify likely susceptible genotypes to exclude from subsequent field testing.

Emergent forest Phytophthora pathogens are routinely managed in a context of uncertainty where basic epidemiological understanding must be developed alongside disease management experiments. Phytophthora agathidicida causes the emergent disease kauri dieback, which threatens kauri (Agathis australis), a southern-hemisphere conifer comprising some of the world's largest trees. Disease management often includes phosphite injections to reduce kauri susceptibility and slow or prevent spread between invaded and uninvaded stands. We adapt an existing spatially-explicit and multi-host forest epidemiological model (SODDr) to understand where incompletely understood treatment and epidemiological mechanisms may result in disease emergence across a treated host barrier. We parameterize our model with published data to evaluate three key areas of uncertainty: (1) the impact of transmission from secondary hosts, (2) the epidemiological impact of phosphite treatments at the host level, and (3) the width (treatment area) of phosphite chemical barriers. Our model experiments routinely demonstrated the potential for unknown secondary hosts to overwhelm phosphite treatment benefits, including any secondary hosts that support relatively little sporulation. Also, it is unclear if phosphite treatments cure trees of infection and reduce transmission in real systems however, in the model when treated trees remain infected the barrier is rapidly overcome. Lastly, increasing the width of the disease treatment area resulted in consistent slowing of disease spread in the model landscape, regardless of the other factors. These results were consistent in a model landscape with uniform host densities and in a follow-up simulation of 2250 unique host landscapes with random host distribution. Our analysis suggests understanding the outcome of chemical treatment at the individual tree level along with a robust understanding of secondary host competency will be critical to effective kauri dieback management.

Decline symptoms, including bark necrosis and dark stem exudates, were observed in newly planted pedunculate oak (Quercus robur) trees in an urban roadside site in Poland. Bacteria isolated from symptomatic tissues were characterised using biochemical tests and 16S rRNA gene sequencing. Among the isolates, strains putatively assigned to Lonsdalea quercina subsp. britannica and Brenneria goodwinii were recovered, co-occurring within the same necrotic lesions. Pathogenicity tests performed on oak seedlings confirmed that L. quercina subsp. britannica induced necrotic symptoms and fulfilled Koch's postulates. This study represents the first preliminary record supported by phenotypic traits and 16S rRNA sequencing of this subspecies in Poland and extends its known distribution to urban environments in Central Europe. The results support the view that oak decline is associated with a polymicrobial complex and highlight the need for further research on the epidemiological role of L. quercina subsp. britannica, particularly under urban conditions where environmental stress may favour disease development.