The contribution of the apple microbiome to the production chain of apple was so far largely unknown. Here, we describe the apple fruit microbiome and influences on its composition by parameters such as storage season, storage duration, storage technology, apple variety, and plant protection schemes. A combined culturing and metabarcoding approach revealed significant differences in the abundance, composition, and diversity of the apple fruit microbiome. We showed that relatively few genera contribute a large portion of the microbiome on fruit and that the fruit microbiome changes during the storage season depending on the storage conditions. In addition, we show that the plant protection regime has an influence on the diversity of the fruit microbiome and on the dynamics of pathogenic fungal genera during the storage season. For the genus Neofabraea, the quantitative results from the metabarcoding approach were validated with real-time PCR. In conclusion, we identified key parameters determining the composition and temporal changes of the apple fruit microbiome, and the main abiotic driving factors of microbiome diversity on apple fruit were characterized.

Ash dieback (ADB), caused by the invasive fungus Hymenoscyphus fraxineus, has spread extensively across Europe, resulting in considerable economic and ecological damage to European forests. Resistance screening is crucial for selecting and breeding common ash genotypes that are resistant to the ADB pathogen. This requires a standardised and effective method for infecting ash trees with the pathogen and subsequently measuring the infection success. However, to date, there is a lack of scientifically sound comparisons of available methods. In this study, three different methods for testing the resistance of ash genotypes against H. fraxineus (i.e., stem infections, rachis infections, and spore germination assays) were compared and benchmarked against crown defoliation and stem infection assessments. All three resistance assays were performed on 12 ash genotypes covering a wide intraspecific variation in ADB resistance. Stem infections correlated best with crown defoliation observed in the field. However, this method requires a long preparation time and experimental duration. The easiest and fastest method in terms of preparation and execution was the rachis infection assay. Although not completely equivalent to stem infections, the results of the rachis infection assay significantly correlated with the stem lesion lengths measured. Spore germination assays were also quick to carry out, but the timing of sporulation and the sampling of the leaves, followed by agar‐medium preparation, proved to be very difficult to synchronise. Following the very small variations in spore germination rates, there was no significant correlation with the stem lesion lengths measured. To obtain meaningful results on the resistance of ash genotypes to ADB, the stem infection method still proved to be the best and most accurate assay. Although new indirect methods for testing resistance in ash trees are arising (e.g., chemistry, spectroscopy, or genetics), direct resistance testing of ash genotypes remains the basis for resistance and breeding research for ash conservation.

The number of non-native invasive species has increased markedly in recent decades, raising the likelihood of co-occurrence and interactions among invaders. Such interactions may exacerbate impacts on native ecosystems, making it essential to understand co-invasion dynamics for effective pest management and conservation. European ash (Fraxinus excelsior) is simultaneously threatened by two invasive species: The fungus Hymenoscyphus fraxineus, which causes ash dieback (ADB), and the emerald ash borer (EAB), Agrilus planipennis, a phloem-feeding beetle. Although their invaded ranges overlap in parts of eastern Europe, it remains unclear how these species interact within the same host. We investigated whether recent ADB stem infections affect EAB performance, and whether such effects are mediated by changes in ash phytochemistry. Our findings indicate that neither larval nor adult EAB performance was affected when feeding on ADB-infected trees. Phloem chemistry showed localized changes near the ADB infection front but remained unaltered 18–20 cm away. Also leaf chemistry was only minimally affected by an ADB stem infection. These limited systemic responses suggest that F. excelsior relies primarily on constitutive phytochemical defenses against ADB. Therefore, ADB-induced phytochemical changes in recently infected trees do not affect EAB performance. We conclude that freshly ADB-infected trees neither promote nor inhibit EAB development. From a pest management standpoint, immediate sanitary removal of ADB-infected trees may not be necessary in stands only recently affected by the pathogen, as potential beneficial ADB infection effects on EAB are unlikely. However, the long-term impact of chronic ADB stress on EAB dynamics remains uncertain and warrants further investigation.