Putting 'X' into context: the diversity of 'Candidatus Phytoplasma pruni' strains associated with the induction of X-disease.

Recurrent epiphytotics of X-disease, caused by 'Candidatus Phytoplasma pruni', have inflicted significant losses on commercial cherry and peach production across North America in the last century. During this period, there have been multiple studies reporting different disease phenotypes, and more recently, identifying different strains through sequencing core genes, but the symptoms have not, to date, been linked with genotype. Therefore, in this study we collected and assessed differing disease phenotypes from multiple U.S. states and conducted multi-locus sequence analysis on these strains. We identified a total of five lineages associated with the induction of X-disease on commercial Prunus species and two lineages that were associated with wild P. virginiana. Despite a century of interstate plant movement, there were regional trends in terms of lineages present, and lineage-specific symptoms were observed on P. avium, P. cerasus, and P. virginiana, but not on P. persica. Cumulatively, these data have allowed us to define 'true' X-disease-inducing strains of concern to the stone fruit industry across North America, as well as potential sources of infection that exist in the extra-orchard environment.

Organic orchard floor management in peach: effects on arthropods and associated fruit injury in the Intermountain West.

Understanding orchard floor management is critical to organic tree-fruit production systems given its impact on weeds, soil fertility, tree health, and crop yield. Several viable options are available to producers for weed management and promotion of organic fertility, including use of turf and broadleaf alleyway covers and living and nonliving tree-row mulches. While these measures can be effective, little is known about how these strategies influence arthropod pests, which cause fruit injury. Here, we assessed 6 organic orchard floor management strategies for their impact on arthropod abundance and diversity in an organic peach production system in northern Utah from 2010 to 2014, using sweep netting and pitfall collections along with observed peach fruit damage. Generally, we found that alleyway and tree-row treatments had no impact on total arthropod diversity, species richness, or community diversity. However, earwig (Forficula auricularia) abundance was significantly impacted by alleyway and tree-row treatments that resulted in increased fruit injury. Trefoil alleyway treatments consistently increased earwig abundance across life-history stages, while mulch or Alyssum (straw) tree-row treatments harbored more earwigs and, as a result, increased earwig fruit injury. Since earwigs are especially prone to damaging young, developing fruits, it is imperative that more work is done to assess earwig abundances and life-history traits. Our results demonstrate that detrimental arthropods are sensitive to orchard floor management and can further inform integrated pest management approaches that complement sustainability goals.

Assessing water stress in a high-density apple orchard using trunk circumference variation, sap flow index and stem water potential.

Introduction: Automated plant-based measurements of water stress have the potential to advance precision irrigation in orchard crops. Previous studies have shown correlations between sap flow, line variable differential transform (LVDT) dendrometers and fruit tree drought response. Here we report season-long automated measurement of maximum daily change in trunk diameter using band dendrometers and heated needles to measure a simplified sap flow index (SFI). Methods: Measurements were made on two apple cultivars that were stressed at 7 to 12 day intervals by withholding irrigation until the average stem water potential (ΨStem) dropped below -1.5 MPa, after which irrigation was restored and the drought cycle repeated. Results: Dendrometer measurements of maximum daily trunk shrinkage (MDS) were highly correlated (r² = 0.85) with pressure chamber measurements of stem water potential. The SFI measurements were less correlated with stem water potential but were highly correlated with evaporative demand (r² = 0.82) as determined by the Penman-Monteith equation (ETr). Discussion: The high correlation of SFI to ETr suggests that high-density orchards resemble a continuous surface, unlike orchards with widely spaced trees. The correlations of MDS and SFI to ΨStem were higher during the early season than the late season growth. Band dendrometers are less labor intensive to install than LVDT dendrometers and are non-invasive so are well suited to commercialization.

Subseasonal prediction for bloom dates of tart cherries in Utah and Michigan, USA: merging phenological models with CFSv2 forecast.

Temperate fruit trees require chilling for rest completion, followed by sufficient heat accumulation for onset of growth and bloom. The application of phenological models to predict bloom dates has been widely used in orchard management. Examples of such application include selecting adapted cultivars less prone to early bloom, predicting needs for frost protection, and preventing damage from late spring freezes. This study merged the Utah (chill) and ASYMCUR (forcing) phenological models by combining chill units and heat units (measured in growing degree hours) to predict bloom dates of tart cherries (Prunus cerasus L.) in Utah and Michigan, the top producing states of the USA. It was found that the modified Utah model improves the estimation of chill units compared with the original one, while the original Utah model may still be suitable for use in the colder winter of Michigan (with its later bloom dates than Utah). The combined models were applied with the temperature predicted by the Climate Forecast System v2 (CFSv2) model. The prediction was applied twice a month, starting from 1 February to 1 May. The Utah-ASYMCUR model using the forecasted temperature from CFSv2 exhibits subseasonal performance in predicting the bloom dates for 6 weeks in advance. The prediction can offer growers a way to mitigate extreme climate anomalies.

Partitioning of nitrate assimilation among leaves, stems and roots of poplar.

Plants differ in tissue localization of nitrate reduction and assimilation. Some species reduce nitrate primarily in the leaves, whereas other species localize nitrate reduction and assimilation in the roots. We determined how nitrate assimilation is partitioned among leaves, stems and roots of poplar (Populus tremula L. x P. alba L.) by comparing tissue differences in in vivo nitrate reductase activity (NRA), nitrate reductase abundance and tissue nitrate concentration. Compared with stems or roots, NRA was greater in leaves, and the highest leaf NRA was found in young leaves. Leaf and root NRA increased with increasing nitrate supply, whereas stem NRA remained constant. Leaf NRA was at least 10-fold greater than root NRA at all external nitrate concentrations. Nitrate reductase abundance increased in all tissues with increasing nitrate availability, and nitrate reductase abundance was at least 10-fold greater in leaves than in stems or roots at all nitrate availabilities. Tissue nitrate concentration increased with increasing nitrate supply and was greater in roots than in stems and leaves. Photoperiod influenced NRA, with leaf NRA declining in nitrate-fertilized plants with short daily photoperiods (8-h). We conclude that different tissues of poplar vary in nitrate assimilation with little nitrate assimilation occurring in roots and the most nitrate assimilation taking place in leaves.