Pistachio bushy top syndrome (PBTS) emerged as a new disease of pistachio in the United States in 2011. The disease, caused by two phytopathogenic Rhodococcus spp., affected clonally propagated ‘UCB-1’ pistachio rootstocks, causing a suite of symptoms including stunting, shortened internodes, swollen lateral buds, bushy growth, altered bark and root morphology, and stem galls. Symptoms developed within 1 year of planting; however, the long-term influence of PBTS on orchard productivity was unknown because conventionally trained pistachio orchards are typically first harvested after the sixth season. The goal of this project was to determine the phenotypic characteristics and productivity of PBTS-affected trees in the seventh and eighth seasons, as trees entered maturity. Symptomatic and asymptomatic trees in a PBTS-affected orchard were evaluated for bark morphology, tree size, suckering potential, yield, nut quality, photosynthetic rate, foliar chlorophyll content, foliar nutritional status, stem water potential, and bloom progression. PBTS-affected trees exhibited at least a 20% reduction in growth, more than three times more suckering, and a greater than 50% reduction in yield compared with asymptomatic trees. In addition, PBTS-affected trees exhibited greater levels of blank nuts than asymptomatic trees; however, percent edible yield was unaffected by disease status. Symptomatic trees exhibited greater variability in tree size parameters, suckering, and edible yield than asymptomatic trees. The heterogeneity of PBTS symptomatic trees undermined the value of planting clonal rootstocks, and the deleterious impact of the syndrome on both yield and nut quality demonstrates that affected trees should be rogued upon initial symptom development rather than managed to maturity.

Very little is known about potential benefits of using AM inoculation as a tool to alleviate drought stress of trees grown under nursery conditions. There is some evidence that benefits of mycorrhiza on gas exchange parameters and growth of trees are proportionally greater when trees are subjected to drought stress. We hypothesized that inoculation with mycorrhiza would increase growth, nutrition, stem water potential and gas exchange of containerized almond (Prunus dulcis (Mill.) D.A. Webb) when young trees are grown under low water availability.In the spring of 2015 and 2016, forty containerized almond trees (Nonpareil scion grafted on Hansen 536, a peach/almond hybrid rootstock) were either inoculated or not with a commercially available arbuscular mycorrhizal inoculum and kept well-watered for 60 days in 2015 and 54 days in 2016, after which half of the plants were exposed to gradually reduced water availability over 38 days in 2015 and 61 days in 2016. Measurements included growth metrics, stem water potential, stomatal density, leaf NPK concentration, light saturated photosynthesis, and parameters calculated from CO2 response curves (Vcmax, J, TPU, Rd, gm, and stomatal limitation).The low-water treatment decreased stem water potential and reduced trunk diameter growth, light saturated photosynthesis (A), stomatal conductance (gs), Vcmax, J, TPU, gm, specific root length, leaf N and K concentration and total leaf N, P, and K content. Inoculation decreased root mass, increased leaf mass fraction, and decreased root mass fraction in 2015, but not in 2016. Neither stem water potential nor gas exchange were significantly altered by inoculation. Inoculation decreased leaf stomatal density, and slightly elevated leaf N and P concentrations in the low water treatments, although not consistently across years. Contrary to expectations we did not find substantially greater benefits from mycorrhizal inoculation in young drought stressed almond trees and conclude that the potential for mycorrhizal inoculant to alleviate drought stress in containerized trees is minimal.

An essential component of integrated pest management is the use of resistant or tolerant host plants. These traits may not always be targeted during cultivar development; however, they should still be evaluated when assessing new genotypes for commercial release. Traits that may require significant pest management interventions increase costs for growers and may not be desirable. Twenty-six almond cultivars were evaluated in three commercial orchards in California. Among several other traits, the cultivars were monitored for two key pest issues. The first is the presence of mummy nuts, which are nuts that are not removed at harvest. These mummies serve as an overwintering site for the most significant insect pest in California nut orchards, Amyelois transitella, and also serve as a food source for first generation larvae. The second is hull rot, an infection of the hull by one or more fungi. This fungal infection causes no negative effect on the current crop, but the fungi release toxins that may kill fruiting wood, reducing future crop potential. There is a wide variation in the number of leftover nuts after harvest among the cultivars, as well as susceptibility to hull rot. Cultivars that consistently suffer from one or both may not be appropriate for commercial release.

The California walnut industry historically utilized two seedling rootstocks for commercial production, Northern California Black (Juglans hindsii) and 'Paradox' hybrid seedling (Juglans hindsii x Juglans regia). Recent research and propagation advances have resulted in commercial availability of three clonal walnut rootstocks, 'RX1', 'VX211' and 'Vlach'. As these rootstocks had been evaluated elsewhere to reveal desirable Phytophthora and nematode-related traits, a trial was planted in California's Central Valley in 2009 to evaluate their horticultural characteristics relative to the control of 'Paradox' seedling. All rootstock under evaluation were grafted to the cultivar 'Tulare'. Yield and size were evaluated annually until 2020. Between 2013 and 2019, 'RX1' rooted trees at this site produced significantly smaller trees in terms of trunk circumference and canopy-shaded area. In the same span of years, 'RX1' yields were frequently significantly lower than trees on the higher yielding 'VX221' and 'Vlach'. However, when these two parameters were integrated to determine yield efficiency, results indicate that trees on 'VX211', 'Vlach', 'Paradox' seedling and 'RX1' produce comparable yields on a per-unit-canopy basis. This was further borne out in 2020, when trees were hedged to create the same sized canopy for all rootstock treatments. In other words, when trees are restricted to the same size, yields are not significantly different. This finding has implications for how rootstocks are evaluated and how growers can optimize orchard design to take advantage of rootstocks with desirable soil-borne pest or disease-related traits.

Because of the vast information on health benefits and the urbanization impact changes in eating habits, the demand for ready-to-use shelled walnuts as a convenient, healthy, and nutritious snack food is increasing all over the world. However, shelled nuts sold as halves or pieces in ‘ready-to-use’ small packages are more susceptible to pellicle darkening and rancidity than their in-shell equivalents. Currently, about two-thirds of the USA crop is exported, and ~66% of these exports are sold as shelled ‘ready to use’, and its demand is increasing. Yet, this package style is generating quality challenges due to the darkening and rancidification of the kernel pellicle. Protection against kernel color quality loss and rancidity during postharvest handling, even at warm temperatures, can be accomplished at 6 kPa (%) oxygen or less for ‘Chandler’ and 3 kPa or less for ‘Howard’ and ‘Tulare’. The application of the ‘cold chain’ principle and/or low oxygen technology is unpractical for large cropping systems. Thus, a useful quality control system to rank the lots based on their potential visual and sensory quality is important. Then, the lots can be selected based on their potential quality, match market destinations, and are subjected to proper postharvest technologies.

California signed the Sustainable Groundwater Management Act (SGMA) into law in 2014. SGMA requires groundwater-dependent regions to halt overdraft and develop plans to reach an annual balance of pumping and recharge. Groundwater aquifers can be recharged by flooding agricultural fields when fallow, but this has not been an option for perennial crops such as fruit and nut trees. While flooding these crops might be possible during the dormant season, it is not known what impact flooding might have on tree-root systems, health and yield. We followed root production, tree water status and yield in two almond orchards in Northern California for 2 years to test the impact of applying captured winter water runoff for groundwater recharge purposes on tree performance. Results showed that more than 90% of the water applied to sandy soil and 80% of the water applied to loamy soil percolated past the root zones, with no measured adverse effects on tree water status, canopy development or yield. Groundwater recharge did not negatively affect new root production and tended to extend root lifespan. Based upon these data, applying additional water in late December and January is not likely to have negative impacts on almond orchards in moderately drained to well-drained soils.

California walnut growers have several nursery product options available including seedling or clonal rootstock, or finished bareroot one-year old June buds or two-year old nursery grafted trees. Typically, after planting, rootstock or finished trees are cut back to three to six buds and trained to a single leader (trunk) during the first leaf. The primary scaffolds develop during the second leaf and secondary scaffolds during the third leaf. June budded 'Chandler' walnut trees were grown for one year in the nursery on 'VX211', 'Vlach' and 'RX1' clonal Paradox rootstocks. Trees were left unheaded when they were dug from the nursery. Growth is being compared in a large replicated trial that was planted in February 2015 in Sutter County, California. Trees were not headed at planting or during first or second dormant season. Tree growth characteristics were noted and rootstock circumference was measured in the fall each year and scions measured in 2017. Midday canopy photosynthetically active radiation interception was measured in 2017. Unheaded June budded 'Chandler' trees on clonal Paradox rootstocks produced functional primary branches during the first leaf. By the end of the second leaf, trees had a well-developed structure much more advanced than a normal two-year old tree. Trees were mechanically harvested and produced high yields in the third leaf with 'RX1' producing significantly higher yields than the other rootstocks. Rootstock circumference for 'VX211' was significantly greater than 'Vlach' which was significantly greater than 'RX1' 2015 through 2017. However in 2017, scion to rootstock ratio was significantly higher for 'RX1' compared to 'VX211'. Using unheaded June buds only grown one year in the nursery with no heading in the first or second dormant periods, allows earlier tree development reducing costs of orchard development while producing quicker returns.

Canopy-intercepted light, or photosynthetically active radiation, is fundamentally crucial for quantifying crop biomass development and yield potential. Fractional photosynthetically active radiation (PAR) (fPAR) is conventionally obtained by measuring the PAR both below and above the canopy using a mobile lightbar platform to predict the potential yield of nut crops. This study proposed a feasible and low-cost method for accurately estimating the canopy fPAR using aerial photogrammetry-based canopy three-dimensional models. We tested up to eight different varieties in three experimental almond orchards, including California's leading variety of ‘Nonpareil’. To extract various canopy profile features, such as canopy cover and canopy volume index, we developed a complete data collection and processing pipeline called Virtual Orchard (VO) in Python environment. Canopy fPAR estimated by VO throughout the season was compared against midday canopy fPAR measured by a mobile lightbar platform in midseason, achieving a strong correlation ( R 2 ) of 0.96. A low root mean square error (RMSE) of 2% for ‘Nonpareil’. Furthermore, we developed regression models for predicting actual almond yield using both measures, where VO estimation of canopy fPAR, as a stronger indicator, achieved a much better prediction ( R 2 = 0.84 and RMSE = 195 lb acre −1 ) than the lightbar ( R 2 = 0.70 and RMSE = 266 lb acre −1 ) for ‘Nonpareil’. Eight different new models for estimating potential yield were also developed using temporal analysis from May to August in 2019 by adjusting the ratio between fPAR and dry kernel yield previously found using a lightbar. Finally, we compared the two measures at two different spatial precision levels: per-row and per-block. fPAR estimated by VO at the per-tree level was also assessed. Results showed that VO estimated canopy fPAR performed better at each precision level than lightbar with up to 0.13 higher R 2 . The findings in this study serve as a fundamental link between aerial-based canopy fPAR and the actual yield of almonds.

The annual dynamics of whole mature almond tree nutrient remobilization in spring and the accumulation of nutrients in perennial tissues during the year were determined by sequential coring, tissue sampling, nutrient analysis, whole tree excavation and biomass estimation for trees grown under four nitrogen rate treatments 140 kg ha−1 N (N140), 224 kg ha−1 N (N224), 309 kg ha−1 N (N309), and 392 kg ha−1 N (N392) over 2 years. Whole tree perennial organ N content was greatest in dormancy then declined through bud swell, flowering and fruit set, achieving the lowest total whole tree nutrient content of perennial organs by March 12 [12–14 days after full bloom (DAFB)] coincident with 60–70% leaf expansion. During this period no net increment in whole tree N content (annual plus perennial N) was observed indicating that tree demand for N for bud break, flowering, fruit set and leaf out was met by remobilized stored N and that there was no net N uptake from soil. Remobilizable N increased with increasing N application up to N309 and was maximal at 44.4 ± 4 kg ha−1 and 37.5 ± 5.7 kg ha−1 for the optimally fertilized N309 in 2012 and 2013 respectively. Net increases in perennial organ N (stored N) commenced 41 DAFB and continued through full leaf abscission at 249 DAFB. Total annual N increment in perennial organs varied from 25 to 60 kg ha−1 and was strongly influenced by N rate and tree yield. N remobilized from senescing leaves contributed from 11 to 15.5 ± 0.6 kg ha−1 to perennial stored N. Similar patterns of nutrient remobilization and storage were observed for P, K, and S with maximal whole tree perennial storage occurring during dormancy and remobilization of that stored P, K, S to support annual tree demands through to fruit set and 70–100% leaf development. Net annual increment in perennial organ P, K, S commenced 98 DAFB and continued through full leaf abscission at 249 DAFB. Organ specific contribution to remobilizable and stored nutrients changes over the growing season are presented. Details of the pattern of perennial organ nutrient allocation, storage, and remobilization provides a framework for the optimal management of nutrients in almond with relevance for other deciduous tree species.

Agricultural productivity is subject to various stressors, including abiotic and biotic threats, many of which are exacerbated by a changing climate, thereby affecting long-term sustainability. The productivity of tree crops such as almond orchards, is particularly complex. To understand and mitigate these threats requires a collection of multi-layer large data sets, and advanced analytics is also critical to integrate these highly heterogeneous datasets to generate insights about the key constraints on the yields at tree and field scales. Here we used a machine learning approach to investigate the determinants of almond yield variation in California's almond orchards, based on a unique 10-year dataset of field measurements of light interception and almond yield along with meteorological data. We found that overall the maximum almond yield was highly dependent on light interception, e.g., with each one percent increase in light interception resulting in an increase of 57.9 lbs/acre in the potential yield. Light interception was highest for mature sites with higher long term mean spring incoming solar radiation (SRAD), and lowest for younger orchards when March maximum temperature was lower than 19°C. However, at any given level of light interception, actual yield often falls significantly below full yield potential, driven mostly by tree age, temperature profiles in June and winter, summer mean daily maximum vapor pressure deficit (VPDmax), and SRAD. Utilizing a full random forest model, 82% (±1%) of yield variation could be explained when using a sixfold cross validation, with a RMSE of 480 ± 9 lbs/acre. When excluding light interception from the predictors, overall orchard characteristics (such as age, location, and tree density) and inclusive meteorological variables could still explain 78% of yield variation. The model analysis also showed that warmer winter conditions often limited mature orchards from reaching maximum yield potential and summer VPDmax beyond 40 hPa significantly limited the yield. Our findings through the machine learning approach improved our understanding of the complex interaction between climate, canopy light interception, and almond nut production, and demonstrated a relatively robust predictability of almond yield. This will ultimately benefit data-driven climate adaptation and orchard nutrient management approaches.

Arbuscular mycorrhizal fungi (AMF) are mutualistic fungi that play important roles in plant nutrition and soil ecosystem functions. While AMF are known to benefit diverse host plants under a range of conditions, little is known about their presence in commercial almond orchards and how frequently used management practices regulate AMF root colonisation. A large-scale survey of almond orchards in the Central Valley of California was conducted to determine the extent of mycorrhizal associations with roots and the impact of orchard management practices and soil properties on AMF root colonisation rates. The roots in all orchards were colonised, with an overall average rate of 64.4%. Organically managed orchards had higher AMF root colonisation rates (73.2%) as compared with conventionally managed orchards (62.1%), primarily due to the presence of soil vegetative cover rather than organic matter inputs. Choice of rootstock and fumigation had only marginal effects while inoculation at planting increased AMF root colonisation of young trees by 27% compared to non-inoculated control. These results highlighted the ubiquitous presence of AMF in commercial almond orchards and significant interacting influences of common management practices on AMF root colonisation under field conditions. Further research into the functional implications of mycorrhizal associations in these orchards will help guide the development of management practices that increase AMF abundance and root colonisation to improve the sustainability of this rapidly expanding industry.

English walnut (Juglans regia L.) kernel color is an important quality factor for producers, processors, buyers and consumers because it affects marketing, sales, brand reputation, and prices. In the field and during postharvest handling, oxidation of phenolics in the pellicle (seed coat) triggers kernel darkening, resulting in amber-colored kernels. Our two-year study confirmed that 'Howard' is highly susceptible to dark kernel color development due to excess irrigation and harvesting at later physiological maturity, unlike 'Chandler'. In 'Howard', the combination that reduced most the percentage of light-colored kernels was later physiological maturity at harvest and excess irrigation: this may account for similar to 40% of losses due to amber kernel color at harvest. The percentage of light-colored kernels is also reduced during cold storage, but the impact of cold storage is small (similar to 10% loss) compared to that from incoming kernel color quality pre-determined by maturity stage at harvest and irrigation. Our data confirm the benefits of carefully monitoring proper irrigation, physiological maturity, and low-temperature storage (similar to 0 degrees C) for California walnut cultivars.