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ARS Home » Southeast Area » Byron, Georgia » Fruit and Tree Nut Research » Research » Research Project #437836

Research Project: Healthy, Sustainable Pecan Nut Production

Location: Fruit and Tree Nut Research

2022 Annual Report

1. Improve pecan nut productivity by analyzing key horticultural issues that disrupt annual consistency, yield, and quality, and developing new or improved mitigation strategies. 1.A. Determine if canopy exposure to nano-particles, in particular zinc and nickel nano-particles can improve health and longevity of pecan tree canopies. 1.B. Characterize horticultural traits of native pecan germplasm and identify genes of interest as a resource for development of new and improved cultivars. 1.C. Characterization of improved pecan rootstocks for uniformity of yield and enhanced productivity. 2. Reduce impacts of the most important pecan diseases on production, quality and uniformity of nutmeats. 2.A. Characterize and identify novel ways to improve management of pecan scab in tree canopies based on inoculum sources, fungicide spray coverage, disease distribution and methods for disease management. 2.B. Determine dynamics of population genetic diversity of Venturia effusa in pecan orchards. 3. Develop new and improved pecan processing technologies, such as pasteurization and cracking/shelling, for improved storage, food safety, nutrition, and marketability. This objective will be coordinated with research on improved crop management to reduce the impact of abiotic and biotic factors causing unstable or reduced yield, and reduced quality nuts, for an integrated, systems approach to pecan production and post-harvest that benefits the profitability of the U.S. pecan industry. Anticipated products include new pecan pasteurization processes that meet market standards while maintaining nutmeat quality and nutrition.

This research aims to provide pecan farmers with improved, sustainable tree and disease management practices that stabilize yield in pecan (Carya illinoinensis) orchards and maximize postharvest nutmeat quantity and quality. The management tools and strategies will enable farmers to mitigate alternate bearing (AB) and yield loss caused by disease. AB is considered the most important biological problem facing pecan production: it is economically harmful, resulting in excessive year-to-year fluctuation in nut yield and kernel quality. Many biotic and abiotic factors can induce or increase the amplitude of AB. How factors associated with canopy health, particularly nutrition, rootstock, and disease affect AB represent some of the knowledge gaps that limit development of suitable tools for stabilizing nut production and reducing yield losses. Losses postharvest include physically damaged kernels due to postharvest processing, loss to biological contaminants and poor shelf life resulting in loss of quality and salability. The research addresses 1. Whether use of nano-fertilizers can provide a basis for more efficient nutrient management, stabilize and increase production of pecan, while ensuring better environmental security. 2. Phenotyping horticultural traits of native and improved pecan germplasm in conjunction with genome wide analyses. 3. Using genomics to identifying markers for horticultural traits for use in the breeding program. 4. Characterizing the role of rootstocks in tree growth and productivity, to provide a basis for more uniform, consistent and thus sustainable production of pecan nutmeats. 5. Determining how inoculum sources of pecan scab (caused by Venturia effusa) contribute to the epidemic, and using this information to develop new disease management tools, thereby reducing the impact of scab in susceptible pecan cultivars. 6. Understanding the population genetics of the scab pathogen to underpin deployment of more durable host resistance in the future. 7. Using genomics to identify effector genes involved in scab pathogen virulence. 8. Developing novel nut shelling and associated processing techniques to minimize physical nutmeat damage, contamination, and loss of quality postharvest. and 9. Improving understanding of pre-harvest and postharvest environmental factors that impact pecan color, texture, oil quality, and phytochemical content to identify conditions that maximize duration of quality. A series of field and laboratory studies over the next five years will address these key areas where knowledge is lacking; the resulting information will provide the basis for novel management products that improve horticultural and disease management and favor canopy health in pecan, and improve the postharvest pipeline to maximize quantity and quality of saleable nutmeats.

Progress Report
Data collection of agronomically and horticulturally valuable traits in a provenance collection of pecan was continued. Characterization of bud break and flowering time has been completed in the provenance collection housed at ARS, Byron, Georgia. Additional tree phenotypic traits including trunk diameter, tree height, leaf area, nut load and tree architecture are being analyzed. Nut quality traits are also being assessed. These traits are important for future breeding programs and will contribute to more improved, more sustainable production and will benefit the health value of the pecan nut. The second year collection of tree phenotypic variables was made in the ‘Lakota x 87MX3-2.11’ mapping population. Variables measured included growth rate (tree height, trunk diameter), leaf area, and bud break. The two different climates in Texas and Georgia will allow for screening of horticultural traits in this population. These data will allow environment x genotype studies, providing a basis for understanding inheritance of different phenotypic traits in pecan. Pecan cultivars. ‘Desirable’ and ‘Stuart’ grafted onto 12 species of hickory in genus Carya were planted at the Southeastern Fruit and Tree Nut Research Station (SEFTNRS) in Byron, Georgia in June 1986 with the aim of assessing graft compatibility. Data collection of agronomic and horticultural traits was continued by ARS scientists at Byron, Georgia. Variables included tree height, trunk diameter, nut load, yield, and nut quality characteristics. The aim is to understand whether different species of hickory used as rootstocks have an effect on tree yield and nut quality of these two common pecan cultivars. The study will provide information on whether non-pecan hickories could have beneficial rootstock traits that can be used to improve pecan production. A field experiment with ‘Pawnee’ trees was conducted to evaluate the effects of a fungicide treated/non-treated and hedged vs a non-hedged pruned orchard on nut quality, yield and gas exchange parameters. Gas exchange parameters include assimilation rate, transpiration rate, and stomatal conductance to water vapor . Second year data collection is currently in progress. The information will shed more light on tree and nut quality parameters on hedged and non-hedged pruned pecan trees. A field experiment was initiated to characterize the microbiome of a hedged vs non-hedged pruned orchard of ‘Cherokee’. Shoot growth and gas exchange parameters are being collected monthly at 3 different heights within the canopy. Soil and leaf samples are being collected in order to get better insight into the bacterial and fungal microbiome. Improved knowledge on the effect hedge-pruning on soil and foliar microbiomes will help make better informed decisions regarding irrigation and fertilization programs to maximize pecan tree health by ensuring a healthy microbiome. Commenced collection of nuts of cultivars ‘Elliott’, ‘Pawnee’, and ‘Sumner. Nuts are being harvested over the developmental period of nut growth, water stage, shell hardening, and kernel fill stages. Allergen analysis will at the protein and RNA levels. This time course study will allow for a better understanding of pecan nut attributes such as lipid content, lipid oxidation, and allergens during growth and development. An experiment was continued for a third season to better understand pecan scab disease epidemic development in hedge-pruned pecan trees. Trees were hedge pruned or not, and were either sprayed with fungicide + insecticide or not, and scab disease incidence and severity assessed on foliage, fruit and shoots. Results will be analyzed and will be used to ascertain how early season shoot infections vary in the differently managed trees, and the impact on scab disease development on foliage and fruit during the hedge-pruning cycle. The information will aid fungicide applications timing to better manage shoot lesions in hedge-pruned and non-hedged pecan trees. A long-term study (10 y) was continued to explore the effect of prolonged cycles of mechanical hedge-pruning on pecan scab and tree health compared to non-hedge-pruned trees (all fungicide treated). Results are showing the benefits of hedge-pruning for management of scab susceptible cultivars in the southeastern United States. The new season’s results continue to be analyzed. Mechanical hedge-pruning is beneficial to reducing tree size and maximizing canopy coverage with fungicide to reduce scab severity. An experiment to explore the systemic nature of phosphite fungicides in the canopy of tall pecan trees was repeated, and disease assessment performed. Knowing how the fungicide redistributes can help with understanding potential for systemic activity in relation to disease control in distal portions of the canopy, and determine how this impacts residues in nutmeats. The information can be utilized to maximize scab control while minimizing regulated phosphite residues in nutmeats. Scab severity was assessed for a second year in a mapping population designed to better understand the inheritance of scab resistance. The cross is the two genotypes ‘Lakota’ x ‘87MX3-2.11’. Assessments were performed for each genotype in the experiment. As part of a multi-year, multi-location study, the information will aid understanding of scab resistance for breeding purposes. An experiment was continued to determine characteristics of populations of the pecan scab pathogen collected from different cultivars. The experiment is designed to establish whether populations from cultivars grown alongside each other are freely recombining and conceivably have the potential to develop a “super race” of scab able to infect multiple cultivars. Results will provide valuable insights into populations genetic dynamics of scab pathogen populations to help guide resistance breeding efforts. An experiment monitoring for the sexual stage of pecan scab in leaf litter and in air samples was continued. Slides were inspected for the ascopores (which have been demonstrated in vitro) of the pecan scab pathogen. Identifying the production of sexual spores will aid understanding the epidemiology of the disease, sources of variability, and potentially additional points to apply control. An experiment was performed to test the effectiveness of pulsed ultra-violet light at reducing Salmonella on the surface of pecan kernels. Data was collected to determine the reduction of Salmonella before and after treatment. Understanding the effectiveness of PUVL at reducing pathogens will help determine if it is a feasible treatment that can be used to reduce pathogen contamination. The SEFTNRS in Byron, Georgia has established a new Biosafety Level 2 lab to perform food safety research on pecans. Construction is underway for a small scale pecan shelling facility at the location for studying processing impacts on pecan food safety and nutmeat quality. Pecan cracking mechanisms were tabulated, and a taxonomy of approaches developed through a comprehensive search of literature, patents, equipment, meeting with equipment manufacturers, and visiting facilities that use shelling equipment. Knowing the various approaches used assists in understanding the mechanics of how cracking and shelling takes place, and that can be used to improve the process for higher shell-out. A series of experiments have been designed by ARS scientists at Byron, Georgia, that will test different impactor geometry and materials during the cracking process. The design of these experiments will provide a basis for understanding and optimizing the shelling process to minimize damage of the nutmeat.

1. Determined that pecan bacterial leaf scorch can be seed transmitted. Pecan bacterial leaf scorch can cause yield loss and decline of some pecan cultivars. The causal pathogen has a wide host range, and therefore movement of the pathogen is a concern. ARS researchers at Byron, Georgia showed that the bacteria causing pecan bacterial leaf scorch could be transmitted from infected seed to the seedlings of pecan. This information has potential ramifications for use of seed for root stocks, and the provision of pecan scion germ plasm or seed to foreign partners.

2. Determined the genetic control of resistance to demethylation inhibitor (DMI) fungicides in the pecan scab pathogen. Fungicide resistance is an issue with the scab pathogen, reducing effectiveness of control. ARS researchers at Byron, Georgia confirmed that a high level of resistance occurred in the field to DMI fungicides, and that the resistance was associated with specific mutations in two genes (CYP51A and CYP51B), that result in abnormalities of the protein, and how much of the protein is made. The knowledge identifies causes of resistance in the pathogen, and provides a basis to develop detection methods to rapidly identify and track specific resistance traits in orchard populations of the scab pathogen.

3. Characterized color change in pecan nutmeat postharvest and the kinetics of the process. Storage of pecan nutmeats is a problem as they can degrade rapidly, resulting in loss of the product. ARS researchers at Byron, Georgia confirmed that higher temperature and relative humidity, and longer storage duration resulted in darker nutmeat color, which reduces the consumer acceptability of the nutmeat. Kinetic constants for the color change (hue, lightness and chroma), and activation energies were calculated. The results allow stakeholders to predict changes in color of pecan nutmeats during storage under various conditions.

Review Publications
Pereira De Melo, V., Da Silva Mendonca, A.C., De Souza, H.S., Gabriel, L.C., Bock, C.H., Eaton, M.J., Schwan-Estrada, K.R., De Carvalho Nunes, W.M. 2020. Reproducibility of the development and validation process of standard area diagram By two laboratories: An example using the Botrytis Cinerea/Gerbera Jamesonii Pathosystem. Plant Disease. 104:2440-2448.
Hofman, C.O., Cottrell, T.E., Bock, C.H., Mizell, R.F., Wells, L., Shapiro Ilan, D.I. 2021. Impact of a biorational pesticide on the pecan aphid complex and its natural enemies. Biological Control. 161/104709.
Wu, S., Blackburn, M.B., Mizell, R.F., Duncan, L.W., Toews, M.D., Sparks, M., El-Borai, F., Bock, C.H., Shapiro Ilan, D.I. 2021. Pupal cell antibiosis suppresses plant and insect pathogenic fungi and is associated with a bacterium related to Serratia nematodiphila i. Journal of Invertebrate Pathology. 184/107655.
Bock, C.H., Hotchkiss, M.W. 2021. Effect of tractor speed and spray application volume on spray coverage at different heights in the canopy of tall pecan trees. Plant Disease. 105:2509-2520.
Cervantes, K., Hilton, A.E., Stamler, R.A., Heerma, R., Bock, C.H., Wang, X., Jo, Y., Grauke, L.J., Randall, J.J. 2022. Evidence for seed transmission of Xylella fastidiosa in pecan (Carya illinoinensis). Frontiers in Microbiology. Article e13:780335.
Li, Z., Di Gioia, F., Zhao, X., Hong, J.C., Rosskopf, E.N., Wilson, P.C., Pisani, C., Paudel, B. 2022. Quantifying the effects of anaerobic soil disinfestation and other biological soil management strategies on nitrous oxide emissions from raised bed plasticulture tomato production. Journal of Environmental Quality.
Pisani, C., Adkins, S.T., Turechek, W., Patel, P.C., Rosskopf, E.N. 2021. First report of Macrophomina phaseolina, Fusarium brachygibbosum, and Lasiodiplodia theobromae causing fungal watermelon vine decline in Southwest and West-Central Florida. Plant Health Progress. 22:544-551.
Moore, L.C., Brenneman, T.B., Waliullah, S., Bock, C.H., Ali, M. 2022. Multiple mutations and overexpression in the CYP51A and B genes lead to decreased sensitivity of Venturia effusa to tebuconazole. International Journal of Molecular Sciences. Vol 47: 43-57.
Del Ponte, E.M., Cazon, L.I., Alves, K.S., Pethybridge, S.J., Bock, C.H. 2022. How much do standard area diagrams improve accuracy of visual estimates of plant disease severity? A systematic review and meta-analysis. Tropical Plant Pathology.
Pravhakar, H., Kong, F., Bock, C.H., Kerr, W. 2022. Pecan color change during storage: kinetics and modeling of the processes. Food Research International.