Location: Fruit and Tree Nut Research
2021 Annual Report
Objectives
Objective 1. Improve pecan nut productivity by analyzing key horticultural issues that disrupt annual consistency, yield, and quality, and developing new or improved mitigation strategies. Objective 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. Objective 1.B. Characterize horticultural traits of native pecan germplasm and identify genes of interest as a resource for development of new and improved cultivars. Objective 1.C. Characterization of improved pecan rootstocks for uniformity of yield and enhanced productivity. Objective 2. Reduce impacts of the most important pecan diseases on production, quality and uniformity of nutmeats. Objective 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. Objective 2.B. Determine dynamics of population genetic diversity of Venturia effusa in pecan orchards. Objective 3: Develop new and improved pecan processing technologies, such as pasteurization and cracking/shelling, for improved storage, food safety, nutrition, and marketability (NP305, C1 PS1B; NP 306, C1, PS1C) Objective 3 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 US pecan industry. Anticipated products include new pecan pasteurization processes that meet market standards while maintaining nutmeat quality and nutrition.
Approach
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 such as, 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.
A ‘Lakota x 87MX3-2.11’ mapping population was created and has been planted in Somerville, Texas and replicated at Byron, Georgia. Collection of tree phenotypic variables such as: growth rate (tree height, trunk diameter), leaf area, branching pattern, and bud break has begun. This will allow environment x genotype studies to begin, providing a basis for understanding inheritance of different phenotypic traits. The two different climates in Texas and Georgia will allow for screening of horticultural traits in this population.
Pecan cvs. ‘Desirable’ and ‘Stuart’ grafted onto 12 species of hickory in genus Carya were planted at the SOUTHEASTERN FRUIT & TREE NUT RESEARCH LABORATORY 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 in 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 two common pecan cultivars ‘Stuart’ and ‘Desirable’.
A greenhouse pot study using two cultivars “Pawnee” and ‘Elliott” was established by ARS scientists in Byron, Georgia. The goal is to observe the effects a root-knot nematode Melodogyne partityla and beneficial entomopathogenic nematode Steinernema rarum have on plant physiological responses such as plant growth parameters (stem diameter, seedling height), plant gas exchange, chlorophyll content, leaf area, fresh and dry weight of leaves, stems, and roots, and root galling. This information will allow us to assess the effects root-knot nematodes can have on two common pecan cultivars (one of which is a commonly used rootstock) and whether entomopathogenic nematodes could be a solution to mitigate the problem.
An experiment was repeated by ARS scientists in Byron, Georgia, to better understand pecan scab disease epidemic development in 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 by ARS scientists in Byron, Georgia, and will be used to ascertain how early season shoot infections vary in the differently managed trees, and how this impacts scab disease development on foliage and fruit early in the season. The information will aid fungicide applications timing to better manage shoot lesions in hedge-pruned and non-hedged pecan trees.
A long-term study was continued by ARS scientists in Byron, Georgia, to explore the effect of prolonged cycles of mechanical hedge-pruning on pecan scab and tree health compared to non-hedge-pruned trees. Results confirm that hedging benefits scab management by allowing a greater proportion of the fruit to be effectively protected by efficacious fungicide coverage. This has ramifications for management of scab susceptible cultivars in the southeastern United States. But long-term effects of hedge pruning on pecan disease development are not known. Results are being analyzed by ARS scientists in Byron, Georgia. Mechanical hedge-pruning is beneficial to maximizing canopy coverage with fungicide to reduce scab severity of the nut crop.
A field experiment was repeated by ARS scientists in Byron, Georgia, to monitor the activity of pecan scab disease lesions throughout the year on three cultivars of pecan. The numbers of spores produced on lesions on shoot samples were monitored to determine periods of lesion activity. The information will help identify the critical period of conidial production from shoot lesions, particularly early in the season which is considered the primary source of infection for the epidemic, which will pave the way for more appropriately targeted fungicide applications.
An experiment was conducted by ARS scientists in Byron, Georgia, to determine characteristics of populations of the pecan scab pathogen collected from different cultivars. This is part of a study to establish whether populations from cultivars grown alongside each other are freely recombining and conceivably have the potential to develop “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 exploring the systemic nature of phosphites fungicides in the canopy of tall pecan trees was conducted by ARS scientists in Byron, Georgia, 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 phosphite residues in nutmeats.
Accomplishments
1. Developed the first four high quality genomes of pecan. ARS scientists at Byron, Georgia, developed the first four high quality genomes of pecan genotypes. Comparative genome analysis revealed substantial adaptive introgressions. The outbred nature of the genomes provided targets for variants segregating within breeding pedigrees. Presence-absence and functional variants were used to identify candidate genes for pathogen resistance. Combined, the analyses and genome resources take a crucial step towards exploiting molecular targets in the pecan genome to accelerate breeding efforts in pecan and form a foundation for functional and quantitative genomics in a highly diverse and outbred crop.
2. Optimizing scab control in mature pecan canopies. ARS scientists at Byron, Georgia, characterized the distribution of fungicide and consequent scab disease control in the canopies of tall pecan trees receiving fungicide sprays applied at different volumes and speeds. The results demonstrated that applying fungicide at 2.5 mph and 50 GPA was as efficacious as slower speeds and higher volumes usually used by the industry. This information is valuable as it provides growers the option to travel faster and apply lower volumes thereby saving time, labor costs, equipment operation and maintenance costs, and water.
3. Use of zinc and nickel nano-fertilizers on pecan physiology. ARS scientists at Byron, Georgia, assessed whether nano-fertilizers of two essential micronutrients to pecan nickel and zinc could supplement traditional salt-based fertilizers. The results demonstrated that nickel was more easily absorbed into the leaf when supplied by nano-fertilizers for both cultivars in the study ‘Zinner’ and ‘Byrd’. Additionally, nano-fertilizers showed a positive impact on gas exchange parameters on ‘Zinner’ plants. Based on these data, nano-fertilizers could be beneficial in pecan production systems in developing new orchard management tools.
Review Publications
Standish, J.R., Brenneman, T.B., Bock, C.H., Stevenson, K.L. 2021. Spatial variation and temporal dynamics of fungicide sensitivity in Venturia effusa within a pecan orchard. Plant Disease. 105: 377-383. https://doi.org/10.1094/PDIS-04-20-0889-RE.
Bock, C.H., Alarcon, Y., Conner, P.J., Young, C.A., Randall, J.J., Pisani, C., Grauke, L.J., Wang, X., Monteros, M.J. 2020. Foliage and fruit susceptibility of a pecan provenance collection to scab, caused by Venturia effusa. Center for Agriculture and Biosciences International (CABI) Invasive Species Compendium. 1:19. https://doi.org/10.1186/s43170-020-00020-9.
Bock, C.H., Young, C.A., Zhang, M., Chen, C., Brannen, P.M., Adaskaveg, J.A., Charlton, N.D. 2021. Mating type idiomorphs and genetic diversity of venturia carpophila, cause of peach scab. Phytopathology. 111(2):408-424. https://doi.org/10.1094/PHYTO-12-19-0485-R.
Luo, W., Posny, D., Kriss, A.B., Graham, J.H., Poole, G.H., Taylor, E.L., Mccollum, G., Gottwald, T.R., Bock, C.H. 2020. Seasonal and post-harvest population dynamics of the Asiatic citrus canker pathogen Xanthomonas axonopodis pv. citri on grapefruit in Florida. Crop Protection. 137:105277. https://doi.org/10.1016/j.cropro.2020.105227.
Charlton, N.D., Yi, M., Bock, C.H., Zhang, M., Young, C.A. 2021. First description of the sexual stage of Venturia effusa, causal agent of pecan scab. Mycologia. 112:711-721. https://doi.org/10.1080/00275514.2020.1759998.
Hilton, A.E., Wang, X., Zhang, M., Cervantes, K., French, J., Randall, J.J., Bock, C.H., Grauke, L.J., Jo, Y.K. 2020. Improved methods for detecting Xylella fastidiosa in pecan and related Carya species. European Journal of Plant Pathology. https://doi.org/10.1007/s10658-020-02050-5.
Bock, C.H., Rains, G.C., Hotchkiss, M.W., Chen, C., Brannen, P.M. 2020. The effect of tractor speed and canopy position on fungicide spray deposition and peach scab incidence and severity. Plant Disease. 104(7): 2014-2022. https://doi.org/10.1094/PDIS-11-18-1950-RE.
