Location: Fruit and Nut Research2012 Annual Report
1a. Objectives (from AD-416):
1. Provide improved horticultural tools for stabilizing pecan nut yields in alternate bearing pecan trees and orchards. 2. Develop an improved approach for orchard management of fungal diseases attacking developing fruit, based on enhancement of the tree's natural disease resistance mechanisms.
1b. Approach (from AD-416):
Objectives are achieved via research and development efforts to introduce to commercial pecan producers new horticultural tools to improve production efficiency of commercial enterprises. The approach focuses on alternate bearing and pecan scab disease via introduction of the following tools: a chemical-based thinning strategy for regulation of crop-load; regulation of flowering and alternate bearing via plant growth regulators; an improved “conceptual tool” for managing alternate bearing; reduction of chemical stresses by improved nutrient element management with emphasis on zinc; and induction of greater natural tree resistance to fungal disease pests (such as pecan scab) via improved tree nutrition and systemic acquired resistance inducers. The approach includes numerous lab, greenhouse, and field based studies, but with a great deal of emphasis on field evaluation of treatments in real-world commercial orchard situations. Extension specialists are included in the research efforts to facilitate transfer of knowledge, strategies, and technologies to farmers. The approach largely relies on adaptation of existing technologies to pecan horticulture, but with new technology potentially being introduced for transition and inner-transition metals, and registration of existing commercial products for usage on pecan. There will be dozens of research studies conducted, with experimental designs structured to test hypotheses associated with objective-linked postulates. Results will be disseminated to scientific, extension, and commercial individuals and groups via appropriate publications/articles, talks, and field-days.
3. Progress Report:
Chemical thinning: Inconsistency in efficacy when using ethephon or calcium hydroxide to fruit-thin requires an approach using indirect chemical thinning using gibberellins in ‘Off’ years to reduce subsequent ‘On’ year crop load. Alternate bearing: Flowering and alternate bearing are found to be regulated via ‘gibberellins-ethylene’ vs. ‘auxin-cytokinins’ balances, followed by level of carbohydrate reserves during vernalization and bud break. Research also examined the role a) of ‘resource switching’ and its impact on flowering; and b) sugar composition of late winter xylem sap and intensity of sap flow. Zinc and nutrient element stress: Special isotopically labeled zinc was applied to enable determination of how foliar zinc moves within trees; thus, providing information for development of a better and cheaper means of controlling zinc nutrition. An investigation of the rare-earth composition of pecan and Carya species, found them to be hyperaccumulators, with one or more rare-earth elements likely playing a beneficial biological role. Pecan scab: Research was aimed at confirming the efficacy of foliar applied Ni at different concentrations, and for assessing the effect of phosphite and method and timing of phosphite (PO3) applications for reducing the impact of pecan scab on pecan yield. Both Ni and phosphite were efficacious against scab, providing additional outcomes for integrated management strategies of pecan scab. However, results showed that trunk applications of phosphite were not efficacious. Pecan scab: The distribution and severity of pecan scab in mature, tall pecan trees, was studied. There was more severe disease in the lower third of trees not receiving a ground-based application of fungicide, and although disease in treated trees is less severe in the lower third, it is similar in the upper third to non-treated trees. This indicates that tall trees need aerially-applied fungicide to manage disease in the upper canopy. Systemically acquired resistance (SAR) to pecan scab: Phosphite reduces scab severity compared to other SAR agents. Several potential SAR agents are being tested on both seedling and as trunk injections in mature trees under field conditions to investigate whether they can act as triggers for resistance to pecan scab. Scab resistance: The genome of the scab fungus was partially sequenced to identify simple sequence repeat (SSR) markers for assessing genetic diversity of the pathogen and guide scab management decisions, and for finding durable disease resistance. Six SSR-markers are now identified for studying genetic diversity. Scab resistance: Standard area diagrams improve accuracy and precision of disease assessments; thus, enabling better quality assessments, resulting in more reliable and accurate data on which to base decisions. Research compared rating scales to using the percent scale for disease assessments, and to identify the optimal number of standard area diagrams to use to assess pecan scab.
1. Nickel Reduces Pecan Scab in Orchards. Pecan scab can cause severe yield loss when environmental conditions are conducive to epidemic development. Furthermore, resistance to certain fungicides used by pecan growers to control the disease demands an integrated approach to disease management. Field studies have previously demonstrated a small but significant effect of foliarly applied Ni at reducing pecan scab. ARS researchers at Byron, Georgia, tested a full range of Ni concentrations to optimize the application strategy. By optimizing Ni applications growers are better able to manage pecan scab in orchards and consequently reduce scab-associated crop loss; thus, adding a new tool to the scab management toolbox.
2. Phosphite as a New Tool for Managing Pecan Scab. There has recently been emergence of scab resistance to certain classes of fungicides used to control this fungal disease in orchards; thus new, alternative fungicide chemistries are needed for controlling this devastating disease. ARS researchers at Byron, Georgia, found that a simple inorganic chemical, phosphite, registered for use on certain other crops, to be highly efficacious for controlling the pecan scab fungus and disease in orchards when applied as a foliar spray, but it was less effective as a trunk application. This research led to registration of phosphite products for use in pecan orchards and its use by pecan growers to manage scab and other diseases, which helps minimize the impact of the disease and also reduce the risk of fungicide resistance developing among existing fungicides used to manage scab.
3. Early Spring Sap Flow and Sucrose Content Predicts Subsequent Flowering in Pecan Trees. There are key knowledge gaps in regards to the flowering and alternate bearing mechanism in pecan trees. ARS researchers at Byron, Georgia, found that flowering and crop load are tightly linked to the volume and the amount of sucrose within late winter and early spring sap flow from tree trunks. This information affirms the importance of previous season canopy management to maximize leaf health in order to ensure following year flowering. Orchards are being increasingly managed to ensure availability of high dormant season carbohydrate and sucrose reserves for completion of the final phase of floral development.
4. Improving Accuracy and Reliability of Pecan Scab Assessment. Pecan scab can cause severe disease on fruit and leaves, which leads to nut crop yield loss. Accurate and reliable methods to assess disease are critical to ensure high quality data for comparing treatments statistically, as is required by researchers, and for growers who may base management decisions on estimates of disease. ARS researchers at Byron, Georgia, developed a ‘standard area diagram set’ that aides scab disease assessment on fruit, and which reduces error and variability in disease assessments, and are preferable to unaided assessments or using disease-category scales. These standard area diagrams are being used to ensure that consistently high quality data is being recorded when disease assessments are made.
5. Improving Canopy Health of Tall Pecan Trees. Growers are faced with challenges managing foliage health and scab disease in tall (> 55 feet) pecan trees. Ground-based sprayers may not provide sufficient coverage to reach the tops of tall trees, and aerially applied sprays can be costly. However, there is no information available on the distribution of scab, nor whether the essential nutrient element concentrations differ in tall pecan trees. ARS researchers at Byron, Georgia, found that disease is most severe in the lower third of trees receiving no fungicide; however, in trees receiving fungicide, disease is either less in the lower third of the canopy, or the same as the upper third of the trees receiving no fungicide. Nutrient element concentrations are also typically less at the top of the canopy. This work identifies a need for growers to use aerially applied fungicides in severe scab years, and to aerial application of micronutrients.
6. Genetic Diversity of Pecan Scab. The ability of the pecan scab pathogen to adapt to new pecan cultivars is unknown, but knowledge of the pathogen’s genetics will aid development of improved cultivars with durable resistance. ARS researchers at Byron, Georgia, have partially sequenced the genome of the scab pathogen to identify SSR markers to study its population genetics. A total of 60 SSR markers have now been screened, of which six have now been identified for using in studies of genetic diversity. Resulting information on the genetic diversity of the scab pathogen is critical to help guide decisions in managing scab resistance in pecan so as to ensure durable disease resistance in newly developed cultivars planted by growers in regions prone to scab epidemics.
Bock, C.H., Brenneman, T.B., Hotchkiss, M.W., Wood, B.W. 2012. Evaluation of a phosphite fungicide to control pecan scab in the southeastern USA. Crop Protection Journal. 34:58-65.