2013 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.
This project has spanned its 5-year cycle and is transitioning to a new Project Plan on ‘Mitigating Alternate Bearing’. The ability for growers to manage alternate bearing by individual trees has been advanced by a) developing a mechanized hedge pruning protocol for the southeastern U.S., b) gaining an understanding as to how to better regulate flowering using gibberellic acid to reduce excessive cropping in the following year; by managing tree canopy health to maximize dormant season sucrose reserves, c) improving tree access to rare-earth elements, d) using plant bioregulators to prevent induction of alternate bearing by canopy damaging by black pecan aphids. A new theory was advanced for flowering regulation in temperate zone trees, with timely regulation of tree hormones increasing or decreasing subsequent year flowering with ethylene and cytokinins being especially promising for future study. New trace element tools were developed for improving crop nutrition by improving nickel and rare-earth element nutritional physiology. Management strategies and tools were developed for reducing June-drop and water-stage fruit-drop by improving tree boron, nickel, and potassium nutrition. It was also found that pecan, and most hickory species, are hyper-accumulators of rare-earth elements and likely possess a physiological requirement for one or more rare-earth elements as either essential or beneficial nutrient elements; thus, identifying management of organic matter in orchard soils as a means of ensuring that trees receive sufficient rare-earths to meet nutritional needs. These elements appear to help trees to better withstand water stress and adaptation to dry sites. It was found that improving tree nickel nutrition increases nutmeat quality, reduces crop loss to pecan scab disease and water-stage fruit-drop. Improving tree potassium nutrition was found to reduce ‘June drop’. Efforts to mitigate alternate bearing by reducing pecan scab found that phosphite is very effective at protecting trees from scab, especially that of foliage, and that the agent triggers a form of systemic resistance to scab. A screening of many other systemic resistance-inducing agents effective in other crops had no activity in pecan. Several co-dominate genetic markers, and other dominate genetic markers, were developed as tools enabling detailed investigation of the genetic diversity of the scab pathogen. Use of these markers indicate that there is far more genetic diversity in the pathogen than thought; therefore, leading to new orchard management approaches for managing scab resistance to fungicides and disease epidemics. An improved disease rating scale was developed that greatly improves visual estimation on disease severity. The above knowledge, tools, and strategies promise to greatly facilitate efforts by U.S. farmers to produce pecans.
Reducing pre-harvest germination (Vivipary) of pecan. Pecan crop loss to the germination of nuts while still on the tree (i.e., vivipary) is a major problem for many pecan farmers, especially those in hot arid climates. There is need to develop management strategies that stabilize year-to-year variability in marketable nutmeat yields due to vivipary. ARS researchers at Byron, Georgia, found that vivipary is greatly influenced by tree exposure to moist soils with high nitrate nitrogen during the nutmeat filling stage of fruit development, and that the malady is regulated by the plant hormone, abscisic acid. Moist soils and high fruit nitrate prevents timely production of abscisic acid in developing seeds, which prevents seed dormancy. These findings also implicate copper and/or molybdenum deficiency as major contributing factors to vivipary through their influence on the activity of the key enzyme producing abscisic acid. This information enables pecan, and possibly grain farmers, to tailor crop management strategies to reduce loss to pre-harvest germination.
Plant and animal nickel nutrition influences the activity of ribonuclease enzymes. The metabolism of nitrogen is key to a multitude of primary and secondary metabolic reactions taking place in both plants and animals. While nickel is an essential trace element in plants and animals, relatively little is known about its metabolic or physiological roles. ARS researchers at Byron, Georgia, found that a key plant/animal enzyme involved in the recycling of nitrogen (Ribonuclease A, RNase A) within organisms functions as a urease enzyme in the presence of nickel. This research demonstrates that a non-metallo enzyme, RNase A, exhibits dual activity depending on the presence or absence of nickel, and identifies a possible new metabolic role for nickel in plants and animals. The presence of both RNase A and nickel in the spring sap of pecan trees indicates that this enzyme is likely important to the cycling of nitrogen during the early growth phase of many crop species; thus, potentially affecting alternate bearing by pecan trees as well as nitrogen and nickel nutrition management strategies of other crops, especially those exposed to urea fertilizers or transport nitrogen as urea.
Phosphite as a new tool for managing pecan scab. Due to the emergence of fungicide resistance in the scab pathogen (Fusicladium effusum) to certain classes of fungicides, there is need for alternative fungicide chemistries for managing this devastating disease. ARS researchers at Byron, Georgia, found that the simple inorganic molecule, phosphite, registered for use on certain other crops, was highly efficacious in controlling the pecan scab fungus. Phosphite is effective in reducing the disease on young trees as a trunk application, but not on older trees. This research led directly to the registration of phosphite products for use in pecan orchards as canopy sprays in the southeastern U.S. The addition of phosphite as an effective alternative fungicide in the pecan ‘disease management toolbox’ enables growers to minimize the impact of scab and reduces the risk of fungicide resistance developing among existing fungicides used to manage scab.
Improving the accuracy and reliability of pecan scab assessment. 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, have demonstrated that use of some older category-type scales produces data that has less agreement with actual values and poorer reliability compared to assessments made using the 0-100% scale. These results provide the basis for stakeholders (scientists, advisors and growers) to use a rating system that provides data with the highest possible accuracy and reliability, thereby minimizing the risk of error in databased decision-making.
Genetic diversity of pecan scab. The pecan scab pathogen is known to adapt to resistant cultivars of pecan, and knowledge of the pathogen’s genetics will provide a basis for improving resistance development in new cultivars to ensure resistance that is more durable. ARS researchers at Byron, Georgia have now screened a total of 130 SSR markers, and have developed an additional 5 UP-PCR marker and 10 RAPD markers that will be used in studies of genetic diversity, which will lead to improved management of available resistance genes and thus more durable scab resistance.
Excessive iron triggers nickel deficiency. Nickel deficiency not only influences alternate bearing by pecan trees, but also reduces yield and quality of many other crops. Factors affecting the cellular bioavailability of nickel in either plants or animals have the potential to influence the health, yield and quality of plant and animal products. ARS researchers at Byron, Georgia, found that nickel deficiency is easily induced by excessive iron fertilization or plant exposure to iron, with iron acting in an antagonistic manner in regards to nickel bioavailability and nutritional physiology. This research identifies an important micro-nutrient interaction in plants that has heretofore been unrecognized, while possessing many practical applications in agriculture. For example, it identifies iron fertilization as a means of alleviating nickel toxicity in crops, especially those growing on highly mineralized serpentine soils. It also highlights the possible existence of iron induced nickel deficiency occurring in many cropping systems where iron is a fertilizer supplement.
Bock, C.H., Cook, A.Z., Parker, P.E., Gottwald, T.R., Graham, J.H. 2012. Short distance dispersal of splashed bacteria of Xanthomonas citri subsp. citri from canker-infected grapefruit tree canopies in turbulent wind. Plant Pathology. 61:829-836.
Bock, C.H., Van Den Bosch, F., Parnell, S., Wood, B.W., Gottwald, T.R. 2013. The effect of Horsfall-Barratt category size on the accuracy and reliability of estimates of pecan scab severity. Plant Disease. 97:797-806.
Bock, C.H., Wood, B.W., Gottwald, T.R. 2013. Pecan scab severity-effects of assessment methods. Plant Disease. 97:675-684.