Location: Fruit and Tree Nut Research
2019 Annual Report
Objectives
1. Mitigation of alternate bearing by individual trees, and affiliated reproductive subunits, by improving canopy health and longevity.
1.A. Determine if canopy exposure to ammonium and/or cytokinin can increase flowering, fruit-set, and improve health and longevity of pecan tree canopies.
1.B. Determine if increasing root exposure to ammonium by way of soil fertilization alone or in conjunction with canopy applied urea and flowering bioregulators can increase flowering, fruit-set, and improve canopy health and longevity.
2. Develop improved integrated management of pecan foliar and fruit diseases, with emphasis on pecan scab.
2.A. Determine ways to provide better control of scab and other pathogens in pecan trees based on an understanding of disease progress, tree stress and canopy health.
2.B. Develop an improved basis for managing scab resistance using a knowledge and understanding of the characteristics of the genetic diversity and population biology of the scab pathogen.
Approach
This research aims to provide pecan farmers improved and sustainable crop-load and disease management practices enabling mitigation of alternate (AB) bearing by pecan orchards.
The goal of this research project is to provide U.S pecan (Carya illinoinensis) farmers with horticultural and disease management tools and strategies that enable them to mitigate alternate bearing (AB). The industry considers AB to be its most important biological problem. AB is an economically harmful, typically biennial, trait that causes excessive year-to-year fluctuation in nut yield and kernel quality. Many biotic and abiotic factors, and the stresses caused by reproduction, interact with innate reproductive physiology to either induce or increase the amplitude of this cyclical pattern. There are many important knowledge gaps pertaining to the biology and management of AB. This project targets whether fruiting in ‘Off’ (low crop load) years can be increased by appropriate and timely use of cytokinin (CK) and/or ammonium (NH4+) in the previous ‘On’ year to enhance production of female (pistillate) flowers the following year. It also targets whether reducing previous ‘On’ (high crop load) year tree stress caused by fungal diseases, particularly pecan scab (Fusicladium effusum), but also other stressrelated pathogens, can increase subsequent ‘Off’ year flowering. A series of field and laboratory studies over the next five years will address these key knowledge gaps and develop greater understanding and horticultural tools and strategies required for mitigation of AB.
Progress Report
Experiments using air-blast sprayers were repeated and disease severity was recorded in pecan tree canopies to relate to spray coverage. Fungicide spray distribution was measured using different sprayer parameters of volume and speed. Results are being analyzed and will be used to ascertain how different sprayer settings from ground-operated air-blast sprayers impacts disease control at different heights in the canopy of tall pecan trees.
An experiment was run to assess the effects of single- and double-sided applications of fungicide to pecan trees using an air-blast sprayers to manage scab. Fungicide spray distribution was measured and disease severity estimated. Results are being analyzed and will be used to ascertain whether single sided spraying of trees has merit at any time during the season for management of scab in tall pecan trees.
An experiment was designed to compare the efficacy of using a double volute orchard air-blast sprayer to a standard non-voluted sprayer for control of scab in large orchard trees was repeated. Preliminary results indicated a small advantage in disease control when using the volute system compared to the non-voluted system. In situations where trees are tall, volutes may provide some benefit to reducing severity of disease.
A long-term study 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. 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 U.S. But long-term effects of hedge pruning are not known. Results are being analyzed. Mechanical hedge-pruning is beneficial to maximizing canopy coverage with fungicide to reduce scab severity of the nut crop.
Experiments were continued to better understand the role of asexually produced spores (conidia) of the scab fungus, Venturia effusa in scab epidemics. Contrary to prior assumptions, the lesions on shoots in the canopy may not play a major role as a source of infection for the early buildup of the scab epidemic in the canopy of pecan trees. Results may impact control options. Preliminary data is being analyzed.
A study to determine scab resistance genes in a provenance collection of pecan was expanded. In collaboration with scientists at the Noble Foundation, DNA was extracted from samples of ~850 trees previously phenotyped for scab susceptibility. To confirm results additional scab severity data is being obtained from all trees in 2019. Bulked segregant analysis/GWAS is planned to confirm sources of resistance to scab that may eventually be utilized in breeding programs.
Research was conducted to better characterize the impact of using ordinal scales for the assessment of pecan scab and other plant diseases to ensure continued improvements in the accuracy of the disease severity data collected (ensuring that analysis will have reduced risk of error). The ramifications of rating methods, particularly disease ordinal scales and analysis of resulting data, were further explored. The work aims to ensure accurate and reliable estimates of disease are obtained in all experiments requiring visual estimates of scab.
A greenhouse study was initiated to determine the effect of canopy exposure to nano-particles, in particular essential micronutrients zinc and nickel. The effects of foliar application of zinc and nickel nano-particles is being compared to conventional micronutrient applications on pecan seedlings as a means to improve health and longevity of pecan trees, while reducing wastage and cost of application. The use of nano-fertilizers will allow more sustainable and precise pecan production.
Data collection of agronomically and horticulturally valuable traits in a provenance collection of pecan was initiated. Characterization of bud break and flowering time has been completed. Nut traits, including fatty acid profiles of trees resistant to scab are being assessed. These traits are important for future breeding programs and will contribute to more sustainable production and can benefit the health value of the pecan nut.
Research to evaluate dwarfing hickory rootstocks on tree productivity is underway. Tree size is a major constraint to pecan production. Two cultivars, ‘Desirable’ and ‘Stuart’ were grafted onto 12 different hickory rootstock species 33 years ago. Graft compatibility and tree yield are being assessed for the first time. The effect of hickory species as rootstocks can provide useful information on their value for tree dwarfing and other traits, including drought tolerance, while maintaining tree productivity and overall tree health.
Accomplishments
1. Established the prevalence of Pecan Bacterial Leaf Scorch (PBLS) in Georgia. ARS researchers in Byron, Georgia, characterized the presence of PBLS (caused by the plant pathogenic bacterium, Xylella fastidiosa subsp. multiplex) in different pecan cultivars in pecan production areas in Georgia. The disease is widespread and in some orchards/cultivars was of high incidence. Previously there was no information on the distribution or prevalence of the disease in Georgia. This information is useful to guide any future management approaches applied to PBLS which is known to reduce yield by at least 30% in some cultivars.
2. Ascertained the relationship between the incidence and severity of pecan scab. ARS researchers in Byron, Georgia, characterized the relationship between incidence (the number of units diseased) and severity (the amount of plant tissue diseased) of pecan scab and demonstrated that incidence may have some potential as a surrogate measure for severity. Currently both disease severity and incidence are measured to assess scab, but disease severity estimates are time consuming and less accurate. Understanding the relationship between incidence and severity can facilitate the determination of disease intensity by using just incidence (saving resource and improving the accuracy of data).
3. Improving assessment of pecan scab and other plant diseases. Accurate and reliable methods for assessing disease severity are critical to both research of disease and their management. Quantitative ordinal scales have been used to assess severity of scab. ARS researchers in Byron, Georgia, investigated the impact of the method of analysis on the ability to differentiate treatments based on different scale types, including ordinal scales. The results give scientists useful guidance for selecting specific methods of analysis appropriate to ordinal scales, and the characteristics to select when developing a scale.
Review Publications
Bock, C.H., Hotchkiss, M.W., Shapiro Ilan, D.I., Wells, L., Brock, J., Brenneman, T.B., Mizell, R.F. 2019. Efficacy of Bordeaux mixture in reducing pecan scab in the southeastern U.S.A. Organic Agriculture. 9(2):189-198. https://doi.org/10.1007/s13165-018-0218-x.
Bock, C.H., Young, C.A., Stevenson, K.L., Charlton, N.D. 2018. Fine scale population genetic structure and within tree distribution of mating types of Venturia effusa, cause of pecan scab in the U.S.A. Phytopathology. 108:1326-1336.
Bock, C.H., Olive, J.E., Chen, C., Hotchkiss, M.W., Stevenson, K.L., Wang, X., Grauke, L.J., Hilton, A.E., Jo, Y., Randall, J.J. 2018. Pecan bacterial leaf scorch, caused by Xylella fastidiosa, is endemic in Georgia pecan orchards. Plant Health Progress. 19(4):284-287.
Chen, C., Bock, C.H., Brannen, P.M., Adaskaveg, J.E. 2018. Mining and characterization of microsatellites from a genome of Venturia carpophila. Mycological Progress. 17(8):885-895. https://doi.org/10.1007/s11557-018-1401-x.
Chen, C., Bock, C.H., Brannen, P.M. 2019. Novel primers and sampling for PCR detection of Xylella fastidiosa in peach. Phytopathology. 109(2):307-317. https://doi.org/10.1094/PHYTO-11-18-0439-FI.
Bock, C.H., Hotchkiss, M.W., Shapiro Ilan, D.I., Brock, J.H., Brenneman, T.B., Wilkins, B., Wells, D.E., Wells, L., Mizell, R.F. 2019. A comparison of organic fungicides: alternatives for reducing scab on pecan. Organic Agriculture. https://doi.org/10.1007/s13165-018-0229-7.
Bock, C.H., Hotchkiss, M.W., Brenneman, T.B. 2019. The effect of number of fungicide applications and application date on phosphonate residue in nutmeats of pecan. Crop Protection. 122:70-78. https://doi.org/10.1016/j.cropro.2019.04.020.
