Location: Fruit and Tree Nut Research
2020 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
This final report serves to document progress of research conducted under the project, 6042-21220-013-00D. ARS researchers at Byron, Georgia, conducted experiments using air-blast sprayers and severity of pecan scab disease was recorded in pecan tree canopies to relate to spray coverage. Fungicide spray distribution was measured by ARS researchers at Byron, Georgia, 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 conducted by ARS researchers at Byron, Georgia, 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 conducted by ARS researchers at Byron, Georgia, 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. 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 by ARS researchers at 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 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 by ARS researchers at Byron, Georgia, 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. Data is being analyzed.
An experiment was initiated by ARS researchers at Byron, Georgia, to understand how populations of scab differ on different cultivars. No studies have been done 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.
Accomplishments
1. Higher doses of phosphonate fungicide improved control efficacy of pecan scab disease. Pecan scab disease is the major disease reducing yield of pecan in the southeastern region. ARS researchers at Byron, Georgia, demonstrated that higher concentrations of safe phosphonate fungicides provided more efficacious control of pecan scab disease. Several of the currently available fungicides are under threat due to fungicide resistance in the scab pathogen, so establishing improved control using higher doses of phosphonate is timely and will result in higher yield of pecan with better quality nutmeats.
2. Aerial applications of fungicide are similarly efficacious as ground applied fungicide to control scab disease. Pecan scab disease is a major issue in tall trees due to difficulties achieving adequate fungicide coverage high in the canopy. ARS researchers at Byron, Georgia, investigated the efficacy of ground rig-applied and aerially-applied fungicide sprays for controlling pecan scab disease in tall pecan trees. Both methods were effective and overall controlled scab to a similar level. Aerial application is a rapid away to apply fungicide and can be useful as an alternative during wet periods. Demonstrating equivalent control of scab disease provides growers with an alternative application method which may be used to achieve control of the disease while maintaining yields of pecan.
3. Understanding pathogenicity in the pecan scab pathogen, Venturia effusa. How the pecan scab pathogen initiates and develops infection in pecan is not understood, but the knowledge will underpin development of durably resistant cultivars. ARS researchers at Byron, Georgia, generated a complete telomere-to-telomere reference genome of V. effusa that represents the first full chromosome-level assembly of any Venturia sp. The genes identified in the genome will provide the basis for understanding the infection process, and ultimately developing new resistant cultivars.
Review Publications
Chiang, K., Liu, H.I., Chen, Y.L., El Jarroudi, M., Bock, C.H. 2020. Quantitative ordinal scale estimates of plant disease severity: Comparing treatments using a proportional odds model. Phytopathology. 110:734-743. https://doi.org/10.1094/PHYTO-10-18-0372-R.
Ramakuwela, T., Hatting, J., Bock, C.H., Vega, F.E., Wells, L., Mbata, G.N., Shapiro Ilan, D.I. 2019. Establishment of Beauveria bassiana as a fungal endophyte in pecan (Carya illinoinensis) seedlings and its virulence against pecan insect pests. Biological Control. https://doi.org/10.1016/j.biocontrol.2019.104102.
Bock, C.H., Brenneman, T.B., Herrington, K., Hotchkiss, M.W. 2019. The effect of phosphonate rate on control of scab on pecan. Crop Protection. 124:104823. https://doi.org/10.1016/j.cropro.2019.05.017.
Liu, H.I., Tsai, J.R., Chung, W.H., Bock, C.H., Chiang, K. 2019. Effects of quantitative ordinal scale Design on the accuracy of estimates of mean disease severity. Agronomy Journal. 9:595. https://doi.org/10.3390/agronomy9090565.
Winter, D.J., Charlton, N.D., Krom, N., Shiller, J., Bock, C.H., Cox, M.P., Young, C.A. 2020. Chromosome-level reference genome of Venturia effusa, causative agent of pecan scab. Molecular Plant-Microbe Interactions. 33:149-152. https://doi.org/10.1094/MPMI-08-19-0236-A.
Bock, C.H., Hotchkiss, M.W. 2020. A comparison of ground-based air-blast sprayer and aircraft application of fungicides to manage scab in tall pecan trees. Plant Disease. 104:1675-1684. https://doi.org/10.1094/PDIS-11-19-2345-RE.
Bock, C.H., Barbedo, J.G., Del Ponte, E.M., Chiang, K., Bohnemkamp, D., Mahlein, A. 2020. From visual estimates to fully automated sensor-based measurements of plant disease severity: status and challenges for improving accuracy. Phytopathology Research. 2:9. https://doi.org/10.1186/s42483-020-00049-8.
