ADVANCING OUR UNDERSTANDING OF ASTER YELLOWS EPIDEMIOLOGY TOWARD IMPROVED DISEASE MANAGEMENT By Kenneth E. Frost A dissertation submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy (Plant Pathology) at the UNIVERSITY OF WISCONSIN – MADISON 2012 Date of final oral examination: 11-02-2012 The dissertation is approved by the following members of the Final Oral Committee: Russell Groves, Associate Professor, Entomology Amy O. Charkowski, Associate Professor, Plant Pathology Jed B. Colquhoun, Professor, Horticulture Paul D. Esker, Assistant Professor, Plant Pathology David Kyle Willis, Associate Professor, Plant Pathology i Acknowledgments I thank my adviser, Russ Groves, for his excellent criticism and support over the course of this research. I also thank my committee members, Amy Charkowski, D. Kyle Willis, Paul Esker and Jed Colquhoun for their time, guidance, and advice. Along the way, conversations with Tom German, Jeff Wyman, Brian Flood, and Paul Mitchell have been enjoyable and helpful. I want to give a shout out to all the current and former Groves Lab members – Carol Groves, Emily Mueller, Scott Chapman, Shahideh Nouri, Anders Huseth, JP Soto-Arias, Chen Zhang, David Lowenstein, Natalie Hernandez, and Sarah Schramm have all been invaluable sources of information and constructive criticism over the last five years. I thank them for their insightful discussion in and around Russell Labs and at the terrace. I should also thank the workers at the Hancock Wisconsin Agricultural Experiment Station for their willingness to lend a hand and the carrot growers of Wisconsin for their cooperation and support. Finally, I would like to thank my family for their support throughout this work. ii Abstract Rational: Each year, Wisconsin vegetable producers grow carrots on an average of 4,500 acres grossing over $6 million dollars in revenues. Unfortunately, this crop is threatened annually by the occurrence of aster yellows (AY), a disease caused by the aster yellows phytoplasma (AYp). The AYp is transmitted primarily by the aster leafhopper (ALH) in a persistent-propagative manner and current disease control focuses on controlling the insect vector. The decision to intercede and implement an insecticide spray is based upon the Aster Yellows Index (AYI), a calculation of the allowable numbers of ALH for a crop at a particular infectivity rate. Since its development, the AYI has been refined to include treatment thresholds for crops with differing susceptibility to AY. We have also advanced our tools for detecting AYp in plants and insects and have come to understand that the components of the index, abundance and infectivity, vary spatially and temporally in Wisconsin. However, these advances in technology and understanding have not been used to their highest potential for managing AY. Project Goal: To advance our understanding of the epidemiology of aster yellows in Wisconsin towards the development and implementation of a comprehensive management plan Objectives: I) To determine the probability that an AYp-infected leafhopper will transmit the pathogen to a susceptible plant. II-A) To evaluate the relative importance of assigned causes of variability associated with aster leafhopper abundance and infectivity. B) To determine seasonal trends in factors associated with AY risk. III) Refine and characterize the current integrated pest management program in processing carrot production to include reduced-risk iii technologies. To address these objectives we developed molecular diagnostic tools to ensure molecular AYp detection was accurate, precise, and reflects biology, used historical pest survey sampling to identify seasonal trends in factors associated with AY risk, created an ALH development model to better understand conditions favorable for the sporadic occurrence of high AY risk periods, and combined this information together with econometric assessments for improved decision making in pest management and risk avoidance. Impact and Outcomes: By ensuring molecular AYp detection is accurate, by identifying seasonal trends in association with AY risk, and by advancing predictive tools to address the sporadic occurrence of high AY risk periods, we can minimize costs associated with unwarranted pesticide applications and reduce yield losses due to advanced preparation for ALH infestations. This project aims to incorporate biologically relevant information about the AY disease system from multiple scales using available and emerging technologies to improve on-farm AY management decisions. These improvements to diagnostic, detection and vector monitoring systems are anticipated to benefit agricultural producers, crop consultants and other stakeholders and will be essential for properly timing vector control measures to times when plant protection is most needed. iv Table of Contents Acknowledgments ............................................................................................... i Thesis Abstract ................................................................................................... ii Chapter 1: Literature Review ............................................................................. 1 Problem defined ................................................................................................................. 2 The aster yellows phytoplasma ......................................................................................... 2 The aster leafhopper .......................................................................................................... 4 Mode of transmission ........................................................................................................ 5 Aster Leafhopper migration .............................................................................................. 5 Aster Leafhopper influence on aster yellows in Wisconsin .............................................. 6 Aster Yellows management ............................................................................................... 7 Research rational and significance .................................................................................... 8 References ....................................................................................................................... 12 Chapter 2: Detection and variability of aster yellows phytoplasma (Candidatus Phytoplasma asteris) titer in its insect vector, Macrosteles quadrilineatus (Hemiptera: Cicadellidae) ........................................................ 16 Abstract ............................................................................................................................ 17 Introduction ..................................................................................................................... 18 Materials and Methods .................................................................................................... 21 Results ............................................................................................................................. 33 Discussion ........................................................................................................................ 39 References ....................................................................................................................... 50 Tables ............................................................................................................................... 57 Figures ............................................................................................................................. 63 Chapter 3: Factors influencing aster leafhopper (Macrosteles quadrilineatus) abundance and aster yellows phytoplasma infectivity in Wisconsin carrot fields ...................................................................................... 67 Abstract ............................................................................................................................. 68 Introduction ..................................................................................................................... 69 v Materials and Methods .................................................................................................... 73 Results ............................................................................................................................. 83 Discussion ........................................................................................................................ 86 References ....................................................................................................................... 96 Tables ............................................................................................................................. 102 Figures ........................................................................................................................... 107 Appendix A .................................................................................................................... 111 Appendix B .................................................................................................................... 116 Chapter 4: Seasonal pattern of aster leafhopper (Macrosteles quadrilineatus) abundance and aster yellows phytoplasma infectivity in Wisconsin carrot fields .................................................................................... 118 Abstract ..........................................................................................................................