By II Bachelor of Science Oklahoma State University Stillwater
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STUDIES ON CYTOSPORA CANKER &1-ffi DIEBACK OF PEACH AND SELECTED ORNA}ffiNTAL TREES By ROBERT TOWNES HOLLAND II Bachelor of Science Oklahoma State University Stillwater, Oklahoma 1976 Submitted to the Faculty of the Graduate College of the Oklahoma State University in partial fulfillment of the requirements for the Degree of N.t\STER OF SCIENCE December, 1981 I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I STUDIES ON CYTOSPORA CANKER AND DIEBACK 0 PEACH AND SELECTED ORNAMENTAL TREES Thesis Approved: ===-- ii '10~98~9 PREFACE Cytospora canker and dieback is not a new disease in Oklahoma. The disease has not been recognized as an important problem in Oklahoma until recently because signs of the pathogen are difficult to detect in the field and symptoms of the disease are easily confused with those caused by other pathogens and insect pests. In addition to the broad range of studies presented in this thesis, a cross-index of Cytospora species, Valsa species, and their reported hosts within the United States has been compiled from the literature and is available separately. The author wishes to express his appreciation to his major adviser, Dr. Kenneth E. Conway, for his guidance and assistance throughout the study. Appreciation is also expressed to the other committee members, Dr. H. A. Melouk, Professor L. S. Morrison, Dr. D. F. Wadsworth, and Dr. C. E. Whitcomb, and to Dr. W. L. Klarman, for their assistance in preparation of the final manuscript. A special note of thanks goes to Mr. Ernie Fischer for the use of his orchard and for his financial support through the B. F. Blackledge Memorial Scholarship, created to further research in Oklahoma peach production. Various staff members of the Department of Horticulture at Oklahoma State University are to be thanked for their cooperation and advice. Plant materials were donated by Mid-Western Nurseries, Iric. Appreciation is also extended to my parents, Dr. and Mrs. Charles K. Holland, for their moral and financial support through the years. iii A final expression of gratitude goes to my wife, Julie, and our daughter, Laurel, for their understanding and perserverance with me throughout this project. ~ · TABLE OF CONTENTS Chapter Page I. INTRODUCTION. 1 II. REVIEW OF THE LITERATURE. 3 Disease Losses . 3 Taxonomy of the Pathogen • • • 4 The Disease Cycle ••.. 6 Pathogenicity ..•..•• 11 Predisposition--The Role of Physiological St~ess . 12 Genetic Control. • • •••• 14 Chemical Control • • . .. 17 Cultural Control • • 19 Biological Control 21 III. M..<\TERIALS AND l'lETHODS • 24 Disease Surveys. 24 Cultural Experiments • . ~ . 26 Mating Crosses. 26 Host Utilization. ••• 26 Induction of the Perfect State. 27 Cross-Inoculation Experiment • • 27 Control Experiments ••.••••.•• 29 Natural Infection of Entire Trees • 29 Artificial Infection of Pruning Wounds •• 30 IV. RESULTS • 33 Disease Surveys. 33 Cultural Experiments • • 37 Mating Crosses .• 37 Host Utilization. • • 38 Induction of the Perfect State .• 38 Cross-Inoculati~n Experiment • • 38 Control Experiments ••.••••.• 42 Natural Infection of Entire Trees • 42 Artificial Infection of Pruning Wounds ••• 42 V. DISCUSSION •••...• 46 Disease Surveys. 46 Cultural Experiments . 48 Mating Crosses •• 48 Host Utilization. 49 v Chapter Page Induction of the Perfect State. so Cross-Inoculation Experiment . 51 Control Experiments •• · ••••..... 54 Natural Infection of Entire Trees 54 Artificial Infection of Pruning Wounds .• 55 LITERATURE CITED • • . 58 APPENDICES 65 APPENDIX A - HOSTS OF CYTOSPORA AND VALSA SPECIES RECORDED IN OKLAHOMA. 66 APPENDIX B - FIGURES 9-14 68 vi .. LIST OF TABLES Table Page I. Number of Cytospora Infection Sites per Peach Tree of Two Age Groups of Three Varieties in the Fischer Orchard. .. 34 II. Number of Cytospora Infection Sites per Peach Tree from Combined Data of Perkins and Fischer Orchard Surveys-- Fall, 1980 .•.•.••. ! • • • • • • • • • 35 III. Recovery of Cytospora Isolates After Cross-Inoculation of Various Hosts . • • . • • • • • . • 39 IV. Effect of Fungicide Treatments on the Number of Cytospora Infection Sites per Peach Tree--Fall, 1980 • . • . • 43 V. Effect of Fungicide Treatments on the Number of Cytospora Infection Sites per Peach Tree--Summer, 1980 . • • 43 VI. Percent Recovery of Cytospora from Treated Pruning Stubs of Peach . • • . • . 44 vii LIST OF FIGURES Figure Page 1. Cytospora Pycnidia on Peach Stem with Adjacent Bark Removed . 5 2. Conidial Tendril Oozing from a Pycnidium .••• 5 3. Infected Peach Twig Causing Perennial Canker of a Subtending Scaffold Limb .. • • • • . • • • . • • • • • 5 4. CytosEora Infection of Peach Limb Originating at a Pruning Wound 0 . 0 0 0 0 0 0 0 0 0 0 0 0 0 . 0 0 0 0 5 5. Diagram of a Peach Tree Showing Categories Used for Disease Assessment. 0 0 0 0 0 0 0 . 0 0 0 0 0 0 25 6. Counties Recorded with Cytos_eora Infection on Peach 36 7 0 Host Discoloration and Cytospora Isolate Recovery After Cross-Inoculation, According to Host ....•. 40 8. Host Discoloration and CytosEora Isolate Recovery After Cross-Inoculation, According to Isolate . • . • . 41 9. ~ytospora Cultures from Mating Group Study. 69 10. Mucilaginous Cluster of Conidiophores and Conidia of C. leucostoma from Peach Produced Freely on Hyphae in Culture. 69 11. Treated Pruning Stub Used in Control of Artificial Infection Study • . • • • . • • . 70 12. Canker and Pycnidia on Peach Stem Inoculated with the Peach Isolate of Cytospora. • . 70 13. Longitudinal Section of a Healed Peach Stem Inoculated with the Cottonwood Isolate of Cytospora . • . • 70 14. Diffuse Pattern of Xylem Discoloration of a Pruning Stub of Peach After Artificial Inoculation with C. leucostoma Conidia . 70 viii CHAPTER I INTRODUCTION Horticultural crops are frequently considered to be of minor importance in Oklahoma, however, a 1978 survey (56) indicates a total State income from ornamental greenhouse and nursery stock of over $80,000,000 annually. Oklahoma has approximately 3,000 acres of commercial peaches (Dr. M. Smith, OSU Dept. Horticulture, personal communication) producing over $1,000,000 worth of fruit annually (6). American Fruit Grower (4) in 1978 reported a 40% growth increase for the Oklahoma peach industry in only two years. Other fruit and vege table crops are grown successfully in many parts of the state. Disease research on horticultural crops has been limited in Oklahoma and studies of the important diseases are needed. The Oklahoma State University Plant Disease Diagnostic Laboratory found that members of the fungal genus Cytospora Ehrenb. were causing damage to commercial peach orchards, nursery stock, and landscape trees in the eastern part of the state. One wholesale nursery suffered an estimated 100% loss of two year old cottonwoods in 1980 from a block of 400 five gallon containers, with a wholesale loss of nearly $3400 or a retail loss of $10,400. The fungus causes a perennial canker and dieback of fruit, timber, and ornamental trees (1, 2, 3, 16, 19, 69, 69, 78). Although Cytospora canker has been shown to seriously limit production in other states (16, 1 2 19, 28, 37), little is known of its activity in Oklahoma. This repre- sents the first comprehensive study of Cytospora canker in Oklahoma. The scope of this thesis is necessarily broad owing to·the lack of .. any previous research on the subject in the state. The objectives were three-fold: first, to determine whether or not the disease is a serious problem; if so, on which tree species and in what parts of the state; secondly, what is the etiology of the disease; and finally, how can it be controlled. More specifically, the research involved: (l) surveys to determine the impact of the disease on crops within the state; (2) isolations from various hosts, laboratory culture studies, and field cross-inoculations to identify the causal agents and determine their host specificities; and (3) chemical and biological control studies to develop an economical and effective means of control. The majority of the work concerns peach although ornamental and forest tree species are also considered. These studies should establish a groundwork of information on Cytospora canker in Oklahoma, from which new insights and new ideas for further research can be generated. .. CHAPTER II REVIEW OF THE LITERATURE Disease Losses Although some Cytospora species exist as very weak pathogens and often only as saprophytes, others can be very destructive, such as those infecting stone fruits and ornamentals in the willmv and poplar family (Salicaceae). In young peach orchards the disease can cause the pre- mature death of trees; older infected trees lose productivity and cankers may eventually girdle the trunk or scaffold branches, thus decreasing tree longevity (67). Girdling and dieback of one year old twigs in the spring is also an important phase (79). Severe losses of cottonwood (Populus deltoides) and willow seedlings (Salix spp.) have been reported from both within the nursery and after outplanting (28). Perennial peach canker caused by Cytospora cincta Sacc. and C. leucostoma Sacc. is the most serious disease of peach (Prunus persica) in Ontario, Canada (43). The disease occurs throughout the United States wherever peach trees are grown. In Illinois it is the greatest limiting factor in production (67). A report from Colorado (46) indicates that 65% of all bearing-age trees are infected and recommends 18-23% of its survey blocks should be removed as commercially unacceptable production units. Cytospora canker is the most threatening disease facing western Colorado stone fruit growers (46). Idaho (37) and New York (40) have long contended with the disease and it is now considered 3 4 a major contributing factor to peach decline in the Hudson River Valley, previously thought to be caused by a mycoplasma (61). Canker on poplar (Populus spp.) has been recorded throughout North America and Europe and has affected poplar (13, 19, 74) and cottonwood (16, 28) grown in nurseries where losses of 75% have been reported.