City Trees

September/ October 2011

City Trees is a premier publication focused on urban + community forestry. In each issue, you’ll learn how to best manage the trees in your community and more!

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Page 29 of 39

AN INTRODUCTION TO Sudden Oak Death and Upcoming Research by Anna O. Conrad and Pierluigi Bonello, Department of Plant Pathology, The Ohio State University, Columbus, Ohio, and David S. Bienemann, Municipal Arborist for the City of Bowling Green, Ohio Photos by Anna O. Conrad dynamics of the mixed evergreen forests of coastal California and southwestern Oregon, and, more recent- ly, of larch stands in the British Isles. Sudden oak death and ramorum blight are two diseases caused by the pathogen Phytophthora ramorum, an oomycete or fun- gal-like organism. Over the past 15 years, Phytophthora ramo- rum has played a significant role in shaping the P. ramorum has an extensive host range. However, sud- den oak death is commonly associated with important forest, plantation, and landscape trees, such as coast live oak (Quercus agrifolia, an oak from the red oak group) and tanoaks (Notholithocarpus densiflorus) in the Western U.S., and beech (Fagus spp.) and larch (Larix spp.) in the British Isles. Ramorum blight primarily occurs on woody ornamentals and understory plants such as rhododendron, viburnum, and camellia. As the name implies, the disease has been deadly for oaks and tanoaks. Symptoms of sudden oak death are variable and depend on the host and the environment (like all other oomycetes, P. ramorum favors cool and moist conditions). Symptoms on coast live oak are best characterized by the presence of dark, viscous exudate (blood) and cankers on the main stem of trees. On tanoak, symptoms are more diverse, and in addition to the presence of blood and stem cankers, leaf and twig blight and tip and shoot dieback may occur. While these symptoms are often associated with the presence of P. ramorum infection, they are not unique to this pathogen. Therefore, to confirm the presence of P. ramorum, one must look for signs of the pathogen, that is to say the presence of reproducing and rest- ing structures belonging to the pathogen, which are essential for its spread. However, not all hosts support the formation of reproducing structures. Hosts that do not support their formation are considered terminal hosts, because the pathogen cannot spread from them. Terminal hosts in California and Oregon include coast live oak and other species of Quercus. (On the other hand, tanoak cannot be considered a terminal host 30 because the pathogen has been observed to produce dissemination structures on its leaves.) Infection of ter- minal hosts is dependent on the formation of spores on understory species, such as California bay laurel and rhododendron, and dispersal via rain splash and air currents. The potential of P. ramorum to infect a wide array of economically and ecologically valuable species in United States forests and ornamental nurseries has resulted in government-imposed quarantines of any plant material that may be carrying the pathogen. Economic conse- quences of P. ramorum include the loss of oak and tanoak timber, losses in wood product harvesting due to compliance with governmental restrictions, extensive losses to the ornamental industry, and decrease in prop- erty values due to pathogen presence and tree death in urban areas. Additionally, ecological consequences include the loss of fundamental forest species, impacts to watershed protection, and the loss of wildlife food and habitat. Because of these consequences, several disease man- agement strategies and tactics have been imposed. The first strategy is to prevent spread. Perhaps the most suc- cessful form of management has been the prevention of long distance pathogen spread outside of natural infestation areas in the Western United States through quarantine regulations imposed by the USDA's Animal and Plant Health Inspection Service (APHIS) on the orna- mental industry. In California and Oregon, halting of local spread has been attempted by destroying the hosts that facili- tate the dissemination of the pathogen, a process called "sanitation." However, these efforts are sel- dom, if ever, successful, and given the prevalence of the disseminating hosts, this strategy is tantamount to destroying the forest to save it. For economically important trees, like those in urban areas, chemi- cal treatments are available to prevent or reduce P. ramorum infection. Chemical treatments, however, are not feasible in natural areas. Therefore, the most rational approach to managing this disease with City Trees

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