376 Sherald: Bacterial Leaf Scorch of Landscape Trees Arboriculture & Urban Forestry 2007. 33(6):376–385. Bacterial Leaf Scorch of Landscape Trees: What We Know and What We Do Not Know James L. Sherald Abstract. Xylella fastidiosa has been recognized as a pathogen of landscape trees for over 25 years. Collectively, these diseases are referred to as bacterial leaf scorch (BLS). Arborists, property owners, and communities are now beginning to recognize BLS as a serious threat to the urban forest. Although advances in symptom awareness and diagnostic techniques have enabled arborists to diagnose BLS, there are many questions regarding host range, transmission, pathogeneses, disease management, and individual tree therapy that remain unanswered. Key Words. Alternative hosts; BLS; leafhoppers; Pierce’s disease; Xylella fastidiosa. Diseases caused by the fastidious, xylem-limited bacterium Xylella fastidiosa have been known for well over 100 years. Pierce’s disease of grape, or the California vine disease, was reported in California, U.S. in 1892 (Pierce 1892) and phony disease of peach, a dwarfing disorder, was reported in Geor- gia in the late 1880s (Cochran and Hutchins 1974). At first, the relatedness of these disorders was not recognized; how- ever, both diseases were thought to be viral in nature and transmissible by grafting and by leafhoppers. The first indi- cation of a bacterial etiology occurred in 1971 when the an- tibiotic tetracycline was shown to suppress the symptoms of Pierce’s disease (Hopkins and Mortensen 1971). Later, elec- tron microscopy showed the presence of rippled-walled, rick- ettsia-like bacteria in xylem tissue of symptomatic grape and peach (Goheen et al. 1973; Hopkins and Mollenhauer 1973; Hopkins et al. 1973; Nyland et al. 1973). Successful isolation of the pathogen by Davis et al. (1978) led to the description of X. fastidiosa as a new species of plant pathogenic bacteria in 1987 (Wells et al. 1987). Since then, X. fastidiosa has been associated with several major diseases (Table 1). Diseases caused by X. fastidiosa are confined primarily to the Americas, where they have significant economic impor- tance for several major crops such as grape, alfalfa, peach, almond, citrus, and coffee. The recent arrival of the glassy- winged sharpshooter leafhopper in California in the 1990s has greatly increased the economic threat of Pierce’s disease, almond leaf scorch, and oleander leaf scorch (Purcell and Saunders 1999a). Xylella fastidiosa has been recognized as a pathogen of landscape trees since 1980 when xylem-limited bacteria se- rologically related to the Pierce’s disease bacterium were found in leaf scorch-affected elms, sycamores, and oaks (Hearon et al. 1980). Over the last 25 years, X. fastidiosa has ©2007 International Society of Arboriculture been found in over 40 species of landscape trees (Table 2). Leaf scorch diseases caused by X. fastidiosa are commonly referred to as bacterial leaf scorch (BLS). Today, BLS of elm, oak, and sycamore are recognized as major diseases of street and landscape trees in the mid-Atlantic and southeastern United States (Sherald 2001; Gould and Lashomb 2005). BLS is a chronic disorder that affects trees over many years. Symptoms first appear in a single branch or segment of the crown and spread through the crown in subsequent years. Leaf symptoms develop in midsummer and progress in se- verity toward fall. Depending on the species, leaves develop various patterns of marginal and interveinal necrosis (Sherald 2001; Gould and Lashomb 2005). Elms develop a distinct undulating marginal necrosis bordered by a chlorotic halo; sycamores develop an irregular interveinal necrosis with zones of light and dark reddish brown tissue. In both elm and sycamore, symptoms usually progress in severity from the older to the younger leaves on the branch and affected leaves abscise early. Some oak species develop a marginal necrosis with a reddish band of tissue separating necrotic from green tissue. However, symptoms are not always distinct. In pin oak, BLS simply appears as early senescence with no distinct pattern of necrosis. Reductions in growth and seed set, as well as flower bud abortion, have all been reported in some hosts. Although BLS is a vascular disease, there is no vascular discoloration. Long-affected trees eventually develop die- back, which may be a consequence of BLS as well as other factors affecting the stressed tree. Infected trees are usually removed before they die because of their appearance or po- tential hazard. Recognition of X. fastidiosa as a pathogen of landscape trees has been an important advance, because many leaf scorch and decline symptoms in trees were previously attrib-
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