Bacterial Leaf Scorch of Landscape Trees Caused by Xylella Fastidiosa

Abstract

Such observations supported the hypothesis that both Pierce's disease and peach phony were caused by xylem inhabiting viruses, an unusual occurrence since viruses were known to occur only in the phloem and parenchyma tissue and not in the xylem. The first evidence that a bacterium rather than a virus was involved in these diseases occurred in 1971 when symptoms of Pierce's disease were suppressed by treating plants with the antibiotic tetracycline (21). Electron microscopy later con- Table 1. Selected list of natural hosts of Xylella fastidiosa. The bacterium Xylella fastidiosa, first de- scribed in 1987 (44), has been associated with leaf scorch and decline of American elm (Ulmus americana) (13,26,37), American sycamore (Platanus occidentalis) (13,38,39), red mulberry (Morus rubra) (31), red maple {Acerrubrum) (41), and several species of red oak (Quercus) (2,11,13,18,28). This article reviews what is known and unknown about this unique pathogen and its effect on landscape trees. X. fastidiosa has a diverse and extensive host range encompassing over 30 families of mono- cotyledonous and dicotyledonous plants (Table 1). While most hosts are asymptomatic, there are a number of species in which symptoms occur and some that are severely affected (Table 2). Pierce's disease of grape and phony disease of peach are the two most thoroughly studied diseases caused by X. fastidiosa. Epidemics of Pierce's disease were first observed in California in the 1880's. The disease is now known to be endemic in the south- eastern United States where it is the major factor limiting grape culture (15). Pierce's disease causes leaf necrosis, decline, and eventually death of the vine. Phony disease of peach was first observed in Georgia during the same period and is found predominantly in the southeastern United States. The characteristic symptoms of peach phony are dwarfing accompanied by profuse lateral branching and flattened dark green foliage. Trees live for many years, but fruit size, number, and quality are reduced (3). Early efforts to isolate the pathogens from grape or peach were unsuccessful. However, the causal agents of both Pierce's disease and peach phony were transmitted by xylem feeding leaf- hoppers (14,22,42,43) or by grafting using tissues that included xylem and not bark alone (3,7,23).