ABSTRACT
Arthropod pests including insects and mites and plant pathogens including viruses, bacteria, and especially fungi severely affect survival, growth and aesthetics of amenity trees in urban environments. Factors such as the loss of plant biodiversity and reductions in the diversity and abundance of natural enemies and antagonists predispose urban forests to pest and disease outbreaks and catastrophic tree loss. The introduction of non-native plants can disrupt ecosystem processes with varying and sometimes contradictory results. Some non-native plants are not consumed by native herbivores in the new range. The result is fewer consumers at higher trophic levels and simplification of predator communities in urban areas. When non-native plants are introduced with their associated non-native insects and mites, eruptive outbreaks of pests can occur in the absence of important natural enemies left behind in the aboriginal range. Moreover, non-native pathogens that arrive on a plant from one region may spread to new hosts in the new geographic range. Non-native plants lacking an evolutionary history with insects and pathogens in a new geographic location may lack defenses and succumb to indigenous pests and diseases. Similar and calamitous tree loss occurs when non-native arthropod pests and pathogens are introduced to evolutionarily naïve plant hosts in a new geographic region. Classic examples of non-native pests and pathogens devastating trees in a new location include emerald ash borer, hemlock woolly adelgid, chestnut blight and Dutch elm disease. Other features of the built environment including impervious surfaces and elevated temperatures stress plants by lowering their defenses or increasing their nutritional value and predisposing them to attack by insects, mites and pathogens. Several anthropogenic inputs including ozone, nitrogen, and de-icing salts are associated with urban infrastructures. These too may increase the susceptibility of trees to attack by biotic agents. In this chapter we discuss several features of built environments that threaten the vitality and resilience of urban forests. We also deal with globally important pests and diseases in urban environments. Arthropod pests treated in the chapter encompass lethal borers, foliar pests, and sucking arthropods including scale insects, lace bugs, and 252spider mites. Specific arthropod case studies include oak processionary moth, Thaumetopoea processionea, emerald ash borer, Agrilus planipennis, and horse chestnut leafminer, Cameraria ohridella. Diseases encompass root rots and wood decays, cankers including the canker stain of plane trees caused by Ceratocystis platani and ash dieback associated with Hymenoscyphus fraxineus, vascular diseases, including Dutch elm disease and oak wilt caused by Ophiostoma novo-ulmi and Ceratocystis fagacearum, respectively, as well as the most important anthracnose and foliar diseases of oaks, planes, maples and horse chestnuts. Pests and diseases are described in their significance, impact and diagnostic characters. Integrated management strategies and tactics are discussed.
