The ecological conditions imposed by the urban lifestyle and urban activities, and their specific unfavorable effects strongly influence tree development and the presence and behavior of pathogens affecting them. It is known that some fungi, such as Ganoderma australe or Abortiporus biennis, occur almost exclusively in areas of human habitation or areas influenced by human activity - the so-called synantropic species. Serious damage is done to trees by traffic, incorrect tree maintenance, construction work near trees and deliberate man-made injuries. This includes damage to trunks, tree crowns, branches and roots. As a result, the wood-destroying fungi are very active, since they penetrate into woody plants through those wounds caused by abiotic or biotic agents. For example, Ganoderma resinaceum and Meripilus giganteus are usually found to produce fruit bodies at the base of trunks, which are often damaged when lawns are cut. All mechanical injuries reduce the vitality of woody plants and so diminish plant resistance to parasites.
Furthermore, the fact that people live in urbanized areas restricts the use of certain control methods. It is quite difficult to prune or spray tall trees in narrow streets or busy avenues when big cranes are necessary, nor is it possible to spray with chemicals that exceed a certain level of toxicity for humans. The intrinsic layout of the town or the ornamental function of the trees frequently determines the shape of the trees, and subsequently the type of pruning, drastic and harmful much of the time, weakening the trees and favoring the entrance and attack of the pathogens. Frequently, in order to offer a high aesthetic value, a mixture of a great number of plant species of diverse colors, shapes, foliage, height, etc. can be found growing together in close contact in parks and gardens, mostly exotic species showing different levels of adaptation to the urban environment. This mixture, although very attractive and desirable in terms of species diversity, has two sides. On one hand, species diversity successfully reduces the risks of serious widespread outbreaks of pests and diseases, whilst on the other hand their different sensitivities to pesticides and different environmental requirements can make the difficult task of keeping pests and diseases under control even more complicated.
Another notable difference between plant protection in urban areas as opposed to in forests or agricultural crops is that sometimes it is necessary to control a pest or disease that does not cause direct damage or is not particularly harmful to the tree, but it causes a great deal of inconvenience to, or it is aesthetically undesirable for, the citizens. The ornamental value of urban trees implies the acceptance of a level of damage or visible symptoms of zero, or very close to zero, which requires the application of more drastic control measures. That is the case with many aphid species or with their associated non-parasitic fungi Capnodiaceae (sooty mould). Trees blacken due to the accumulation of sooty mould on tree surfaces and the honeydew the insects produce falls on vehicles parked underneath, street benches, pavement cafés, sidewalks, etc., which get dirty and sticky.
Commonly, the main concern for a plant/forest pathologist or entomologist when working on a certain disease or pest is not its effect on a single host plant but on the majority of the population (the crop or the forest). Within the urban pathosystem, both approaches can be necessary; either the pest/disease affecting a group of trees (in a park, garden or urban forest) and also the problems affecting a single tree of special importance - with a high historical, social, cultural or aesthetical value.
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