树木生长期长,在它的一生中,可能同时或先后受到多种生物的和非生物的致病因素的影响,其中有些因素的作用不很明显,但作用的时期很长。树木受到这些因素的综合影响,会逐渐生长衰退甚至死亡。在这一过程中,很难确定那一个因素是主要病原。这种现象 Trees grow long and may be affected simultaneously or sequentially by a number of biological and non-living virulence factors, some of which are less pronounced but have a longer duration of action. Affected by the combination of these factors, trees will gradually decline and even die. In this process, it is difficult to determine which one is the major pathogen. this phenomenon