Which of the following is an example of a pathogenic tree disease?

The correct answer identifies fire blight, verticillium wilt, and powdery mildew as examples of pathogenic tree diseases. These diseases are caused by various pathogens, such as bacteria and fungi, which invade the plant and lead to harmful effects.

Fire blight, for instance, is caused by the bacterium Erwinia amylovora and primarily affects fruit trees like apple and pear. It presents as a wilting and blackening of blossoms and shoots, significantly impacting tree health. Verticillium wilt, caused by the fungus Verticillium dahliae, affects a wide variety of plants, causing wilting and dieback due to vascular system blockage. Powdery mildew, a fungal infection, is characterized by white powdery spots on leaves and can weaken the overall vigor of trees.

Other options include various symptoms and conditions, but they do not accurately represent pathogenic tree diseases as exclusively as the correct choice. For example, while option A lists some serious tree diseases like root rot and crown gall, it is less specific about their pathogenic nature in the context of tree health relative to option C's focus on established tree pathogens.