Infectious tree diseases—those diseases that involve not only a tree host, but also an infectious pathogen such as certain fungi, bacteria, nematodes, phytoplasmas or viruses—are among the most vexing maladies that we deal with as horticulturists. Let's take a look at why this is the case. First, of what do we speak? Dutch elm disease. Verticillium wilt on maple and smoketree and tuliptree and many other hosts. Oak wilt. Fire-bhght on pear, mountainash and other members of the rose family (Rosaceae). Plum black knot on plums, cherries, peaches, almonds and their ornamental cousins. Pine wilt disease. Phytoph-thora root and crown rot of beech and other hosts. Apple scab on crabapple. Tar spot of maple. Anthracnose on sycamore, anthracnose on dogwood and anthracnose on oak. Bacterial leaf scorch. Sudden oak death. Thousand cankers disease. Powdery mildews on many different trees.
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