Fig. 1. Pear Blight, healthy and diseased twigs. serious American plant diseases such as potato blight, grape black rot, grape downy mildew and powdery mildew have made the European tour or even the world tour "personally conducted" under some unwitting guides.
The chestnut bark disease (Fig. 3) illustrates well the rapid and destructive invasion that is possible in case of new diseases. First noted in 1904 by Murrill in New York, and now well known in New York, New Jersey, Delaware, Connecticut, Rhode Island, Maryland and Virginia, it is rapidly spreading in every direction (Fig. 4). In Brooklyn 16,695 trees were killed on 350 acres, and the loss in and about New York city is placed at five to ten million dollars. The chinquepin and chestnut alone are susceptible. The attack is made upon the bark through wounds, but twigs and leaves are not directly affected. From the point of attack the disease spreads in all directions until the diseased parts meet on the opposite side of the branch, thus girdling the twig and killing it.
Sometimes it happens that a newly introduced disease causes much loss in its first years and later sinks to comparative insignificance. Such was the Fig. 2. A Potato half Destroyed by "Wart Disease"; the large "warty" outgrowth is soft and rapidly decaying. history of the carnation rust which about 1892 caused the loss of entire houses of plants, but which in a few years spent its force until it is now regarded as a disease of no unusual menace. In other instances imported diseases continually remain serious, as have the numerous grape diseases introduced into Europe from America and from Europe into America.
The diseases mentioned in Diagram I. are nearly all of great destructiveness. The potato blight is that which caused the famous potato famine in Ireland in 1845, in which year it swept Great Britain as well as much of Europe and America with a