Chapter XXII. 



Diseases of Wild Plants. 



Gall disease of the blueberry, cranberry and other heaths 



[Exobasidium vaccmii (Fckl.) Wor.]. Plants of the heath fam- 

 ily, e. g., cowberries, blueberries and true cranberries, may be at- 

 tacked by a fungus which produces malformations of branches 

 and leaves. Flowers and flower-stalks are also attacked in some 

 cases, and the formation of fruits prevented. Sometimes the leaf 

 swells up into enormous kidney-shaped, fleshy bodies which are 

 many times thicker than the normal leaves. The hypertrophied 

 portions, where exposed to the light, are colored red. A cut 

 through such a leaf shows it to be composed of a fleshy mass, 

 through which the branching veins can be clearly seen. These 

 veins are much reduced in structure from the normal and contain 

 but a small amount of woody material. The fleshy portions have 

 almost completely lost the power of starch-making, as is shown 

 by the small amount of leaf-green present and by the entire ab- 

 sence of large air-spaces. The fungus threads are very fine and 

 are found only in the attacked regions. They run between the 

 cells of the host, and at the surface of the leaf form a rather 

 dense weft, just underneath the cuticle of the external layer of 

 cells. From this weft numerous spores are produced on basidia, 

 which are arranged in a palisade. The spores can be seen with 

 the naked eye as a fine, white powder on the surface of the leaf. 

 Under the microscope the spores are seen to be produced on small, 

 finely-pointed stalks, four arising from a single cell. When these 

 spores fall on the young leaves or stems of the host-plant they 

 germinate into fine germ-tubes, which penetrate into the leaf 

 through an air pore, or directly through the walls of the outer 

 row of cells, and from this point spread into the mature mycelium. 

 The presence of the mycelium immediately stimulates the leaves 

 to the above-described abnormal growth, which takes place at 

 the expense of the neighboring parts. All diseased plant parts 

 should be burned. (See Fig. 37.) 



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