56 Diagnosing of Diseases 



The existence of the root rot which produces stunted 

 growth and eventually death, may be determined by close 

 observation in finding a whitish, yellowish, or dark brown 

 growth of loosely interwoven, hair-like myceHum threads 

 on the root surface and in decaying tissues, or in the soil 

 itself. This mycelium will also be found between wood 

 and bark in whitish leather sheets, and will grow into the 

 bark at the base of the tree above ground, finally producing 

 here and on exposed roots the readily recognized fruit- 

 body. 



There is at least one insiduous disease, known to be due 

 to bacteria, a disease peculiar to North America, and famil- 

 iar under the name of fire-blight, which attacks the trees of 

 the apple family, and especially the pears and quinces, but 

 also the mountain-ash, service-berry, and hawthorn. It is 

 evidenced by the browning and subsequent blackening, first 

 of the leaves and finally of young twigs and shoots, starting 

 at the tips and progressing downward until even the larger 

 branches may be infected. The bark of the diseased 

 branches cracks, and a thick, blackish, gummy fluid exudes, 

 the bark later drying up and shrinking. The disease may 

 even descend into the trunk, then resembling sun-scald. 



Curiously enough, healthy, vigorous, well-fed, and well 

 cultivated trees are more liable to this malady than less 

 thrifty ones, new succulent growth favoring the disease. 

 There is but one remedy, namely, to cut out and burn the 

 affected parts well below the discolored portions. This 

 should be done during winter or spring, or in the fall after 

 the trees have stopped forming wood. Checking the amount 

 of moisture in the soil, as, for example, by the grow^th of 

 grass or clover, will reduce the succulent growth and pre- 

 vent the disease. 



Physiological Diseases. Whenever local causes of the 



