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 mycelium 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 growth of 
grass or clover, will reduce the succulent growth and pre- 
vent the disease. 
Physiological Diseases. Whenever local causes of the 
