180 MANUAL OF GARDENING 
attack is commonly not so much in the individual leaves as in 
the stems, the sources of food supply being thereby cut off from 
the foliage. The symptoms of this classof gy. _. 
diseases are general weakening of plant when jax 2 
the disease affects the plant as a 
whole or when it attacks large 
branches; or sometimes the 
leaves shrivel and die about 
the edges or in large irregu- 
lar discolored spots, but 
without the dis- 
tinct pustular 
marks of the 
parasitic fungi. Ry 
There is a gen- 214. Leaves and fruits injured by fungi, chiefly apple- 
eral tendency neab. 
for the foliage on plants affected with such diseases to shrivel 
and to hang on the stem foratime. One of the best illus- 
trations of this type of disease is the pear-blight. Sometimes 
the plant gives rise to abnormal growths, as in the “ willow 
shoots”? of peaches affected with yellows (Fig. 215). 
Another class of diseases are the root-galls. They are of 
various kinds. The root-gall of raspberries, crown-gall of 
peaches, apples, and other trees, is the most popularly recognized 
of this class of troubles (Fig. 216). It has long been known as 
a disease of nursery stock. Many states have laws against 
the sale of trees showing this disease. Its cause was unknown, 
until in 1907 Smith and Townsend, of the Bureau of Plant 
Industry, United States Department of Agriculture, undertook 
an investigation. They proved that it is a bacterial disease 
(caused by Bacterium tumefaciens); but just how the bacteria 
gain entrance to the root is not known. The same bacterium 
may cause galls on the stems of other plants, as, for example, 
on certain of the daisies. The “hairy-root”’ of apples, and 
