390 The Principles of Fruit-growing. 
leaf, or even the entire plant, or a large part of 
it, shows a general weakening and disease, as_ if 
there were some cutting off of the accustomed 
source of nourishment. Such diseases are very likely 
to be seen in a general yellowing and death of the 
leaf, in the dying of the leaf along the main veins 
and around the edges, showing that the difficulty is 
one which affects the entire leaf, and not any par- 
ticular part of it. In general, there is a tendency 
for the foliage in plants so attacked to wither up 
and hang on the tree for a time. The peach yel- 
lows and pear blight are diseases of this kind. 
There are no specific treatments for troubles of this 
sort. They must be approached by what physicians 
call prophylaxis,—that is, by methods of sanitation 
and prevention. The diseased plants or parts are 
eut away and burned. All those conditions which 
seem to favor the development of the disease are 
removed. Varieties which are particularly susceptible 
are discarded. Careful management in matters of 
this sort is often much more important than any 
attempt at specific treatment. 
Before taking up a discussion of spraying, it 
may be said that the best treatment for borers and 
similar pests is to watch the plants carefully, and 
to dig the insects out at least twice every year. 
In grounds which are kept in clean tillage, such 
insects are rarely as troublesome as they are in 
neglected areas. This is both because the insects 
find places of lodgment in neglected orchards, and 
because the fruit-grower is so seldom present that 
