164 DISEASES OF FRUIT TREES 



as the apple tree does, out of air and water and various other 

 ingredients, but it allows the tree to do this and then it comes 

 in and steals the manufactured foods. It is a robber pure and 

 simple. And in order that it may absorb these manufactured, 

 foods such as sugar and starch, it has to establish a very intimate 

 contact with the host plant (and a very unwilling host it is). 



Sometimes it grows on the surface with very slight attach- 

 ment to the host, as in the case of mildews, and such a disease 

 may be treated by the use of dry sulfur dusted upon the leaves 

 after the fungus has become established, because practically all 

 of the fungus is spread out there open to attack. 



Again, the fungus has a much more intimate connection with 

 the host, although still growing on the surface. Such a type is 

 the apple scab. In this case a large part of the fungus is im- 

 bedded in the host, and treatment, after the fungus has become 

 established, is of relatively little value. 



Lastly we have a type of fungus which grows wholly or 

 largely within the host. The black knot of the plum is such a 

 fungus. Here the fungus is entirely safe from attack after it 

 once enters the host and until it emerges in the black knots of 

 the fruiting stage. 



The most rational treatment for all fungous diseases is that 

 which attempts to prevent their ever gaining a foothold on the 

 host, and we are enabled to give our trees this kind of protec- 

 tion because men have discovered certain substances which are 

 harmless to the host plant but which will kill the fungus. Fre- 

 quently the margin of safety is very slight and a substance to be 

 effective against the fungus must be of such a strength or such 

 a composition as to come very near to injuring the host plant. 



Sometimes varying conditions of weather or of the plant 

 cause the fungicide to pass the margin of safety and become 

 injurious to the host as well as to the fungus. Bordeaux mixture 

 is one example of this. As sprayed upon the apple tree it is 

 probably in the form of copper hydroxide, or some similar com- 

 pound, and in this form it does not hurt the apple tree but does 

 destroy the apple scab. But under certain weather conditions 

 the chemical form of this fungicide undergoes a change which 



