262 ELEMENTS OF AGRICULTURE 



have previously learned, the fungi are small plants that 

 live on our crops. We might say that there are two kinds 

 of weeds that we have to control, those that grow on 

 the ground and those that grow on our crops. For the 

 one kind we cultivate, for the other we spray. Fungi 

 must be killed by a fungicide that hits them. It is just 

 like spraying for wild mustard. We can apply a spray 

 that is so strong as to kill the mustard, but that is not 

 strong enough to harm the oats. If we applied it too strong, 

 the oats would also be killed. So, in use of the Bordeaux 

 mixture, we can use it strong enough to kill the fungi 

 without hurting the tree or crop. Certain trees, as peaches 

 and plums, are so tender that it is very difficult to spray 

 without killing the leaves also. 



Certain weather conditions favor the growth of fungi, 

 just as certain kinds of weather favor the growth of corn, 

 but weather cannot create the one any more than the 

 other Close, damp days, with frequent showers, are 

 favorable for the growth of most fungi. 



Another essential in spraying for fungi is that we spray 

 before they enter the host plant. When a fungus seed 

 (spore) grows and gets inside the plant, it is too late to 

 spray. Spray on the outside of a leaf does not hurt a fungus 

 that is already inside. Ordinarily it is some time after the 

 fungus has gained entrance before the disease is apparent. 

 We must, therefore, know the life history of the particu- 

 lar fungus, know when it is likely to enter the plant, and 

 spray before that time. However, it is sometimes worth 

 while to spray when the first plants show a disease, if 

 we have not started earlier. In this way, some of the crop 

 may often be saved. Spraying is insurance, it should be 



