SCAB 165 



makes it dangerous to the apple and we have the apples on the 

 tree "russeted" and the leaves of the tree damaged so that they 

 turn yellow and fall. 



Action of a Fungicide. — ^Most fungicides become effective 

 against the fungus by entering its cells and destroying its 

 tissues. For example, the spore of the fungus becomes lodged 

 upon the leaf of the host plant and, the conditions of heat and 

 moisture being favorable, it germinates much as a grain of 

 wheat might do, sending out a little germ tube which grows about 

 over the surface of the leaf and finally enters the tissues, either 

 by way of one of the breathing pores or by actually working its 

 way through the tissues. Now if the germ tube is able to do all 

 this without encountering any injurious substance, it establishes 

 itself within the host and goes on thriving. But if the tree 

 has been properly sprayed, then the little germ tube in its 

 wandering prior to entering the host comes in contact with 

 some of the fungicide, absorbs it into its tissues and is thereby 

 destroyed. 



This is the whole story in a few words and the aim of the 

 man who sprays should be to do his work so thoroughly that 

 no wandering fungus can escape coming in contact with a 

 particle of the fungicide used, whether this be lime-sulfur or 

 Bordeaux mixture or plain copper sulfate. 



SPECIFIC DISEASES 



Now let us consider a few of the more important specific 

 diseases. They may perhaps be considered in two sections, those 

 attacking the pome fruits and those attacking the stone fruits, 

 because it so often happens that a particular disease attacks 

 both the apple and the pear, for example, or the peach and 

 plum. 



DISEASES OF POME FRUITS 



Scab. — This list may be very appropriately headed by the 

 scab or black-spot, which stands in about the same relation to 

 apple diseases as the codling moth does to apple insects. Slightly 

 different but closely allied forms attack the apple and pear. It 

 attacks fruit (Fig. 71), leaves and twigs. On the fruit it 



