266 



FUNGOUS DISEASES OF PLANTS 



The relation of host and fungus. These fungi commonly 

 affect fruit and leaves, but they may also be found upon leaf 

 stalks, flowers, and twigs. Upon the leaves (Fig. 118) in each 

 case the spots are more abundant, as a rule, upon the lower 

 surface. Where the fungus is made evident by an olivaceous, 

 velvety, superficial growth, or when the disease is very abundant, 

 both surfaces of the leaf may be covered and considerable curling 



FIG. 118. APPLE SCAB ON LEAVES: DIFFERENT TYPES OF INFECTION 



may result. Upon the fruit there are at first small, circular, oli- 

 vaceous spots, especially upon the pear, but as a rule the appear- 

 ance changes as the fungus spreads, the epidermis is killed, and 

 the familiar scabby spots are produced. At times practically the 

 whole fruit may show indications of the fungous growth, and a 

 general puckering of the tissues may result in an abnormal form 

 of the fruit. Some varieties of pear may develop cracks or 

 fissures extending halfway to the core. Fig. 116 shows a severe 

 attack of scab on Flemish Beauty. 



