34° 



FUNGOUS DISEASES OF PLANTS 



very pronounced shot-hole effect. Varieties of Primus americana 

 are frequently free from this fungus. 



Where a species or variety is subject to shot-hole diseases a shot- 

 hole effect may also be produced upon the leaves by spraying with 

 any substance injurious to the leaf. When the 

 leaves are so severely injured that the spots coa- 

 lesce, the large irregular pieces may fall out in 

 the same manner as just indicated. In any 

 case the effects of shot-hole troubles on the leaf 

 are frequently very severe, so that practically 

 complete defoliation of the trees may take place 

 by midsummer. 



The fungus. In many cases the development 



of the acervuli of the fungus is not evident before 



the diseased areas have fallen away, but varieties 



in which the injured areas are persistent exhibit 



the fruiting pustules in great quantity. In such 



cases the spores may be seen to issue from the 



acervulus in tendril-like masses which are quickly 



spread out over the surface by dew and other 



agencies, appearing at first as a pale or ashen 



coating, becoming darker after a few days. The conidiophoric 



layer is often extensive and closely beset with the minute conidio- 



phores (Fig. 164). The spores are curved and measure ordinarily 



Fig. 163. Culture 

 of Cylindrospo- 



RWM PaDI 



Fig. 164. Cylindrosporium Pad/ 

 a, section of acervulus ; <5, conidia, some germinating 



