276 



FUNGOUS DISEASES OF PLANTS 



produced. Some observers have recorded extreme sizes, 6-40 x 

 l\-jp. More frequently, however (Von Schrenk), they are 12- 

 16 x 4-6/*. The conidia germinate readily, and upon germination 

 almost invariably become septate. Under unfavorable conditions 

 a germ tube may develop at its tip a brown resting cell termed a 

 secondary conidium or appressorium. It is believed that the germ 

 tube may obtain entrance to the fruit through the uninjured skin 

 of the apple, and certainly artificial infection may result without 

 noticeable surface injury. Nevertheless, infection can be hastened 

 by injuring the surface, and it is possible that some slight injury or 

 abrasion may be essential to penetration, although the belief is cur- 

 rent that entrance may be effected through the stomates of the fruit. 



Fig. 124. Glomerella rufomaculans: Conidial and Ascigerous Stages 



This imperfect form was for a long time the only known fruiting 

 stage of the fungus. It was referred to the genus Glceosporium 

 and was generally known as Glceosporium fructigenum Berk. 



The perithecial stage of this fungus, found by Clinton in 1902, 

 may be readily developed in artificial culture, though Clinton has 

 also reported having found it frequently upon the fruit. In cul- 

 tures it may be developed within two weeks on various nutrient 

 media, while in nature it develops apparently only the following 

 spring upon fruit which has been upon the ground throughout the 

 winter. In artificial culture the perfect stage develops promptly 

 and vigorously upon apple agar corn meal. The mycelium first 

 forms small black nodules which become stromatic cushions about 

 one fourth inch in diameter. Within this stroma one or many peri- 

 thecia might be developed. The various stages in the development 



