1 84 



FUNGOUS DISEASES OF PLANTS 



general characteristics of the abnormalities and special peculiarities 

 of the fungus may be mentioned. The mycelium attacks the fruit 

 buds and causes remarkable hypertrophies in the developing 

 ovaries. The mesocarpic tissue is invaded, whereby it is stimu- 

 lated to the production of an abundant spongy growth and the 

 whole form of the plum is enlarged and distorted. Apparently the 

 connection of the stone with its usual source of nutrition is broken 

 up and no stone is developed, or else only a rudimentary structure. 

 It is claimed that the mycelium is perennial, that here infection 

 results by the growth of this mycelium into the young shoots and 



Fig. 68. Witches' Broom on Cherry, produced by Exoascus 

 (Photograph by F. C. Stewart) 



ovaries in the spring. This point requires further investigation. 

 As in the case of the peach leaf curl fungus, the ascogenous cells 

 are developed beneath the cuticle and the elongating asci rupture 

 the latter. The asci are densely crowded together and do not all 

 mature at the same time. In general, the asci are 30-60 x 7-1 2 fi. 

 Robinson notes a certain dimorphism in the asci, slender ones 

 measuring 43-60 x 5.5—7, and stout forms 27-35 X 9-12/4. In 

 several instances I have attempted to inoculate young plums with 

 spore-bearing material received from farther south, but such experi- 

 ments have invariably failed. In general, a study of infection phe- 

 nomena in the Exoascaceas would seem to be of much interest. 



