GRAPE DISEASES 245 



in an irregular fashion. As described for the canes the outer 

 cells are injured, which accounts for a halting in the growth of 

 affected portions. Sometimes this irregularity of development 

 results in a cracking of the fruit, in which case the berries either 

 become entirely dry and never ripen, or are greatly reduced in 

 size and quality. The fruits are not attacked by the pathogene 

 after they enter the ripening-period. On older, but still im- 

 mature, berries brown spots appear; subsequently mildew 

 becomes perceptible. The peduncles and pedicels also show 

 the mildew. Very commonly on these parts the pathogene 

 makes profuse growth. By harvest time the peduncles are 

 dwarfed and withered. The development of the grayish mildew 

 is followed by the appearance of black perithecial bodies as 

 described for the other susceptible organs. 



Cause of powdery-mildew. 



Sexual fruit-bodies, perithecia, of the causal fungus, Uncinula 

 necator, probably remain on the canes and leaves or in the 

 soil until the return of summer. Within each perithecium 

 twenty-five to fifty ascospores are found. These are liberated 

 from their asci, and are carried to the growing shoots and leaves. 

 The ascospores are not all ejected at the same time ; some may 

 not be discharged until a year later. It is thought that in many 

 cases the fungus hibernates by means of the asexual (conidial) 

 stage ; this opinion is based largely on the scarcity of the peri- 

 thecia in certain regions. 



Whether the fungus begins its spring activities by means of 

 ascospores or conidia, the result is a mildewed spot. The 

 whitish mildew is composed of mycelium and conidiophores. 

 When a spore falls on a leaf it germinates, and a copious develop- 

 ment of mycelium results. Haustoria are sent into the epider- 

 mal cells for nourishment. As a result of this action the cells 

 turn brown, as described under Symptoms. From the mycelium 

 elongated, erect conidiophores extend into the air ; each bears 

 a chain of conidia. The conidia are light and are easily dis- 



