THE FUNGI WHICH CAUSE PLANT DISEASE 181 



U. necator (Schw.) Burr.i"'' i"*- ^o^ 



Amphigenous; mycelium subpersistent; perithecia usually epi- 

 phyllous, occasionally hypophyllous or on the inflorescence, 

 more or less scattered, 70-128 ii; cells distinct, rather irregular 

 in shape, 10-20 ii; appendages very variable in number and 

 length, 7-32, rarely up to 40, 1 to 4-times the diameter of the 

 perithecium, septate, thin walled, light or dark amber-brown bas- 

 ally, rarely branched, asci 4-6 rarely up to 9, broadly-ovate or 

 ovate-oblong to subglobose, with or with- 

 out a short stalk, 50-60 x 30-40 n; spores 

 4-7, 18-25 X 10-12 /i. 



Conidial form (=Oidiumtuckeri), coni- 

 diophores short; conidia elliptic, oblong, 

 or obtusely rounded, 2 to 3-catenulate, 

 hyaline, 25-30 x 15-17 n. 



Hosts Vitis, Ampelopsis and Actinidia. 

 One of the worst pests of the family. 



The mycelium is thin walled and spar- 

 ingly septate. The haustoria arise from 

 lobed lateral swellings of the hyphai, 

 penetrate the epidermis with a filamen- 

 tous projection and swell within the host 

 cell to a bladder-like body. The para- 

 sitized cells and later the neighboring pig. iso.— u."necator. ii. 



ones turn brown and die. Perithecium showing/, ap- 



pendages, and a, asci. IV. 

 The conidia germinate readily in moist Group of asci removed 



, J. i: .i /• from perithecium emit- 



air or m water, sending lortn irom one ting «, ascosporea. After 

 to several germ tubes. ^*^' 



The perithecia are found well developed as early as June or 

 July in the United States and are rather evenly scattered over 

 the affected surfaces. Bioletti ^"^ says that a period of warm moist 

 weather which favors luxuriant mycelial growth, followed by sud- 

 den lowering of temperature to about 50° F., favors their most 

 rapid formation. They are at first hyaline, later brown. After 

 their form and walls become definite, usually during winter, the 

 appendages develop as outgrowths from the outer walls. During 

 winter the appendages break off. Galloway^"* failed to secure 

 germination of ascospores earlier than February or March, but 



