m] ERYSIPHALES 8l 



prevalent in Europe in 1845-6, but perithecia were not observed till 47 years 

 later, when they appeared during two successive seasons in various localities 

 in France, and the fungus was identified with the American vine mildew, 

 Uncinula necator, in which perithecia are common. The unusual production 

 of the perithecial stage was attributed to the sudden alternation of high and 

 low temperatures which characterized the seasons in question. The survival 

 of the fungus in the absence of ascospores has been attributed to the persist- 

 ence of the mycelium, and also to the development of conidia capable of 

 passing the winter in the resting state. Fortunately the disease is readily 

 kept in check by the application of appropriate sprays. 



A very similar case is that of the oak mildew. In or about 1904, oak 

 scrub in England and many parts of Europe became infected with the 

 conidial form O. Quercinmn; in 191 1 perithecia were found in France, and 

 the parasite was identified with the common American form Microsphaera 

 Alni which frequently occurs on oaks in the United States. Here again 

 exceptional seasonal conditions appear to have been necessary for the 

 formation of the perithecial stage. 



In a different manner climate has affected the development of the goose- 

 berry mildew, Sphaerotheca mors-uvae, which was introduced into Europe 

 from America about 1900. Very numerous perithecia are developed but a 

 considerable proportion either fail to mature or fail to survive the winter. 

 Infection of the young shoots in the spring appears to depend on the earliest 

 formed perithecia, which alone have had time to mature and lodge in the 

 bark or between the bud scales of the host. For this reason prevention is 

 here much more difficult than in the case of the vine mildew, since the 

 mature perithecia are very difficult to kill and the spread of the disease 

 must be combated by the removal of infected shoots in the autumn, or by 

 appropriate methods of cultivation. 



The perithecia of the Erysiphaceae appear in the late summer or autumn ; 

 'they are spherical or subglobose, 50 — ■300/i(o'05 to 0"3 mm.) in diameter 

 and furnished with simple or branched appendages; they are fixed in 

 position by means of a secondary mycelium. When quite young the peri- 

 thecia appear white and glistening like the vegetative mycelium ; later they 

 become clear yellow and finally brown in colour. Their development is by 

 no means simultaneous, so that a considerable range of stages can often be 

 ^een within the field of an ordinary hand lens. 



During development the wall of the perithecium is differentiated into 

 inner and outer la3'ers (fig. 39). The inner layer is several cells thick ; its 

 cells are rich in cytoplasm with thin, apparently unmodified walls; it is in 

 contact with the developing asci, about which it forms a packing, and to 

 which it supplies food material. Outside this layer is a strengthening and 

 protective zone of several series of cells with scanty contents, the walls of 



G.-v. 6 



