62 A Monograph of the Erysiphaceae 



Magnus' experiment is valuable in showing that the conidiaof 

 5. htumili will germinate and produce a conidia-bearing mycelium 

 when shown on a house plant upon which (it is almost safe to 

 say) the ascigerous stage never occurs in nature. Unfortunately, 

 the experiment was not continued long enough to ascertain 

 whether perithecia w^ould be produced. It is possible that the 

 fungus would not have been capable of producing these on the 

 unusual host-plant. This supposition would explain the fact of 

 5. hionuli never having been recorded on Taraxaaim, althoucrh in 

 nature its conidia may frequently germinate upon this extremely 

 common plant. On the other hand, it is possible that the conidia 

 of S. humidi might have ultimately produced upon the Taraxaaun 

 the large-celled perithecia which we at present consider distinctive 

 of the var. ///%/;/m (which is very common on Taraxacum); in 

 other words, it is possible that the difference in size of the cells 

 of the perithecial wall is due to the effect of certain different 

 host-plants on the same species of a parasitic fungus. We know 

 that in some forms of certain species of the Erysiphaceae, char- 

 acters to which even specific value has been given are directly 

 correlated with the occurrence of the fungus on a certain \vo?,t- 

 ^\^nl—'' Microsphacra pnlchra'' o\\ Coriius altcrnifolia affords a 

 good instance. In this plant which is now allowed by all mycol- 

 ogists to be only a form of M. ahii, a characteristic and more 

 elaborate branching of the apex of the appendages is connected 

 with the occurrence on Corniis ; in the case of S. humuli a similar 

 correlation may exist between the size of the cells, of the perithe- 

 cium and the host-plant. 



S. huinuli, in addition to causing the hop- disease, sometimes 

 seriously attacks cultivated strawberries. In America the disease 

 is known as the " strawberry mildew," and is stated (8) to attack 

 the fruit (both ripe and unripe) and fruit-stalks, as well as the 

 leaves of strawberries. The fungicide recommended is a mixture 

 composed of a quarter of an ounce of sulphide of potassium in a 

 gallon of water; or, according to Humphrey (169), a preparation 

 made as follows : one ounce of carbonate of copper, mixed with 

 five ounces of carbonate of ammonia, and dissolved in a quart of 

 hot water ; when dissolved, sixteen gallons of water are added. 



In the Journal of the Board of Agriculture for 1898 (183) a 



