March 1896.] INJURIOUS INSECIS AND FUNGI. 



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much as, like the potato fungus, belonging to the same genus, it 

 forms a mycelium within the tissues, and lives upon its host plant. 



When swedes are thickly covered with Oidium halsamii 

 their growth is arrested, and they make very little progress 

 afterwards. Generally speaking, the attack begins about the 

 last week of July, and by the third week of August, in favour- 

 able conditions for its development and progress, the leaves 

 are white. Instead of the rapid root-growth usual in the 

 latter part of August and throughout September, the plants 

 come to a standstill, and there is no further gain of weight. 

 Some persons go so far as to say that mildewed swede leaves 

 are injurious to sheep folded on them, causing them to scour ; 

 but this has never been verified, though it is extremely likely. 



Cucumber plants and marrows, both in the open air and ki 

 frames, are subject to this species of Oidium. 



Several complaints of injury caused by this fungus to cucum- 

 bers were made during the last summer by market gardeners 

 who grow them on a very large scale in the open air. Vegetable 

 marrow growers also complained of the " white powder " upon 

 the leaves of these plants. It seemed to them that the unusual 

 heat, and drought accompanying it, made the plants unhealthy 

 and caused this affection. 



Description and Life History. 



Oidium halsamii is, like some other species of fungi, and 

 particularly Oidium Tuckeri, which is destructive to vines, 

 an imperfect fungus whose life history is not fully known. 

 Frank defines Oidium as a genus whose perithecia have not 

 yet been distinguished, and the species of Oidium here 

 described does not develop perithecia. It is not known whence 

 the spores come that first infect the swedes, nor in what 

 form the fungus passes the winter. De Bary says "it was 

 supposed that in Erysiphe the conidiophores were a species 

 of the genus Oidium, and that only the perithecia were 

 assigned to Erysiphe,'' and he adds, *' The researches of Tulasne 

 have led to an understanding of the real condition of things 

 to which he gave the name of pleomorphism, and to him we 

 are chiefly indebted for the distinguishing and naming of the 

 possible forms in the development of a species."* 



At first, the fungus appears as white dust on the lower, as 

 well as the upper, surfaces of the swede leaves. The microscope 

 shows this white dust to be composed of innumerable threads 

 thickly interwoven upon the leaf surfaces. From these mycelial 

 threads sporophores arise vertically, having thickened ends shaped 

 like long narrow barrels (A.). These are conidia, or spores, and 

 if put upon a glass slide damped with water they will germinate 



* Comparative Morphology and Biology of the Fungi, Mycetozoa, and Bacteria, 

 by A. De Bary. 



