AN OIDIUM MILDEW ON CARNATIONS. 



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AN OIDIUM MILDEW ON CARNATIONS. 



By W. B. Mercer, B.Sc, Adviser in Agricultural Botany, 

 Armstrong College. 



In the early summer of 1914 specimens of mildewed carnations were 

 sent in to Armstrong College from the greenhouses of a garden in the 

 Tyne Valley. The fungus proved on examination to be a member 

 of the Erysiphaceae. 



The outbreak at first threatened to be a serious matter, as it quickly 

 spread to all the houses in the garden, and a number of plants became 

 so infested with the fungus that they had to be burned. It was soon 

 found, however, that the disease could be held in check by means 

 of repeated sprayings. Though it has not yet been stamped out 

 entirely, it has been reduced, by these means, to insignificant pro- 

 portions. The writer, indeed, has recently been able to obtain speci- 

 mens for microscopical examination only after careful and prolonged 

 search. 



The object of this note is to draw attention to the existence of 

 the disease — of which there do not appear to be previous records — 

 and to indicate the means by which it has been controlled in the case 

 under discussion. 



The disease is characterized by the appearance on the leaves and 

 calyx of the plant of patches of white mould, which gradually spread 

 and eventually assume , a yellowish hue. Occasionally the mildew 

 appears upon the petals, but this is rare. It has been observed in 

 greater or less degree upon a number of varieties, but ' Lady 

 Alington,' ' Bridesmaid/ and ' British Triumph ' have on the whole 

 suffered the worst. It has been especially noticeable upon young 

 flower -buds of the last-named variety. 



The fungus, like other members of the family, is confined to the 

 outside of the host plant. Its mycelium forms a weft of threads over 

 the surface of the leaf or sepal, and here and there produces a short 

 branch or appressorium, which applies itself closely to the epidermal 

 wall. From the appressorium a fine threadlike branch, known as 

 a haustorium, bores its way through the thick cuticle with which 

 the outer wall of the epidermis is provided and penetrates into an 

 epidermal cell beneath. 



Inside the epidermal cell it becomes greatly distended, presumably 

 for the sake of increasing the feeding surface. A single epidermal 

 cell frequently contains two or three of these sacculate endings 

 to haustoria, the cell cavity in some cases being thereby almost 

 filled. 



