1909-1910-] Parasitic Moulds and Mildews. 1 97 



lY.— PARASITIC MOULDS AND MILDEWS 



By Mr D. A. BOYD, Corresponding Member. 



{Read Jan. 26, 1910.) 



Although the British species of parasitic moulds and mildews 

 are comparatively few in number, they nevertheless possess 

 considerable economic importance. In many seasons, for 

 example, the devastation occasioned by the fungus of the 

 potato-disease {PhytopMliora infestans, De Bary) causes wide- 

 spread disaster to growers of the tuber ; while various other 

 species attack garden and field plants, so as to render them 

 unsightly or unmarketable. 



Many of these parasitic moulds and mildews bear consider- 

 able resemblance to one another, not only in their common 

 habit of preying upon living plants, but in their producing in 

 greater or less abundance, on the surface of the affected leaves 

 or stems of their host, numerous hyphse or threads upon which 

 the conidia are developed. When occurring plentifully, the 

 hyphse and conidia usually impart to the affected surface a 

 downy, floccose, or mealy appearance. In their life-history, 

 however, these parasites exhibit considerable diversity. Some 

 belong to the Phycomycetes and others to the Hyphomycetes, 

 two great groups which differ materially in their modes of 

 development and reproduction. The former are endophytal 

 parasites, and produce a mycelium which permeates the in- 

 ternal tissues of the host. On account of the destruction of 

 the chlorophyll in the cells, the affected parts generally assume 

 a somewhat blanched or pallid appearance, often accompanied 

 with considerable thickening or distortion of the tissues. 

 Moulds of this class are capable of reproduction by two 

 methods — viz. (first), sexually, by means of oospheres and 

 antheridia formed by the portions of the mycelium which 

 permeate the internal tissues of the host ; and (secondly), 

 asexually, by means of conidia formed upon those portions of 

 the mycelium which pass outward to the external surface of 

 the host. As the result of the fertilisation of each oosphere 

 by an antheridium, an oospore is formed which serves the 



VOL. VI. 



