202 'ON THE PERONOSPOR^. 



P. ViCE^. Pea Peronospora. 



Fertile threads densely coespitose, erect, equal, six to eight 

 times dichotomous ; ultimate rumuli shortly subulate, acute ; acro- 

 spores ellipsoid, very obtuse at apex ; membrane violaceous. The 

 under-surface of the leaves of peas and also of tares is liable to 

 attack from this fungus, and in 1846 it appeared amongst vetches 

 in some districts to such an extent as at one time to threaten the 

 destruction of the crops ; but a succession of dry weather at once 

 abridged its power and limited its mischief Mouldy vetches and 

 mouldy peas, evils well known to the agriculturist in damp seasons? 

 are due to this fungus. 



P. Trifolium. Described by De Bary, attacks trefoil and 

 allied plants, and was found by Cooke plentifully on lucerne in 

 some localities. 



P. NivEA. Parsnip Peronospora. 



, Threads of mycelium stout, often torulose ; suckers numerous, 

 vesicular, obovate ; fertile threads fasciculate, dwarfish, tapering, 

 or subulate, or once or twice shortly bifurcate, rarely trifurcate, 

 with one to four horizontal branches near the summit ; once, twice, 

 or three times bifurcate ; acrospores subglobose or ovoid, with an 

 obtuse papilla at the apex. 



The plants infested with this parasite are first attacked in the 

 leaves, but afterwards the roots become spotted and diseased, in a 

 similar manner to the potato tubers attacked by the potato fungus. 

 It is found on many umbelliferous plants ; hence the name given 

 to it by De Bary of P. iimhdliferariim. Its attacks on the pars- 

 nips are, however, most to be deplored, from an economic point of 

 view. 



P. Effusa. Spinach Peronospora. 



Fertile threads fasciculate, short, thick, two to six times dicho- 

 tomous above \ acrospores broadly ellipsoid, membrane with a 

 violaceous tint. Oogonia irregular and variable in size. This 

 fungus affects the under-surface of the leaves of spinach, goose- 

 foot, knot-grass, etc. It forms effused spots two to six lines broad, 

 generally rendering the leaf yellow. Beds of spinach are some- 

 times utterly destroyed by this fungus. It was figured by Sowerby 

 in his British Fungi some fifty years ago. 



