132 PHYCOMYCETES. 



may cause considerable damage to the lettuce (Zaetiica sativa), 

 this being especially the case in France. The parasite is most 

 dangerous in forcing-houses during winter or early spring, and 

 spreads rapidly, favoured by the damp atmosphere. The young 

 diseased plants are stunted, and take on a pale colour. Early 

 removal and destruction of diseased plants is to be recommended; 

 also abandonment for lettuce-xjultivation of infected houses or 

 frames. 



In addition to lettuce, this fungus attacks a number of 

 Compositae, e.g. Cineraria, Sonchus, etc. 



Peronospora. 



The mycelium is intercellular in living plants. The haustoria 

 may be simple, button-shaped, or thread-like, or may branch 

 inside the host-cell. The long and much-branched conidiophores 

 produce conidia singly at the ends of their branches. The 

 conidia produce a germ-tube. The oospores are brown-coated 

 and are formed singly in the oogonia ; they germinate in spring. 



Peronospora Schachtii, Fuck.^ is injurious to the inner 

 leaves of sugar beet and mangold {Beta vulgaris), while young 

 seedlings are killed by it. The mycelium . hibernates in the 

 roots ; as yet oospores have not been found. 



P. effusa (G-rev.) This causes injury to spinach (Spinacia) 

 oleracea) and other Chenopodiaceae (Britain and U.S. America). 



P. Schleideni, Ung. Kills the leaves of cultivated and wild 

 species of onion {Allium) (Britain and U.S. America). 



P. dipsaci, Tul. Injures stems and leaves of Dijosacus Sylvester 

 and D. Fullonum. 



P knautiae. Fuck., of Knautia and Scabiosa, is probably identi- 

 cal with last. 



P. viciae (Berk.) (Britain and U.S. America). A dangerous 

 species to many Papilionaceae (especially peas, beans, tares, 

 lentils, etc.), often causing great damage to field crops. In 

 recent years the new fodder-plant Lathyrus sylvestris has been 

 frequently attacked.^ 



P. trifoliorum, De Bary* (Britain and U.S. America). Dis- 

 tinguished from the preceding form by its irregularly marked 



'Kiihn, Botan. Zeitung, 1873. 



^ Zntuchrift f. Pflanzenhrankheiten, ii., p. 225 and 283. 



'Smith, Diseases of Crops, London, 18S4. 



