OOMYCETES 89 



in which they do not project above the substrate but cut off conidia 

 under water (Pethy bridge, 1914). Also as regards the germination of 

 conidia, transitions are very gradual; thus P. Arecae germinates generally 

 with a sac as Pythium palmivorum (Rosenbaum, 1917). In contrast to 

 Pythium, however, all species of Phytophthora so far known are parasitic. 

 P. infestans causes the late blight of potatoes; it goes over to various 

 other Solanaceae and Scrophulariaceae; in damp earth, the conidia 

 remain capable of germination up to 4 weeks (Murphy, 1922.) P. 

 erythroseplica causes a similar disease of potatoes in which the infected 

 tissue of the tubers becomes salmon color after a half-hour and in a few 

 hours assumes a deep purple brown color. The group of P. omnivora 

 occurs in several strains, rather obscure in choice of host but morphologi- 

 cally distinct in pure culture, which have earlier been given special names, 

 as P. Syringae, P. Cactorum and P. Fagi (Himmelbaur, 1911). It appears 

 on all kinds of hosts and causes particularly rots, in cacti, pear, buds of 

 lilac, beech seedlings, etc. P. Faberi causes a bast rot of cacao, Hevea, 

 etc., P. Phaesoli, an abscission of the pods of Phaseolus lunatus, and 

 Phytophthora Arecae a heart rot of Areca palms. 



Both Pythiomorpha gonapodioides and Pythiogeton utriforme have 

 been found exclusively on rotting fruits and other plant parts in water. 



The Albugineae are characterized by short sterigma-like conidiophores 

 which cut off successively chains of conidia; they are generally laid down 

 in a thick sorus under the epidermis of the host and are liberated by its 

 rupture. Up to the present only one parasitic genus is known, Albugo, 

 whose species cause hypertrophies and malformations; thus A. Candida 

 on crucifers (Melhus, 1911), A. Tragopogonis on composites, A. Ipo- 

 moae-Panduranae on sweet potatoes A. Bliti on Amarantaceae and A. 

 Portulacce on Portulaca. 



The Peronosporeae, finally, are distinguished by the develop- 

 ment of the sporiferous hyphae into true conidiophores; this takes place 

 so gradually that systematic classification, which here rests only on the 

 sporiferous hyphae, can only be certain of the most marked types and 

 must rely on subjective measurements on many transitional forms (Wil- 

 son, 1914). The only species of Basidiophora, B. entospora, lives on 

 leaves of composites. Some species of Sclerospora are dreaded in India, 

 the Dutch East Indies and the Philippines, as a cause of leaf disease of 

 maize. Another species, S. graminicola, appears on various grasses in 

 Europe and North America. Plasmopara is economically important 

 as a cause of plant disease. Its conidiophores and conidia vary markedly 

 in form and size from one host genus to another and sometimes even in 

 the same genus (Wartenweiler 1918). In P. nivea and P. viticola, 

 the downy mildew of grapes, the conidia germinate generally with 

 zoospores; in P. pygmaea, on various Ranunculaceae, in P. alpina on 

 Thalictrum alpinum and P. densa on Scrophulariaceae, generally with 



