88 



THE FUNGI WHICH CAUSE PLANT DISEASE 



conidia washed into the soil by rain; possibly sometimes by the 

 mycelium migrating by way of the stem. 



The fimgus was extensively studied by Jones in pure culture and 

 a decided difference in luxuriance of growth was observed on blocks 

 cut from different varieties of potatoes, Fig. 57. 



The mode of hibernation is not thoroughly known but undoubt- 

 edly hibernation occurs in part in live mycelium in infected 

 tubers.™ The conidia are short-lived, especially when dry. 



P. omnivora de Bary. Cbnidiophores simple or branched; 

 conidia ovoid or lemon-shaped, 50-60 or even 90 x 35-40 n, ger- 

 minating by as many as fifty zoospores; oospores smoothish or 

 wrinkled, light-brown, transparent, 24—30 /i. This species which 

 includes forms previously described as P. cactorimi (Lebert & 

 Cohn) Schr., P. fagi Hartig, and P. sempervivi Schenk is foimd 

 upon seedlings of some fifteen families ranging from Pinacse to the 

 higher Angiosperins. It is of considerable economic importance 

 in Europe especially in the seed beds of the forester. Recently it 



has been found on ginseng in 

 Japan and the United States.'^ 

 The same fungus is credited with 

 destructive rotting of apples ^^ 

 and pears'' in Europe and with 

 causing two wide-spread tropical 

 diseases, the cocoa pod rot and a 

 palm disease. From the studies 

 of de Bary '* and from the nature 

 of the more recent outbreaks cred- 

 ited to this fungus it appears that 

 P. omnivora is a composite species 

 which will eventually be segre- 

 gated. Indeed segregation has 

 already been begun. Coleman" has described the pahn in- 

 fecting fungus of India as P. omnivora var. arecse while Maub- 

 lanc™ has gone further and described the cocoa disease as P. 

 faberi. See also "• '»• 



P. syringae recently described by Klebahn is a closely related 

 species, which is very destructive in the propagating beds of the 

 hlac m Germany. 



Fio. 60. — Formation of swarm-spore 

 of Phytophthora. After SmiUi. 



