PHYCOMYCETES 173 



Phytophthora cactorum (Leb. & Cohn) Schroet. This species 

 of Phytophthora, if it is a single species, shows as great a range 

 of host plants as the common damping-off fungus, Pythium de 

 Baryamnn. It was first described as a disease of certain Cactaceae 

 grown under greenhouse conditions, also of succulent species be- 

 longing to the genus Sempervivum. Hartig 1 and other forest 

 botanists have found it to be one of the most disastrous fungi 

 known upon seedlings of such trees as the pine (Pinus), beech 

 (Fagus), and many others. Additional hosts among herbaceous 

 plants have also been well established, and the fungus may be 

 regarded as unusually widespread and important. The conidia 

 (zoosporangia) are unusually large, often averaging 60 /u- in di- 

 ameter. Upon germination numerous zoospores are produced. 

 Oospores are present in this species. These are small, often 

 24-30/4. 



1 Hartig, R. Der Buchenkeimlingspilz, Phytophthora Fagi, m. Unters. a. d. 

 forstbot. Institut, Miinchen. 1 : 33-57. pi- 3- 1880. 



