84 



THE FUNGI WHICH CAUSE PLANT DISEASE 



timate branchlets arise radially; 

 germ tube produced from the apex 



of the conidia 7. Bremia, p. 95. 



Conidiophores without subapical en- 

 largements; conidia germinating 

 from the side 8. Peronospora, p. 95. 



Phytophthora de Bary (p. 83) 



This genus is of especial interest on account of its one exceed- 

 ingly destructive representative, P. infestans, which occupies an 

 historic position in phytopathology as one of the earliest of para- 

 sitic fungi to receive study in any way complete or adequate; 

 study moreover which did much to turn attention and interest 

 toward plant pathology. 



A distinctive character is that the conidiophores have irregular 

 thickenings below the apparently lateral conidia. The conidio- 

 phore is at first simple and bears a single apical conidium, after 

 the production of which a lateral branch arises below the conidium 

 and grows on in such a way as to give the first conidium a lateral 

 appearance. This process is, in some species, repeated until a 

 large scorpioid cyme is produced. The genus 

 contains seven or eight species, all parasitic. 

 The mycelium is much branched, non-septate, 

 hyaline; the conidiophores arise singly or in 

 groups from the stomata, or break through 

 the epidermis; conidia oval, papillate; zoo- 

 spores oval, biciliate, escaping by rupture of 

 the papilla; oospores, when present, with the 

 epispore more or less ridged. 



P. phaseoli Thax.^i Mycelium well de- 

 veloped, intracellular; conidiophores single or 

 in clusters from the stomata, simple or 

 branched below, apparently simple above 

 but really one to many times cymosely 

 branched; conidia oval or elliptic, papillate, 35-50 x 20-24 fi; 

 germination by about fifteen zoospores. Oogonia in the seed 

 coats or cotyledons of seeds, rarely in the pods, thin walled, 

 sUghtly folded; subspherical 23-28 n; oospores spherical or 



FiQ. 56. — Structural de- 

 tails of P. phaseoli. 

 After Thaxter. 



