96 



THE FUNGI WHICH CAUSE PIANT DISEASE 



radish, and others of minor economic importance. It is cos- 

 mopolitan in distribution. 



The fungus covers any green part of the host with a dense white 

 growth, often causing hypertrophy especially in oospore forma- 

 tion; conidiophores 200-300 x 10-12 m, bushy branched, stout, 

 deliquescent, with 5-8 main branches, each from 3-7 times 

 branched, ultimate branchlets slender, more or less curved, 

 usually arising at acute angles, about 12-15 x 2-3 m; conidia 

 broadly eUiptic, blimtish, often becommg globose, about 12-22 x 

 24-27 II, hyaline or very light; oospore 

 globose, yellow-brown, 26-45 ju, epispore 

 smooth or wrinkled; oogonium thick, color- 

 less. 



P. effusa (Grev.) Rab. causes a serious 

 disease of spinach.*"* It also occurs on a 

 wide range of weeds of the Chenopodiaceae. 

 The species was formerly made to include 

 all the effusae forms of the genus so that 

 literature aboimds with references to it on 

 Viola, Plantago, Polygonum, etc. 



Hypophyllous, causing yellowish or brown- 

 ish discolorations, the mass of conidiophores 

 of a violet cast; conidiophores 150-400 x 

 7-9 Mj much branched, the ultimate branches 

 at right angles, usually recurved, 8-15 x 

 3-4 n; conidia ellipsoid to globose 17-18 x 

 22-24 n, violet or smoky; oospores globose, 

 30-40 II, epispore Ught brown, more or 

 less regularly wrinkled; oogonium thin, 

 brown. 



P. schleideni Ung.*"^ was first described as a Botrytis in 1841. 

 It was noted in America in 1872 by Taylor, '"^ later by Trelease ^"^ 

 and by many others.*"* A very complete description was given 

 by Whetzel *"' in 1904 under the name P. schleideniana. 



The conidia in mass present a purplish tint. The conidio- 

 phores usually emerge singly through the stomata. The slender, 

 branched haustoria aboimd in the parasitized part often with 

 their ends wrapped around the nuclei. In water the conidia 



Fig. 66.— p. effusa on 

 spinach. After Hal- 

 sted. 



