THE FUxNGI WHICH CAUSE PLAi\T DISEASE 97 



germinate directly to form an infective tube (Fig. 67) which 

 grows into the stomata. According to Whetzel oonidia retain 

 their germinating power only a few hours. Shipley beUeved them 

 viable for a much longer time.^'^ Fertilization occurs much as in 

 P. parasitica (Fig. 67) and the sexual spores, which abound, 

 serve for hibernation. They may live several years. 



It is found on onion, garlic, etc. (Allium sps.) everywhere, 

 covering leaves with a dense growth; conidiophores, 3-6 times 

 branched, 300r700 x 12-15 ii; branches 2-5, scattered, ultimate 

 branchlets subulate, 15-20 /x, more or less recurved; conidia large, 

 obovate to pyriform, basally papillate, 45-58 x 20-25 /i, the 

 membrane violet; oospore globose, light-brown, about 30 /x, 

 epispore smooth or slightly wrinkled. 



P. sparsa Berk, is parasitic on roses ^ and constitutes a serious 

 pest in Europe, though not so common in America. 



Hypophyllous, with a whitish growth; conidiophores about 

 9 times branched, the ultimate branchlets reflexed; conidia sub- 

 elliptic, pale gray. 



P. trifoliorum de Bary. Hypophyllous, forming a dense grayish 

 or dirty-white growth over the host; conidiophores slender, 360- 

 600 X 9-11 n, 6-8 times branched at acute angles, the primary 

 branches rather erect, the secondary more spreading, flexuose, more 

 or less recurved, ultimate branchlets at right or obtuse angles, 

 straight, subiilate, 7-12 x 7-3 n; conidia globose to broadly elliptic, 

 15-20 X 18-36 Ai, violet; oospores globose, 24-30 /», epispore light 

 brown, smooth. 



It causes serious loss to clover in Europe. Species of related 

 genera also suffer. Recently it has assumed a r61e of importance 

 in America by its attacks upon Alfalfa ^^° on which it occurs from 

 New York to California. 



It differs from P. viciae in the branching of the conidiophores, 

 the lighter color of the spot and fungus, and the smooth oospores. 

 P. viciae Berk. Hypophyllous or caulicolous, covering the host 

 with a grayish-violet growth, epiphyllous discolorations yellowish 

 or inconspicuous; conidiophores fasciculate, 300-700 x 9-11 ii, 

 5-8 times branched, the main branches arising at acute angles, 

 erect, the ultimate subequal, slightly flexuose, arising at right or 

 obtuse angles, the lateral recurved, 10-17 x 2-3 n; conidia elliptic 



