THE FUNGI WHICH CAUSE PLANT DISEASE 91 



solitary or fasciculate, from the stomata of the host, monopodi- 

 ally branched, the branches arising at right angles to the main 

 axis, as do also the secondary branches (at least never appearing 

 truly dichotomous) the ultimate branches apically obtuse; conidia 

 globose to ovoid, hyaline or smoky, germinating by zoospores or 

 the entire protoplasmic mass escaping and then sending out a 

 germ tube; oospore globose yellowish-brown, the epispore va- 

 riously wrinkled sometimes appearing somewhat reticulate; oogo- 

 nium persistent, but free from the oospore. 



P. viticola (B. & C.) B. & d T.,^^' **• ^^- "'- "' first collected in 

 1834 by Schweinitz and regarded as a Botrytis was first published 

 in 1851." 



Hypophyllous, caulicolous, or on young fruits, covering the 

 infected areas with a white downy growth; on the leaves epiphyl- 

 lous discoloration yellowish; on the fruit often causing a brown rot 

 without producing conidia; conidiophores fasciculate, 250-850 x 

 5-8 II, 4-5 times branched, the ultimate branchlets about 8 n long; 

 conidia ovate-elliptic, very variable in size, 9-12 x 12-30 n; 

 oospores 30-35 y,, epispore brown, wrinkled, or almost smooth; 

 oogoniiun thin-walled, hyaline or light yellowish-brown. 



The mycehum is found in all diseased tissues except the xylem. 

 The conidiophores issue from stomata. The conidia germinate 

 readily in water,' producing in about three-fourths of an hour 

 biciliate zoospores. These after fifteen to twenty minutes activity 

 cease motion, round off, become walled, then germinate by a tube. 

 This bores through the epidermis and develops into the internal 

 mycelium. Infection is almost exclusively from the lower side of 

 the leaf.*' Oospores are much more rare than conidia but are often 

 found in autumn, sometimes two himdred to a square millimeter of 

 leaf surface. Though hibernation is doubtless chiefly by oospores 

 it has been shown that the mycelium can perennate in old wood, 

 and even form oospores therein. The fungus is dependent on 

 abundant moisture. 



P. nivea (Ung.) Schr. attacks various species of umbellifers in- 

 cluding the parsnip and carrot. It has been reported in America 

 only from the region of San Francisco. 



P. halstedii (Fari.) B. & d T. 



This form is quite variable and should perhaps be separated 



