ENVIRONMENTAL FACTORS 69 



Strawberry mildew is also far more abundant in shaded localities. 

 It is a matter of common observation that while cucumbers fre- 

 quently mildew under greenhouse conditions, yet in the open the 

 cucumber mildew is very seldom observed upon this host, at least 

 in the eastern and central United States. It is claimed that cer- 

 tain greenhouse plants are more subject to the attack of the com- 

 mon gray mold, Botrytis, when partially etiolated, and De Bary, it 

 seems, was able to predispose Petunia to the attack of Botrytis 

 through etiolation. 



It is apparent that in so far as screen mechanisms largely pre- 

 vent the formation of dew, it is probably in large part through 

 this change in the moisture relation that they are important. 

 'There may also result a number of direct effects of light, for 

 in the case of strongly etiolated or yellowed and attenuated 

 plants, bud and stem diseases seem frequently to be more com- 

 mon. Very little experimental study has been bestowed upon 

 these relations. 



III. SPECIAL RELATIONS TO ENVIRONMENTAL FACTORS 



Temperature. Very little work has been done which bears 

 more particularly upon the relation of parasitic fungi to various 

 conditions of environment than to fungi in general. The results 

 of the work available, however, will be of assistance to the stu- 

 dent and investigator in using his culture work to the greatest 

 advantage. 



The optimum. The optimum temperature for growth of a 

 particular fungus in culture may not be the optimum tempera- 

 ture for spore germination or for spore formation. It is difficult 

 for observers to agree precisely in giving what is termed the 

 optimum temperature for any fungus, since one observer may 

 emphasize the total growth (dry weight), another the abundance 

 of spore production, etc. The optimum temperature as given 

 for the growth of various species of bacteria usually refers to 

 the temperature at which the extent of the colony is greatest. 

 Wiesner has studied in detail the relation to temperature, of 

 (a) germination, (b) visible mycelium development, and (c) spore 

 formation in the saprophytic fungus Penicillium glaucum. In 

 the absence of data equally as good for parasitic fungi, the 



