342 Journal of Agricultural Research voi. \a, no. w 



measuring the degree of moisture, only approximate data regarding 

 moisture reaction can be given. It was noted in field studies that a 

 higher percentage of infection occurred in the lower and more moist 

 sections of the field than in the higher areas, and that in laboratory 

 infection experiments the fungus develops best when the surface of the 

 tuber is kept moist but not supersaturated. By placing tubers sufficiently 

 near water so that a heavy film of moisture was constantly present, 

 it was found that sporulation was inhibited to a greater degree on the 

 side of the tuber near the water than on the opposite side, which indi- 

 cates that excess moisture may check the growth of the fungus. 



Although the fungus prefers moisture for growth, it can withstand 

 drying without the entire loss of its virility. This was shown by the 

 fact that transfers from agar cultures i6 months old continued to grow, 

 although only a small percentage of the conidia germinated. Notwith- 

 standing the fact that these cultures had been kept at room tempera- 

 ture and were dried to such an extent that simply a dry, brittle mass of 

 media and fungus remained, both viable conidia and mycelium were found. 



REACTION OF THE FUNGUS TO TEMPERATURE 



Conidia in corn meal and oat agar and in water and naturally infected 

 and artificially inoculated potato tubers were used in studies to determine 

 the effect of temperature on 5. atrovirens. In the case of media spore- 

 dilution plates were prepared, the spores being sufficiently far apart so 

 that individual colonies were retained. The same dilution was used on 

 each plate and all were inoculated at temperatures ranging from 2° to 

 31° C. The water cultures were used in making hanging-drop prepara- 

 tions on Van Tiegham cells and in small Petri dishes, the spore suspensions 

 in this case also being made in such manner that some of the spores re- 

 mained on the surface, although germination occurred to a slight extent 

 also beneath the surface. The naturally infected and artificially inocu- 

 lated tubers were placed in pint bottles containing some pebbles and a 

 few cubic centimeters of water, with a piece of cheesecloth extending from 

 the contents of the bottle to its mouth, thus forming a moist chamber. 

 These bottles were incubated in the same way as the media cultures. 



In the eight series of Petri-dish cultures microscopic germination was 

 noted at 3°, 4°, and 5° C, but no macroscopic colonies developed; at 

 temperatures ranging from 6° to 28° macroscopic colonies were obtained, 

 21° to 27° being the optimum for abundance of growth; while at 30° or 

 31° no macroscopic growth was apparent (Pi. XLVII). These tem- 

 perature limits for growth were confirmed by the water cultures, which 

 were used as checks on the media cultures subjected to the highest and 

 the lowest temperatures. In the case of three series of these water cul- 

 tures which were subjected to a temperature of from —5° to —10° C. 

 for four days and then brought to room temperature, 80 per cent of the 



