EXPERIMENT STATION BULLETINS. 



593 



the air and makes less oxygen available, would tend to still further restrict 

 the susceptible area, though relatively wet conditions are requisite for my- 

 celial proliferation. As has been showTi, lack of sufficient moisture in the 

 air in nearly all of the celery gro-wing districts in Michigan where the fungus 

 is know^l keeps it from the aerial parts. Thus, the phj^siological relations 

 of the fungus to these two environmental factors, namely water and air sup- 

 ply, determined the type of disease which is produced, and the part of the 

 host plant which is dangerously subject to attack. 



RELATION TO THE REACTION OF THE MEDIUM. 



Several different media have been used in determining the relation of 

 acidity and alkalinity to the growth of PJioma apiicola. Celery agar, nu- 

 trient broth, and Coons' sjTithetic solutions have been employed in several 

 experiments. 



With the solid media used, a neutral or slightly acid reaction is' most favor- 

 able for growth. The same is true of the liquid media used but the initial 

 reaction for production of the greatest amount of growth varies with the 

 chemical composition of the medium. This can be brought out more clearly 

 by a description of a typical experiment. Flasks of nutrient broth were 

 prepared to range in reaction from pH. 2 to pH. 9.6 by adding N/20 hj^dro- 

 chloric acid or N/20 sodium hydroxide until the desired reaction was ob- 

 tained. Each flask was inoculated with a small bit of mycelium and in- 

 cubated at room temperature. In the following table, results are given for 

 different periods of time on a percentage basis, reckoning the flask showing 

 best growth as 100 per cent. 



TABLE 7. EFFECT OF ACIDITY AND ALKALINITY ON MYCELIAL GROWTH. 



At the beginning, there was a much more rapid growth at the neutral 

 point. As development continued, those flasks having an initial reaction 

 of pH. 6.5, 6.0 and 5.5 respectively replaced the neutral flasks in quantity 

 of mycelium produced. This came about both through a gradual accelera- 

 tion of rate of growth in the acid and a slowing up in the alkaline media. 

 Flasks, neutral and alkahne, had practically ceased growth at the end of 

 four weeks. In those having an acid reaction, the fungus continued a fair 

 rate of growth up to the end of the eighth week. The slowness of growth 

 in the acid flasks at the begimiing was no doubt due to an unfavorable reac- 



