of fluctuations to 40° F. With the program of two constant temperatures, 

 growth increased with an increase in the range of fluctuation to 10° F. 

 and decreased with an increase in the range of fluctuations to 20 or 40° F. 

 Growth with the continually changing temperature program was greater 

 than growth with the program of two constant temperatures, at all ranges 

 of fluctuations tested. 



The increased growth in the 65-75° F. regime suggests that the rela- 

 tionship between growth and temperature is not linear: that the increase 

 in growth with an increase in temperature from 70 to 75° F. is greater 

 than the decrease in growth with a decrease in temperature from 70 

 to 65° F. 



At the upper and lower temperature extremes of the 60 to 80° F. and 

 50 to 90° F. regimes, growth was apparently slower than at temperatures 

 nearer to 70° F. Hence, growth with the continually changing tempera- 

 ture program was greater because the environmental temperature was at 

 the extreme temperatures only a very small part of the 24-hour period, 

 whereas with the programs with two constant temperatures the tempera- 

 ture was at each extreme for 12 hours. Therefore, temperature programs 

 that approximated the temperature variation in the natural environment 

 caused the greatest changes in growth and probably most closely predict 

 the response of the isolates studied to temperature in their natural en- 

 vironment. 



Growth varied significantly among some of the isolates of F. solani: 

 from 40.7 mm. for F5P2, a canker-causing isolate, to 44.7 mm. for F4S, 

 the isolate from soil. However, growth of two of the canker-causing 

 isolates was not significantly different from growth of the soil isolate. 



The variation in growth between isolates of F. solani is probably due 

 to genetic variation within the species and is not related to the source 

 of the isolate. Even thpugh the soil isolate grew significantly more than 

 three of the canker-causing isolates, this relationship will have to be 

 tested more thoroughly with many isolates before any conclusions can 

 be made. 



3 



