68 Journal of Agricultural Research voi. xxii.no. 2 



The writer, working with two strains of Fusarium trichoihecioides 

 in Petri dishes, found 25° C. to be the optimum temperature, and with 

 one strain he was able to obtain sHght growth, 7 mm. in diameter, at the 

 end of one week at 30° C. At 5° C. germination took place and there 

 was slight growth. 



The optimum temperature for Fusarium radicicola was 30° C, the 

 same as for F. oxyspornm. The minimum was at 5° C, where a very 

 slight growth was produced in 10 days. At 35° C. the growth was greater 

 than with F. oxysporum, although the rate of growth was slower. Edson 

 and Shapovalov (5) report a similar optimimi temperature, with germi- 

 nation but no growth at 5° C. They found that at 39° C. a transforma- 

 tion from normal spores to chlamydospores took place. 



In general it can be said that at 25° C. the growth for all three species 

 is nearly equal, Fusarium oxysporum, and F. radicicola increasing in 

 growth up to 30° C. and F. iriclwihecioides decreasing. The minimum 

 temperature for F. oxysporum is higher than for the other two, and in 

 general F. trichoihecioides appears to be more tolerant of the lower tem- 

 peratures than the others. 



Preliminary experiments, using liquid media and determining the 

 growth by dry weights, have been conducted with a number of strains of 

 these three species. While on certain media the results have in general 

 corroborated the foregoing cardinal points for growth, they indicated that 

 these cardinal points may vary with the medium used. For instance, 

 with an nutrient solution made up of ammonium nitrate (NH^NOj), 

 potassium phosphate (KHoPOJ, magnesium sulphate (MgSOJ, ferric 

 chlorid (FeCls), and sucrose, the results compared well with those ob- 

 tained on agar in Petri dishes. With a nutrient solution made up similarly 

 to the potato extract medium used by Link (9), the total growth at the 

 higher temperatures was considerably less than the growth obtained in 

 the first nutrient solution, while at the lower temperatures the growth 

 was much greater. The optimum temperature for growth of Fusarium 

 trichoihecioides in the first nutrient solution was 25° C, with no growth tak- 

 ing place at 5° C. With Link's potato-extract medium the optimum lay 

 between 15° and 20° C, and there was weighable growth at 5° C. These 

 results would possibly account for the considerable discrepancy between 

 the results obtained by Link (9) with liquid media and those obtained by 

 the writer and by Edson and Shapovalov (5) with agar cultures. 



EXPERIMENTAL INFECTION OF TUBERS 



The cultures used in the following experiments, with their origin, are 

 listed below. In practically all cases the various strains of the same 

 species behaved alike. Several other strains of Fusarium oxysporum, iso- 

 lated by the writer, were also used in the experiments in addition to the 

 ones listed below. 



