jaii.i3. I9I9 Morphology of Urediniospores of P. graminis 



69 



Table XIII. — Results showing the effect of temperature on the physiology and mor- 

 phology of urediniospores of Puccinia graminis tritici on wheat 



Temperature. ' 



Ex- 

 peri- 

 ment 



No. 



Daily mean. 



Maxi- 

 mum. 



°F. 



92-3 

 103- 5 

 78.0 

 79-4 

 89.7 

 72.0 

 62.9 



Mini- 

 mum. 



Aver- 

 age for 

 genera- 

 tion. 



°F. 



81.8 



79-5 

 69.7 

 69-3 

 66.4 

 55-8 

 55-8 



Degree of infec-tion. 



Moderate 

 ...do... 

 Heavy... 

 ....do... 

 ....do... 

 ....do... 

 do... 



Result, 



Spore dimensions. 



Size limits. 



23. 04-36. 48X 16. 32-24. 00. 

 23. 04-40. ooX 16. 32-24. 96. 



04 25.92-40.00X16.96-23.36. 



18 

 18 

 16 



18 



Tg \ 22.40-40.32X16.32-23.68. 



Tg 25.60-40.00X16.00-23.04. 



Tg 25.28-40.00X18.24-22.72. 



12 



To I 26.24-40.00X16.96-22.72. 



Modes. 



29.44X19.84 

 30. 72X20. 16 

 32.64X20. i6 

 32.00X19.84 

 33.28X19-84 

 32.64X20.48 

 31.36X19-84 



It was found that the most favorable temperature for shortening 

 the incubation period, hastening the maturity and obtaining a vigorous 

 infection, appeared to be between 66.5° and 70° F. Fromme's tabula- 

 tion {4, p. 507-509) shows that this is in accordance with the results 

 obtained by Wiithrich on P. graminis and by Ward on P. dispersa. This 

 temperature is also the optimum for the production of the largest uredinio- 

 spores. The reason the spores in No. 6, Table XIII, became so large 

 is on account of the high maximum temperature. 



At this temperature (66. 5^-70° F.) rust flecks appeared in from five 

 to seven days and uredinia developed within another day or two. At a 

 higher temperature the development of the uredinia was retarded at the 

 rate of one day for every 10 degrees of rise of temperature, but rust de- 

 veloped at as high temperatures as the host endured, although the size 

 of the spores was considerably decreased. At low temperatures the de- 

 velopment of the uredinia was retarded at the rate of one day for every 

 5 degrees of fall in temperature. Infection resulted at as low tem- 

 peratures as the host could possibly stand. The spores were rather 

 smaU, but the difference was not as great as in the case of high tem- 

 peratures, with moderate temperature as a basis for comparison. 



The uredinia produced under high temperatures were darker in color 

 than those produced under moderate temperatures, while those pro- 

 duced at low temperatures were hghter than those produced at moderate 

 temperatures. The color of the uredinia developed at high temperatures 

 varied from Brussels-brown to argus-brown; at moderate temperature 

 it varied from Sudan-brown or antique-brown to Brussels-brown, while 

 at low temperature from amber-brown to Sanford's-brown. 



