66 Journal of Agricultural Research voi.xvi.no. 2 



Johnson (<5, p. 8) gives the urediniospore dimensions of timothy-rust 

 as 18 to 27 ju in length and 15 to 19 m in width. Stakman and Jensen 

 {12, p. 214) found that on timothy the spores ranged from 17 to 31 /i in 

 length and from 14.5 to 23 n in width, the modes falling at about 26 and 

 18 /!• On barley they found them to be smaller than those produced on 

 any other host, ranging from 18.5 to 28.3 /x in length and from 13 to 20 ju 

 in width, with modes at about 23 and 17 m- In the present work it was 

 found that all the modes, except those of spores cultured on barley, 

 fluctuated about those of a strain of P. graminis phleipratensis , which 

 had been confined to timothy for more than a year. The modes fluc- 

 tuated about 24 and 17 fi, varying perceptibly with the existing climatic 

 and edaphic conditions. 



That barley is not a congenial host for timothy-rust is shown by the 

 slight virulence of infection (Table VIII), very small uredinia, and con- 

 siderably decreased size of the urediniospores, with average mode of 

 21.28 by 14.24 ju. The results of inoculation with P. graminis phleipra- 

 tensis, obtained from two different sources — timothy {Phleum praiense) 

 and orchard-grass {Dactylis glomerata) — and grown on three different 

 hosts (timothy, orchard-grass, and oats), show (Table VIII) that the 

 urediniospores retain their characteristic size, except for small negligible 

 variations, whether parasitizing very congenial or merely tolerant hosts. 

 The spore shape is predominantly pyriform, and the color is dull, dirty 

 yellow to grayish. 



The infection capabilities of P. graminis agrostis are similar to those 

 of P. graminis phleipratensis and P. graminis avenae The uredinio- 

 spores of this rust are the smallest of all the biologic forms of P. graminis, 

 especially in width. In shape and color they resemble very closely 

 timothy-rust spores although not quite so dominantly pyriform. The 

 spore dimensions are given in Table IX, 



The results given in Table IX show clearly that the influence of host 

 on the size of the spores was negligible. 



Attempts to Unify Spore Sizes of Different Bioi,ogic Forms by Culturing 

 Them on the Same Host 



P. graminis tritici has a number of hosts in common with P. graminis 

 secalis. P. graminis avenae and P. graminis phleipratensis also have 

 several hosts in common. The work was directed toward an attempt to 

 determine whether the spore morphology of these biologic forms could be 

 made identical by the use of common hosts. Table X gives the results 

 of using Hordeum, vulgare as a common host to unify P. graminis tritici 

 and P. graminis secalis, and in Table XI are given the results of using 

 Avena sativa as a common host for P. graminis avenae and P. graminis 

 phleipratensis. A condensed tabulated summary of these results is given 

 in Table XII. 



