492 Journal of Agricultural Research voi. x, no. 9 



employ differential hosts to determine the identity of the forms. P. 

 graminis tritici and P. graminis secalis have been found associated 

 most often. 



(5) Different strains of the same bioligic form sometimes differ in 

 virulence on the same hosts; but the differences are usually in degree 

 only. 



(6) There seems to be no sharp geographical specialization of bio- 

 logic forms in the upper Mississippi Valley and Northern Great Plains 

 area, where the biologic forms are quite uniform. 



(7) On the basis of parasitism the biologic forms can be divided into 

 two groups: (i) P. graminis tritici, P. graminis tritici compacti, and 

 P. graminis secalis; (2) P. graminis avenae, P. graminis phleipratensis, 

 and P. graminis agrostis. 



(8) Wheat, club wheat, rye, and Agropyron repens are differential 

 hosts for group i . The tritici form infects wheat and club wheat readily 

 and rye and A. repens weakly; the tritici compacti form attacks club 

 wheat readily and the other three weakly; the secalis form develops 

 normally on rye and A. repens, but very rarely attacks the other two. 

 All three develop well on barley, Hystrix patula, and Bromus tectorum 

 and on a number of species of Agropyron, Elymus and Hordeum. 



(9) Differential hosts for the forms in group 2 are oats, Phleum pra- 

 tense, and Agrostis spp. The avenae form develops normally on oats, 

 infects Phleum pratense weakly, and develops fairly well on Agrotis alba; 

 the phleipratensis form grows normally on Phleum pratense, infects oats 

 rather weakly, and has not yet infected Agrostis alba; the agrostis form 

 develops normally on Agrostis spp., infects oats very weakly, and has 

 not yet infected Phleum pratense. All three infect barley and rye weakly, 

 but develop well on Koeleria cristata, Holcus lanatus, Dactylis glomerata, 

 Alopecuris geniculatus, and A. pratensis. 



(10) Barley, rye, and Bromus tectorum have been infected by all six 

 biologic forms. Oats have been infected by all but P. graminis tritici 

 compacti, but not enough trials have been made with this form. 



(11) All gradations in susceptibiUty occur, from complete immunity 

 to complete susceptibility to various biologic forms. The following reac- 

 tions may be made to inoculation : No visible effect ; appearance of small 

 flecks; production of very minute uredinia without any flecks; produc- 

 tion of minute uredinia in either small or large dead areas ; development 

 of moderately large uredinia in small, medium, or large flecks; produc- 

 tion of large uredinia surrounded by small dead areas or by apparently 

 healthy tissue. 



(12) The biologic forms can be distinguished from each other morpho- 

 logically as well as parasitically. The size, shape, and color of the ure- 

 diniospores are the distinguishing characters. The determination of 

 biometrical modes permits identification with a reasonable degree of cer- 



