58 
IOWA ACADEMY OP SCIENCE 
During the summer of 1908 there were placed under observation in the 
greenhouse, plants of the following grasses: Lolium perenne, Phleuwi pratense, 
Poa pratensis, Agropyron repens, Panicum crus-galU. Bromus secalinus, Setaria 
glauca, Dactylis glomerata, and Hordeum jutatum. Wheat plants bearing abun- 
dance of ErysipUe graminis were placed in contact with the leaves of these 
plants, none of which became affected by it. 
Prom the work of Salmon®^ it appears that different forms of bpTiaerotheca 
humuli occur upon Pyrus Aria, Potentilla, and other hosts, these forms having 
individual characteristics. “While these forms present slight morphological 
differences, these are not sufficiently marked to prevent the forms being con- 
sidered as belonging to the species.” 
Dr. Salmon®® experimented with mildew of Erysipfie graminis. The form 
found on wheat will not normally infect rye; but when the leaves of rye were 
injured by cutting or bruising they could be infected by the fungus from the 
wheat. 
Dr. G. M. Reed who has carried on a series of experiments with Erysiphe 
cichoracearum^^ D. C. has shown quite conclusively that the fungus “occurs on 
at least eleven species of the cucurbits belonging to seven genera, infection 
occurring in these cases in fifty per cent or more of the trials. Only three 
species belonging to two genera are entirely resistant to the mildew.” It is 
evident also that this fungus will produce the mildew upon other forms of 
plants. In the record by Dr. Reed out of fifty-four leaves of common plantain 
inoculated with Erysiphe cichoracearum from squashes ten were affected. 
The sunflower was likewise infected in 35 per cent of the trials, but he failed 
to inoculate mildew of cucurbits on asters and goldenrod. 
Mr. Griffiths®^ . observes that while the favorite habitat of E. cichoracearum 
appears to be on the composites, it is not confined to this group. The entire 
absence of the fungus on hosts which are generically related, is therefore a 
matter of surprise. Mr. Griffith observed such a case in a garden where three 
species of Artemisia, A. Ludoviciana, A. tridentata, A. longifoUa, grew in pro- 
fusion, and in closest proximity. A. Ludoviciana was loaded with E. cichorace- 
arum, but search failed to find it upon the other two species. No positive 
reason can be given for the absence of this fungus on the two species in ques- 
tion, under such favorable conditions. 
Although closely related there is much difference in the aromatic principle 
and the development of trichomes in the two species. 
A parallel case was observed at Buffalo, Wyoming, where A. dracunculoides 
and A. Canadensis were grown together. The former had an abundance of E. 
chichoracearum upon it while the latter was entirely free. 
Rusts also show peculiar relation to hosts. The Puccinia coronata does not 
grow equally well on the orchard grass, and on some other hosts on which it 
has been recorded. The forms of Puccinia ruMgo-vero behave differently on 
various host plants. The form on rye is different than on wheat. 
J. Ericksson®® who has made a special investigation of cereal rusts considers 
that the several forms of Puccinia graminis on different host plants are distinct. 
®^Monog-raph of the Erysipliaceae. 56. 
«•> Annals of Bot. 19 :125. 
®®Reed, G. M. Infection experiments with Erysiphe cichoracearum D. C. Separate 
Bull. U. Wis. 250:341. 
«'^GrifRths. Some Northwestern Erysiphaceae. Bull. Torr. Bot. Club 26:138-144. 
®®Ueber die Spezialsierung des Getreidesschwarzrostes in Schweden und in anderen 
Landern. Centralb. f. Bakt., Abt. 2.9 :590-607, -654-658. 1902. 
