Minnesota Plant Diseases. 289 



year. The rapidity of formation of successive crops of summer 

 spores would make this manner of spreading easily possible. 



Eriksson, an eminent European specialist on rusts, has pro- 

 posed a theory known as the mycoplasm theory, which supposes 

 that the fungus threads live over in the grain of the wheat as 

 an amorphous substance, totally unlike the ordinary fungus 

 threads. This mycoplasm he supposes to be diffused amongst 

 the protoplasm of the cells of the grain, until the latter com- 

 mences to germinate, when it again assumes the form of threads 

 and causes an infection of the seedling wheat plant. We can- 

 not here go into the details of the evidence, but in the opinion 

 of a great majority of rust specialists this theory has not suffi- 

 cient foundation. Infection by the spore methods, described 

 above, is at present the only known method. 



Recent work points to more probable success in combating 

 rusts along another line. It has been found that many of the 

 rusts are so closely adapted to the conditions found in the plants, 

 on which they occur, that they have great difficulty or fail en- 

 tirely in growing upon other varieties. At present there are 

 recognized, by the best authorities on the rusts of cereals, more 

 than a dozen distinct varieties and species. Many of these are 

 indistinguishable from other forms, as far as external characters 

 are concerned, even with the aid of powerful microscopes, but a 

 great difference is soon found when the spores are germinated 

 and infections attempted. It is then seen that some forms are 

 so especially adapted to the species or variety of the host plant, 

 upon which they occur, that they refuse to develop on other 

 varieties or s|>ecies of the same genus of host. Such rust forms 

 are called "biologic" species. On the other hand it will be seen 

 that some species and varieties of grasses and cereals may thus 

 be immune from some forms of rusts. A "rust-proof" variety 

 would not of course be proof against all forms of rust but might 

 be immune from certain forms. Consequently something bene- 

 ficial may be expected from the efforts of plant breeders in the 

 production of rust-proof cereals. With intelligent care in the 

 selection of cereal varieties and with a broader and letter knowl- 

 edge of the habits and life-history of the parasitic plants causing 

 rusts, it is very probable that the ravages of this disease can be 

 at least considerablv checked. 



