DISEASES OF CULTIVATED PLANTS 671 



antitoxins) which destroy such toxins or enzemes or to excrete 

 chemotactic substances which repel fungus protoplasm. 



The work of Ward and others indicates the great probability 

 that the hindrance to attack by fungus spores or bacteria in plants 

 which remain healthy under infection conditions is not in any 

 sense a mechanical one depending on the size or number of breath- 

 ing pores, thickness of epidermis, hairyness, &o., but resides in 

 the peculiarities of the protoplasm and is in no sense different 

 from the conditions that render animals immune to disease. It 

 would then seem possible if difficulties in technique did not stand 

 in the way to vaccinate plants or introduce protective serums that 

 would immunize them against disease. 



The principle that I have stated would indicate that if plants 

 can be bred that are active in producing the anti-bodies and so 

 overcoming parasites, fungi can also develop that will overcome 

 the new varieties, since the fungi are only small plants, and sub- 

 ject to the same general laws of plant life. Another possible hin- 

 drance to securing disease resistant varieties is that the very charac- 

 ters in certain varieties that make them desirable may be the same 

 that would favor disease. 



It seems to me very probable that we shall be able to obtain 

 strains of most of our crops, resistant against all diseases produced 

 by parasites that confine themselves to particular species. I think 

 it much less probable that we shall be able to get varieties resistant 

 to such diseases as some of the sclerotinia troubles and mildews that 

 attack indiscriminately a number of different kinds of crops. 



A great deal of practical work has already been done in selec- 

 tion of varieties less subject to diseases, and recently much attention 

 has been given by plant breeders to the securing of disease resistant 

 crops. It may be of some practical value to you to mention some of 

 this work though in the limited time I cannot do more than give 

 some examples of what is being done. Much has been done by 

 observation on disease resistance while breeding plants for other 

 purposes. 



Prof. Hanson, of South Dakota, one of the leading plant 

 breeders of America, in producing hardy fruits for the cold North- 

 west has found mildew resistant forms of the sand cherry, one of 

 their promising fruits and is working for a strawberry, whose leaves 

 will not rust. Hanson also noted an interesting fact regarding mil- 

 dew. Some of the wild roses subject to mildew, produced mildew 

 free hybrids with cultivated ones. 



J. B. Norton in his work in producing new varieties of oats has 

 noted the variation with respect to smut in the product of his crosses 

 and one good production is practically free from smut, a disease 

 which causes hundreds of thousands of dollars loss annually in the 

 West. 



On the other hand selection and breeding of disease resistant 

 varieties has resulted in securing forms valuable in other ways. 

 Carleton's efforts to get wheat varieties that would not be seriously 

 injured by rust, have resulted in the introduction of drouth resistant 



