162 PLANT PATHOLOGY 



during the last season, and indicating what should be done to pre- 

 vent a recurrence of these fungi in the next crop. 



But great as has been the service to agriculture and horticulture 

 by this development of spraying and its related operations as well 

 as the reciprocal service to the science itself, yet it is the outgrowth 

 of but one division of the science, and that not necessarily the largest 

 one. 



Much work has been done, and still more is contemplated and 

 under way, in the breeding of resistant varieties of various kinds of 

 crops, with the double purpose in view of securing larger products 

 and at the same time evading the destructive attacks of certain fungi, 

 insects, and other small enemies. This work is likely to yield excellent 

 results, especially where parasitic forms show adaptations within 

 narrow limits. It must be borne in mind, however, that our know- 

 ledge regarding the range of adaptability of parasites is not large. 

 Even the whole meaning of parasitism is not yet available to guide 

 the plant-breeder. Much progress has recently been made toward a 

 knowledge of parasitic adaptations and variations by the researches 

 of Klebahn, Eriksson, Marshall Ward, and others, on the grain- and 

 grass-rusts. Especially significant in this connection are the results 

 obtained by Ernest S. Salmon at Cambridge University, who found 

 that by mutilating or otherwise partially killing the tissues in the 

 vicinity of the spot on which the spores of the grain mildew, Erysiphe 

 graminis, were sown, a "biologic" form of barley mildew could be 

 grown on wheat, which under normal conditions would be entirely 

 immune to it. Moreover, when a form was once established by thus 

 lowering the vitality of an immune host, as we may assume was 

 the main effect of the mutilation, spores from the growth thus 

 produced would infect uninjured individuals. Thus through the 

 presence of a wound a parasitic fungus was enabled to gain a foot- 

 hold and maintain itself on a crop, wheat in the instance cited, which 

 had before been completely immune to it. This is surely a highly 

 interesting situation. What assurance have we that after years of 

 work in establishing an immune variety of grain or vegetable some 

 mishap, like an untimely frost, a hail-storm, or an army of locusts, 

 may not lower the vitality or cause injuries that will give just the 

 right opportunity for the fungus, which we have taken so much 

 pains to circumvent, to gain a foothold in our supposed immune 

 crop, and profiting by the greater vigor of the new host, become 

 after a short period of adaptation a greater pest than it was to the 

 old varieties. Thus at the end of our effort comes worse disaster 

 than preceded it. The moral of this fable is clear. Pathologists 

 should turn more attention than at present to a study of the con- 

 ditions underlying parasitism, of the degree of adaptation, and of 

 the range of variation, among fungi which cause the diseases most 



