PROBLEMS AND PROGRESS IN PLANT PATHOLOGY IO5 
relation not only to the normal physiology and morphology of the 
plant, but in relation to its history and its variations under culture. 
Progress requires that we have specialists on types of host plant as 
well as of parasite. 
And, passing to the cellular relations of host and parasite, how 
little we know. The very simplicity of the plant's organization 
makes the pathological reactions harder to investigate than with the 
animal. In the plant the unit in the more fundamental pathological 
relations is not the organism but the cell, an object so minute as to 
make the study of the chemical interrelations highly difficult. We 
recognize the cell membrane as the first barrier to be overcome by the 
invader and we believe the cytolytic enzyme the first weapon in the 
attack. Yet, save with certain soft rot diseases, we know little that 
is definite about these enzymes in their action. We see evidence of 
other disturbing effects of parasite upon host cells, even in advance of 
actual invasion. Sometimes these are inhibitory or fatal, sometimes 
stimulating. But we have scarcely sufficient basis for a suggestion 
as to the nature of the agents involved. Such problems call for the 
combined skill of pathologist, physiologist, cytologist and chemist. 
The variation in the occurrence of disease with environment is 
one of the commonest observations and a thing of the greatest practical 
moment. Yet how little progress we have made in understanding the 
factors. Climate and soil, both are composites of many variables, 
which in turn may react on either host or parasite. Why is it that 
Rhizoctonia diseases and Blattrollkrankheit of the potato claim so 
much attention in certain sections of the United States while in others 
pathologists are skeptical as to their existence? Why is it that the 
bacterial black leg of the potato develops so much worse in the south 
than in the north? Why is it that with the melon the Fusarium wilt 
is the scourge of the one section and the bacterial wilt of another? 
Why is it that the yellows disease of cabbage exterminates the crop 
under certain conditions and is of minor importance under others? 
It would seem that here are problems to challenge the attention 
of every pathologist. Yet if one turns to them he is balked at the 
outset. We have inadequate data as yet regarding the occurrence 
and distribution of even the commonest economic diseases in the 
United States. Let us unite in urging that in the reorganization of 
the work now in progress in the Bureau of Plant Industry the entire 
attention of at least one expert pathologist be given to collecting and 
