DISEASES OF CULTIVATED PLANTS 315 
particular disease, is found in the specific disarrangemen\. in the 
host plants. This discovery and announcement of these causal 
relations are undertaken that proper measures for the control of 
diseases may be finally devised and applied. We must always bear 
in mind that under favorable conditions plant diseases become epi- 
demic and their rapid spread is to be expected. 
The host plant, with its climatic adaptations and the parasites 
of our crops with their mutual adaptations to their hosts are biolog- 
ical factors which are capable of being influenced by prevailing 
atmospheric conditions. With cool,'rainy.weather we have brought 
about conditions favorable to certain parasitic diseases which will be 
inclined to spread while these continue. Other diseases spread under 
the conditions which favor them. The more rapid development of 
diseases of plants under these favoring circumstances is not beyond 
reasonable understanding; there is no mystery about it any more than 
in outbreaks of typhoid fever or diphtheria. By apprehending the 
differing conditions we may learn to separate the causal from the 
merely adventitious factors and thus be the better able to master 
the diseases which result. 
While we may properly look upon infection by microscopic or 
other parasites as the general and usual cause of plant diseases, 
there are diseases of wide importance which arise from internal or 
physiological disarrangements in the plant. (See Enzymatic. Dis- 
eases). In all cases whether of parasitic attack or of physiological 
disarrangement due to other causes, the host plant is weakened and 
predisposed to death. 
GROUPS OF PARASITIC DISEASES 
Parasitic diseases may be grouped ina way, according to the 
groups of fungi which cause them. ‘This is helpful to the plant 
pathologist, though of limited practical guidance, since it requires 
microscopic study to determine the causal organisms. A more 
useful, limited grouping as is hoped, is proposed below and consists 
in making such groups or classes of diseases, as are descriptive of 
the general behavior. Such areseedinfesting diseases, soil infest- 
ing diseases, root diseases, diseases of foliage, wound troubles, 
timber rots, etc. The great mass of diseases are treated under 
each host in the descriptive portion arranged alphabetically. The 
objects to be attained by this method of arrangement are obvious 
and call for no discussion. 
NAMING PLANT DISEASES 
Plant diseases are named with due regard to the symptoms and 
cause of the disease. In the case of enzymatic diseases wherein we 
have peculiar variations or yellowing of the leaves, the names given 
are mare ar lece deerrintive The same applies to the diseases that 
al 
