432 INSECTS AFFECTING VEGETATION 



striking arrest of stem elongation takes place in lettuce rosette 

 wherein the roots are destroyed so largely by the fungus in the soil. 

 (See Soil Infesting Diseases.) 



In cankers of branches upon orchard trees the final death of the 

 immediate branch is preceded by a depressed area invaded by the 

 parasite. 



PLANT DISEASES NOT BEYOND EXPLANATION. 



The old mystery attached to disease prevalence can scarcely be 

 maintained in our day. We have worked out in recent years or had 

 determined for us the causal relations between the ferment or para- 

 site and the effects upon the host plant or crop. So far as we can 

 now discover the reason for the spread of diseases, or of a particular 

 disease, is found in the specific disarrangements in the host plants. 

 This discovery and announcement of these causal relations are un- 

 dertaken 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 epidemic 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 biological 

 factors which are capable of being influenced by prevailing atmos- 

 pheric conditions. With cool, rainy weather we have brought about 

 conditions favorable to certain parasitic diseases which will be in- 

 clined 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 physio- 

 logical disarrangements in the plant. (See Enzymatic Diseases.) 

 In all cases whether of parasitic attack or of physiological disarrange- 

 ment due to other causes, the host plant is weakened and predisposed 

 to death. 



GROUPS OF PARASITIC DISEASES. 



Parasitic diseases may be grouped in a 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 mak- 

 ing such groups or classes of diseases as are descriptive of the general 

 behavior. Such are seed infesting diseases, soil infesting diseases^ 

 root diseases, diseases of foliage, wound troubles, timber rots, etc. 

 The great mass of diseases are treated under each host in the descrip- 

 tive portion arranged alphabetically. The objects to be attained by 

 this method of arrangement are obvious and call for no discussion. 



