DISEASES OF PLANTS. 391 



often find the breakage of a limb to be but the open door by which 

 rot is introduced into the interior tissues. We may consider, however, 

 that with most of the mycelial forms of fungi their action is more or 

 less localized, as in the smut of corn ( TJstilago maydis), or the disease 

 called cedar-apples {Gymnosporangium Sabince), or the curl of the 

 peach-leaf (Exoascus deformans). So far as they are localized, there- 

 fore, their treatment is a simple matter, since it only involves cautery 

 or removal of the affected part. Owing, however, to their peculiar 

 habits of growth, and the insidious rapidity with which the spores 

 may be disseminated, they may cause a disease of the general system 

 when the conditions of the latter are favorable. But it is not such an 

 easy matter to dispose of all these organisms. In the animal, it is 

 now well demonstrated that disease may be directly produced by the 

 action of certain schizomycetes, such as the micrococci and allied germs ; 

 and it is even claimed by some that they have a corresponding patho- 

 genic function in the vegetable organism. These latter views, however, 

 rest upon insufficient evidence at present ; but, in considering certain 

 diseases of plants at least, analogy would dictate measures of caution 

 in formulating an opinion which wholly disregards the importance of 

 these minute structures as pathogenic agents. Whether actually the 

 cause of disease, or only of secondary features, in either case they are 

 most difficult elements to deal with. 



The third class embraces a variety of causes which may be directly 

 controllable by man or not. Injuries may be inflicted by insects, as 

 so generally occurs in the formation of galls upon leaves ; in the punc- 

 tures which various boring insects, as the scolytus and cegeria make 

 for the deposition of their eggs ; and, more especially, as in the subse- 

 quent action of the larvae. There are, also, injuries which may be in- 

 flicted by animals and man, either by accident or design, and which 

 permit the operation of fungoid growths with the development of 

 secondary features. All these are of a strictly local nature, and the 

 question whether or not the entire system will be involved in disorder 

 must largely depend upon the extent and nature of the injury in the 

 first instance. 



The treatment may or may not be difficult. Where insect action 

 is strictly local, as in galls, the amputation of the parts is sufficient ; 

 but, where the injury is inflicted by boring larvae, the grub must first 

 be destroyed, and this requires certain knowledge of the habits of 

 these insects in the different stages of development. In the case of 

 the scolytid borers the treatment is especially difficult, as the beetles 

 are very small, and hard to destroy ; but it is an interesting fact 

 that the ovipositing of these insects is in itself indicative of an 

 already diseased condition,* so that the surest and best remedy is a 



* Professor Riley tells me that, so far as he knows, these borers oviposit only in 

 diseased trees, though they may feed on healthy trees ; and, in my observations of the 

 last two years, I have been unable to collect a single fact opposed to this view. 



