Pkoceedings, 1915. _ 19 



inpid change of temperaturo, etc. On the other hand, we have diseases directly 

 due to a parasitic organism obtaining an entrance into the tissues of the plant, 

 and growing there, bringing about malformation or death of the attacked parts. 

 Obviously, insects can only be concerned in the spread of diseases of this type when 

 an organism or " germ " can be conveyed from one plant to another. Disease- 

 producing organism are of two kinds, fungi and. bacteria. While both of these are 

 for the most part microscopic, they are nevertheless very different In their relative 

 size. A fungus often forms a "body" or mycelium of considerable extent, while 

 bacteria are always exceedingly minute, and their destructive action depends upon 

 their rapid multiplication, large masses of individuals being soon formed. 



Fungous diseases are not usually directly spread by insects, although the spores 

 may be carried about and distributed to some extent by their agency. Usually a 

 liarasitic fungus produces quantities of spores which are most effectively distributed 

 by wind and rain. To produce infection, however, these siiores must have suffleient 

 moisture to enable them to germinate, and the germ-tube so produced has either to 

 dissolve its way through a very resistant membrane, the cuticle of a plant, or to 

 grow along until it finds some natural opening like a stoma or a place where the 

 cuticle has been broken. It is in this latter respect that insects may greatly favour 

 infection. It is probable that the fieanbeetle in this way aids the spread of the 

 early blight of the potato and tomato due to Macrosporium solani. Perhaps the 

 best instance, however, is the spread of the brown-rot {Sclerotinia fructigena) In 

 plum and peach orchards in the East through the agency of the plum-curculio 

 iConotrachelus nenuphar). The punctures made by this insect result in an exuda- 

 tion of gum which affords an excellent situation for the development of the spores 

 of the fungtis, which, moreover, are often carried and introduced into the wounds 

 by the insects. The control of the plum-curculio has hence been found to be essential 

 in the control of browu-rot in the East. 



Itt the case of bacteria we find that these organisms have usually no power of 

 penetrating the cuticle of a plant. They are for the most part dependent for an 

 entrance on natural openings or injuries, except when the tissues are exceedingly 

 delicate and without cuticle, as in the nectaries of flowers, root-hairs, and perhaps 

 the young growing tips of shoots. They also do not produce externally anything 

 corresponding to the spores of a fungus, but live internally in the host-plant. Hence 

 the role of insects as agents in the distribution of such disease is much more 

 important. 



A very good example is the wilt disease of Curcubita, which attacks cucumbers, 

 mnskmelons, pumpkins, or squashes. This is caused by Bacillus trachcipliilus, which 

 develops in the sap-vessels of infected plants to such an extent that the vessels 

 are completely plugged up for long distances. Later the walls of the vessels are 

 destroyed and the conducting system brolceu down. As a consequence the whole of 

 the plant above the infected vessels wilts and dies. So far as is known, natural 

 infection can only take place through insects feeding on an infected plant, getting 

 their mouth-parts smeared with bacteria and then biting into a healthy plant. The 

 two insects mostly held responsible are the striped cucumber-beetles. Diahrotica 

 vittata and D. 12-punctata. The chief gap in our knowledge of the life-history of 

 the disease is with regard to the way in which the organism passes the winter. 

 Probably tlie remains of attacked plants in some cases retain bacteria in a living 

 condition over the winter, and the first infections of the season may be, so to speak, 

 more or less accidental. I do not know that either this disease or its insect carriers 

 are found in this Province, but the disease is known in Nebraska and Colorado, and 

 the insects, I believe, have been found in Washington, so it is quite likely we sliall 

 ultimately have to deal with this disease. 



A disease which comes nearer home to most of us, and in the distribution of 

 which insects play an exceedingly important part, is the fire-blight of apple, pear, 

 and quince. So far as we know, the first infections of the season are always pro- 

 duced by insect Infection of the blossoms. The exudate from hold-over cankers 



