60 THE REPORT OF THE ' No. 36 



INSECTS AS AGENTS IN THE DISSEMINATION OF PLANT DISEASES. 

 Lawson Caesae, 0. A. C, Guelph. 



The following was delivered as the President's Address : 



There are three great classes of plant diseases in the dissemination of which 

 insects play a part. These are : — first, diseases due to fungi ; second, diseases 

 due to bacteria; and third, diseases whose cause has not been discovered but which 

 are of a decidedly communicable or infectious character. This last class is often 

 called " Physiological Diseases," or " Diseases of Unknown Origin," the latter term 

 being preferable. 



Before discussing the role of insects as disseminators it may be well to give 

 a list of the common agents in the spread of plant diseases. They are : wind, rain 

 (especially wind-driven rain), infected seed, infected manure, infected soil, insects, 

 slugs, man with his teams and implements, birds and a few other animals. 



Of these various agents every plant pathologist would say that so far as the 

 dissemination of spores of fungi is concerned, wind and rain are, with very few 

 exceptions, such as perhaps Ergot of Eye, vastly more important agents than in- 

 sects. In the case of diseases that have been introduced recently and that are not 

 yet widely spread, insects may play an important part in long-distance distribution 

 and in the establishment of new centres of infection, especially if the spores of such 

 diseases are of the type that is held together by a gelatinous substance which pre- 

 vents their distribution by wind alone, though after being dissolved in moisture 

 they may be blown short distances by wind-driven rain. 



In the dissemination of plant diseases insects may function in three ways. 

 First, they may serve as mere carriers of the spores or other causal organisms 

 from plant to plant. The amount of disease thus spread compared with that by 

 other agents is probably small. Second, they may cause wounds of various kinds 

 which afford ideal conditions for spores or bacteria to germinate in and establish 

 new infections. This is of course a very important function because many kinds 

 of spores and a large percentage of bacteria seem unable to enter plants in any other 

 way than through wounds. Third, they ma}' serve as direct inoculators, not only 

 bringing the organism with them upon or within their body but actually inserting 

 it, when feeding, into the tissues where it finds favorable conditions for develop- 

 ment. This last is on the whole the most important of tlie three methods. 



IxsECTS AS Disseminators of Fuxgus Diseases. 



Sucking insects with a few exceptions, such as in the case of the spread of 

 Ergot of Eye by flies and of Downy Mildew of Beans by bees, do not appear to 

 play nearly so important a part in the spread of fungus diseases as do biting insects. 

 This is probably because such sucking insects as feed upon plant tissues have minute, 

 needle-like mandibles and maxillae and in feeding make very small wounds. 

 These wounds do not expose the moist inner tissues or afford much better places 

 for spore entrance and germination than do stomata and lenticels. Moreover, 

 insects with such mouth-parts are not adapted for feeding upon spore masses and 

 so seldom get their mouth-parts contaminated and act as direct inoculators of 

 healthy plants. Biting insects are therefore much more important in the dis- 

 semination of fungus diseases of plants. 



It is worth noting here that several species of Coleoptera and Orthoptera 

 as well as some Lepidopterous larva? feed freely uj^on spore masses, and not only 



