RELATION OF INSECTS TO DISSEMINATION OF DISEASES. 
a 
discovered the cause of the yellow disease of hyacinths (Pseudomonas 
hyacinthi). But it-was later investigators like J. C. Arthur, M. B. 
Waite, and, above all, Erwin Smith, who established the sure founda- 
tions of the science of bacteriology as related to plant diseases. Many 
of the Germans who were well advanced in the study of diseases due to 
fungi were opposed to the idea that bacteria could possibly cause 
diseases in plants, and as late as 1900 R. Hartig, Alfred Fischer, O. 
_ Wehmer, and others were still writing against such a cause. Other 
German writers, such as Flugge (1896), in his book on WMicro- 
organisms, 1,385 pp., and Frank (1898), in Plant Diseases, 1,213 pp., 
dismiss the subject of bacteria as the cause of plant diseases in three 
and thirteen pages respectively. Sorauer, however, as early as 1886, 
accepted the doctrine of bacterial diseases of plants, without reserve. | 
Migula also, in the first volume of his System (1897), mentions twenty- 
ine such diseases, and considered that eight were of proved bacterial 
origin; and in his second volume, in which 1,850 species of bacteria 
are described, gave 380 that are of interest to the plant pathologist. 
Very little was mentioned by English writers twenty years ago. In 
1899 and 1901, Massee and Marshall Ward, recognised authorities 
and writers on diseases of plants, only briefly mention the subject in a 
few pages. One of the chief reasons for believing that bacteria could 
not cause diseases in plants was the fact that-cell sap is practically 
always acid, and it was thought that this was inimical to the growth of 
bacteria, which, bacteriologists thought, required an alkaline or at most 
a neutral medium for their development. It has taken-a vast amount 
of exacting scientific work to show that bacteria are responsible for 
many destructive diseases of plants, and we might say that plant 
bacteriology as a science is the development of the last twenty yéars. 
It is not to be wondered at, therefore, that the active part taken by 
Insects in the dissemination of such diseases is little known, as many 
of the diseases themselves have only quite recertly been determined. 
With respect to investigations on human and stock diseases, and the 
nature of the secondary host, European workers are as active as 
Americans, but in the domain of plant diseases and their distribution, we 
find that the greatest amount of work has been accomplished in the 
United States of America, and we are indebted to such authors as 
Erwin Smith, F. Rand, and W. Dwight Pierce. We have to beware 
of accepting American results as being applicable in Australia, where 
climatic and ecological factors in controlling insect. distribution may 
be very different. We shall have to work out our own problems of 
insect development and control, the relative limits of ege production, 
the number of generations in a year, the periodicity of insect appear- 
ance, and the factors that bring it about, the relation of insect to 
host plant, and the manner in which ‘insects are involved in the 
transmission of diseases. Let us turn, then, to the question of the 
general relations of insects (including also the Acarina) to parasitic 
diseases. 
The functions of a plant or of an animal may be disturbed directly 
by an insect without the intervention of any parasitic organism. A well- 
known example is the so-called “ tick worry” of cattle in Queensland. 
The mere presence of large numbers of ticks causes some fever, In 
plants, a condition long known as melanose of the orange in New South 
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