172 SECTIONAL ADDRESSES 
I do not subscribe to this view. While full of admiration for the achieve- 
ments of the geneticists in disease-control, I think there will always be an 
extensive field for plant pathologists in the elucidation of the problems of 
disease in plants, which must precede control, and in direct attack on many 
diseases. The plant breeder is sometimes faced with almost insuperable 
difficulties, for there is often complete or almost complete linkage between 
susceptibility to a specific disease and high quality, which is extremely 
difficult to break. ‘This is notably true of potatoes, in which the best 
varieties are almost invariably susceptible to Blight (Phytophthora in- 
festans). In passing, however, it may be mentioned that Dr. Salaman 
and Miss O’Connor have recently achieved some desirable syntheses 
between domestic potatoes and species of Solanum which are immune to 
Blight, and that these productions shew considerable promise for the 
future. Again, many crop plants are liable to severe attack by several 
different diseases, and since susceptibility and resistance are often trans- 
mitted independently for each specific disease the work of the geneticist 
becomes well nigh endless. With arborescent plants breeding for disease- 
resistance is necessarily slow even when possible, and the time can hardly 
be visualised when the services of the pathologist will not be required for 
the control of the diseases of woody plants ; for example, it is difficult to 
conceive of the breeding of a strain of the rubber tree (Hevea brasiliensis) 
which will not be liable to attack by the root and bark diseases that com- 
monly afHict it. Furthermore, disease-resistance, although of great gene- 
tical significance, is not usually unmodifiable under diverse environmental 
conditions. A variety bred for resistance in one locality may become 
susceptible if transferred to a different environment. It must be remem- 
bered also that parasitic organisms are liable to evolutionary change just as 
their hosts are, and one cannot postulate therefore that their parasitic 
proclivities will remain constant over long periods of time. There is 
evidence that new physiologic forms of parasitic fungi arise both by 
hybridisation and by mutation, and this capacity of micro-organisms to 
change must always be borne in mind by geneticists and plant pathologists. 
Lastly, it is sometimes hardly worth while to try to build up disease- 
resistant varieties, for completely efficacious treatment of the disease in 
question by simple mycological means may be already available. Bunt in 
wheat, for instance, can be entirely eliminated in most countries by fungi- 
cidal treatment of the grain before sowing, so it is scarcely worth while 
for the plant breeder to labour in this field. 
Another advance in the control of plant diseases lies in the greater 
attention now paid to plant sanitation or plant hygiene. Such preventive 
treatment, following the same lines as in medical and veterinary sanitation, 
aims at the abolition of the sources of infection wherever possible. The 
efficacy of plant sanitation is best seen in intensive cropping in fruit 
plantations and under glass. For instance, by preventing the fungus 
Stereum purpureum from sporing within and on the confines of fruit 
plantations the risk of Silver-leaf disease is appreciably reduced. 
The elimination of certain seed-borne parasitic diseases by fungicidal 
treatment of the seed before sowing without impeding germination, the 
control of some epidemic diseases by spraying the shoot system with 
