E. J. BUTLER 25 



climate are so great and there are so many different crops grown, that there 

 is scarcely any considerable area without some parasite liable to attack a 

 valuable economic plant in India. Further there are as yet few exact records 

 of introduced diseases to suggest where danger may be looked for. We have 

 probably received the potato blight and vine mildews from Europe ; chrysan- 

 themum rust may have reached us direct from Japan or through Europe, the 

 downy mildew of maize {Sclerospora Maydis) perhaps from Java, and that of 

 cucurbits possibly from Ceylon. Most of the recently introduced diseases in 

 Europe seem to have come from North America, East Asia or the temperate 

 parts of South America, but no such well-defined danger zones can be suggested 

 for India. Hence in order to secure ourselves against the introduction of 

 exotic diseases, there seems to be no alternative at present but to take each 

 of our important economic plants and examine the records of the parasites 

 that attack it in other countries and that aie not yet known in India. In this 

 way it may be possible to draw up lists of the dangerous diseases that should 

 be prevented from entering India on their host plants. Fortunately India is 

 as yet largely self-contained as regards seed supply and nursery produce ; 

 comparatively few living plants or parts of plants arr; imported ; and from the 

 lists furnished by the Customs authorities it would appear to be a fairly easy 

 task to decide what produce enters the country, through private agency, of a 

 kind likely to bring in dangerous plant diseases. 



In preparing a list of the diseases that should be guarded against, certain 

 difficulties arise. In the first place the diseases that occur in many countries 

 and colonies are not well known. Even in Great Britain no complete plant 

 disease survey has been made and it is consequently sometimes difficult, or 

 even impossible, to ascertain whethe r a particular disease occurs in that country 

 or not. This is still more the case with other areas, such as China, from which 

 there may be a danger of diseases being imported into India. In most of the 

 discussions on international problems of jshytopathology in recent years, 

 reference has been made to the need for establishing a plant disease survey in 

 each country. At the least a card index should be maintained in some central 

 location, in which all diseases of economic plants are entered under the parasite 

 and under the host plant ; and all new diseases should be recorded as they are 

 reported, together with the localities affected as far as known. The mainten- 

 ance of an index of this nature requires no great amount of work and enables 

 any country desiring it to receive a list of the diseases and the plants affected 

 which would greatly help it in deciding what imports should be kept out. 

 Only one word of warning is necessary. Such an iiidex cannot be prepared 

 in the laboratory or office alone ; the knowledge required must be gained 



