24 DISSEMINATION OF PARASITIC FUNGI 



diseases of recent introduction, such as citrus canker and the blister-rust of 

 the white pine. In Ireland one of the most hopeful efforts as yet undertaken 

 is directed against warty disease of potatoes, which was introduced into one 

 small area. On the whole, tliough it is as yet too soon to speak definitely, 

 it is clear that treatments of extinction are extraordinarily difficult to apply 

 successfully, and that it would be much easier and more effective to keep the 

 disease from getting a footing in the country. 



Against the second class of cases, where the disease has not yet reached 

 us though known in neighbouring countries, India is peculiarly well situated. 

 It is almost completely isolated by the sea or the impassable barrier of the 

 Himalaya. Only along the coastal tracts around the Bay of Bengal and possibly 

 through Baluchistan are there routes by which continuous spread might 

 occur, and there aie few cultivated plants which extend along these tracts 

 between India and the countries outside. From Ceylon we might perhaps 

 get infected, as it is possible that such agencies as the wind or contaminated 

 insects could carry spores across the nariow gap from that colony. With 

 these exceptions, however, India is practically immune from foreign diseases 

 unless they are allowed to come in on their host plants. Such countries as 

 are not geographically isolated from their n.sighbours are in a less favourable 

 position, since they are exposed to attack by parasites that can use the methods 

 of continuous dissemination and there is, as a rule, little chance of keeping 

 these from spreading across the frontier, except in the few cases, such as warty 

 disease of potatoes, wheie the ]3arasite cannot ordinarily travel unless on its 

 host. 



The third class includes all those diseases which can only reach us by the 

 methods of discontinuous spread, which, from the evidence given above, may 

 be practically reduced to dissemination with horticultural produce or packings. 

 If all imports of plant produce could be checked there would be little chance 

 of such diseases entering the country. ^ So drastic a measure is not required 

 provided that we can ascertain what diseases should be guarded against. For 

 this it is important to know : (1) from what parts of the world we might receive 

 dangerous diseases, and (2) what diseases exist in each area that could be 

 imported and might prove injurious. 



There is no direct evidence to indicate from what parts of the world India 

 might import dangerous diseases. The country is so large, the variations of 



1 It is scarcely necessary to mention that fungus diseases can escape detection and dis- 

 infection at the port of entry, in so many cases, that this method of attempting to obtain security 

 has been abandoned. 



