E. j. BUTLER 23 



recent to have been recorded points to the preponderating part played by 

 man. Probably at no period in the history of the world has the movement 

 of plants of economic worth, to and from distant countries, been carried on so 

 extensively as at the present day. The time taken in transit has been 

 continually shortened since the introduction of steam ; and whether the 

 parasite is best fitted to travel as spores or as mycelium, there is no doubt that 

 there are few parts of the world that have not been brought near enough 

 to allow the great majority of the fungi that cause disease to reach us, with 

 tlieir host plants, in a living condition. 



For continuous or short-range spread, parasitic fungi make use of a number 

 ol; different methods, the chief of which have already been enumerated. Few 

 travel in the mycelial stage, most having spores suitable for the purpose. In 

 a general way these spores are disseminated chiefly by the wind, but each 

 case must be considered individually, as there are some quite unsuited for 

 air transit and no general statement would cover all groups. For discontinuous 

 or loag-rango spread, on the other hand, the parasites, whether they travel 

 in the mycelial condition or as spores, are bound up with the ovements of 

 their host plant?, and the general statement maybe made that, if we can secure 

 the absence of parasites from imported or exported plants, we can control 

 the discoiitinuous spread of the great majority of parasitic diseases. 



With these conclusions in mind, the possibility of controlling the 

 diss3rainatioi\ of diseases caused by parasitic fungi may be considered. Three 

 cases may bo distinguished : (I) where the disease has succeeded in gaining a 

 footing in the country ; (2) where it has only reached neighbouring countries 

 not separated from us by natural barriers ; (3) where it is still confined to 

 areas isolated by the ocean or by large tracts with dissimilar climate and 

 vegetation. 



Diseases already present in the country can ordinarily make use of the 

 methods of continuous spread, which are mostly uncontrollable. There have 

 b(;eu attempts in the past (and several are in progress at the present moment), 

 to exterminate newly introduced parasites in various parts of the Avorld. It 

 is doubtful if any have succeeded, while some, such as the campaign against 

 black rot of the vine in France and coffee leaf disease in Fiji, have failed 

 completely. The American gooseberry mildew is still sjjreading in England, in 

 spite of restrictive measures. The Godavari palm disease campaign has 

 been more successful, for it has kept the attack confined practically to the 

 same area, but it has, so far, failed to stamp it out wthin that area. In the 

 United States very active measures are at present in force against several 



