4 DISSEMINATION OF PARASITIC FUNGI 



th« fungi wliicli cause wilt and alli(!(l diseases and may reach other fields in 

 various ways, as on the boots of field-workers and pedestrians. But the 

 majority of these and other minor means of dissemination can only be invoked 

 to account for spread over comparatively short distances and may be at once 

 ruled out as probable causes of extension of disease to distant countries. 

 Leaving out dissemination throuLdi the air, all the others require the susceptible 

 plant to be within short range of the source of iiifection. The normal flight 

 of insects, the flow of irrigation water, or the range of labourers and pedestrians, 

 is through a limited area ; and though we could imagine diseases crossing land 

 frontiers or even passing, say, from Ireland to England in this manner, spread 

 to Europe from America or East Asia would be rare indeed. 



For such long-distance spread, for what may be called " discontinuous 

 spread," the methods are much fewer. They may practicall}^ be reduced to 

 three : dissemination through air ; dissemination by animals that travel long 

 distances rapidly, as migrating birds and certain insects ; and dissemination 

 on plants and other horticultural produce. The relative importance to be 

 assigned to each has not, so far as I know, ever been adequately discussed and 

 exact data for such a discussion are scanty ; yet they yield results of considerable 

 significance. 



To take, what must be of the least importance first, birds are probably the 

 chief animals that could materially contribute to the discontinuous spread of 

 parasitic fungi. The distances to which they travel are remarkable. 

 Richard's Pipit, which breeds in Eastern Siberia from Lake Baikal to the sea 

 of Okhotsk, regularly visits Eurof)e, as far sometimes as England, and passes 

 through Spain even to Western Africa. Several other Siberian birds visit 

 Western Europe, some, such as the Hooded Crow, in enormous numbers. The 

 Knot, which breeds within the North Polar basin, has been found in winter 

 in Australia and New Zealand. Individually marked storks, which breed in 

 Central Europe, have been identified in South Aftica in winter. Still it is 

 perhaps possible to show that they cannot be a serious danger. The wanderings 

 of migratory birds which travel long distances are usually between areas of 

 very dissimilar climate ; they pass in a general direction from north to south 

 and back again ; and between the extremes of theii journeys the climates and 

 vegetation are so different that the agricultural and horticultural products 

 are usually sufficiently distinct to have very few parasites in common. WTiere 

 birds pass through areas with partially similar agricultural and horticultural 

 crops, as from south to north Europe and the southern to the northern parts 

 of North America, the parasites which they could convey would sooner or 



