o DISSEMINATION OF PARASITIC PUNGt 



land. These would exceed in size and weight the spores of many fungi. It 

 would seem natural to suppose that spores might equally be caiiied for at 

 least some hundreds of miles, were it not that there is a great deal of circum- 

 stantial evidence against this to support the direct evidence mentioned above. 

 This evidence is based on the known distribution of the minute plants 

 which cause disease. It may be divided into two classes. The spread of a 

 limited number of diseases has been closely observed and has not been found 

 to be such as would be accounted for by long-distance spread through the air. 

 The geograj)hical distribution of some groups of parasitic fungi has also been 

 worked out and seems equally to exclude this possibility in a certain number 

 of cases. 



Amongst the diseases whose spread has been closely observed and is 

 evidently not due to their spores having been carried by the wind over large 

 areas, the following examples may be selected : — 



The chestnut bark disease has been carefully watched, since soon after 

 it first appeared in the United States. It is quite well suited 

 for wind dissemination, as the parasite liberates considerable 

 quantities of ascospores into the air. Nevertheless its spread 

 has been continuous in the main and, as already stated, it 

 was unable to leap a 30 or 40 mile belt free from chestnuts 

 in the Catskill mountains, at least up to 1913, though a little 

 further south it spread continuously along the southern border 

 of New York State, where there is a narrow strip of chestnuts, 

 and so reached the large chestnut area of the south-west of the 

 State. In the ten years after it was first observed in New 

 York City in 1904, it had spread from New Hampshire to 

 Virginia and as far west as western New York and 

 Pennsylvania, but had not reached Ohio or Indiana. Where 

 " spot " infections {i.e., isolated attacks not due to continuous 

 spread) have been observed, the majority have been traced to 

 the introduction of diseased nursery stock. 



The American gooseberry mildew was introduced into Europe about 

 1900. There were piobably at least three distinct introductions 

 about tho same time, into Ireland, into Denmark, and into 

 Russia. In 1905, it is known to have extended to six counties 

 m Ireland, to ten widely separated localities in Russia and also 

 to Finland, to Posen in Germany near the Russian border, to 

 Norway and to Sweden. It attracted so much attention 



