26 DISSEMINATION OF PARASITIC FUNGI 



largely by " field " work, as it is generally useless to rely on the chanc^ 

 notification of diseases ; and the " field mycologist " is the only person who 

 can undertake this work. 



Tlien in deciding whether a disease is likely to prove dangerous or iiot 

 the problem of variation in virulence in different localities has to be taken into 

 account. There is unfoitunately abundaiit evidence that diseases that are 

 relatively mild in one continent may prove exceedingly virulent in another. 



The vine mildew and oidium are cases already referred to above. The 

 chestnut bark disease, perhaps the most completely destructive fimgus disease 

 at present under observation, is another, since ii^ its original home in East 

 Asia it is comparatively innocuous. That a parasite introduced to a new home, 

 where conditions are to its liking, may develop accentuated virulence is now 

 well known ; the rnle holds equally for animal diseases ; and small-pox, for 

 instance, is known to rage with exceptional violence when first introduced 

 into a new commimity, as amongst the North American Indians. An American 

 phytopathologisti has recently brought together some cases observed during 

 a tour in Europe of diseases whose destructiveness differs in the two continents. 

 He found the brown rots of fruit (Sclerolima or Movilia) did mucli more damage 

 in Em'ope than America, as also the stiawberry mildew and some others. On 

 the other hand, the black rot of apple {Sj^hcerojJsis m<tloram) docs little injury 

 in Europe (where it is known from Italy to England) while it is a widespread 

 and destructive disease in the United States, and this is true also of the l^itter 

 rot of the same fruit {GlcEosporkmi fnictigem/m) aiid asj^aragus rust. "VMiile 

 it is tempting to think that these difi'erences aie due to the diseases being 

 comparatively recent introductions to the country in wliich they are most 

 destructive (as is true of the asparagus rust), there is no evidence whatever that 

 this is the case with the brown rots, the strawberry mildew or the two apple 

 diseases. A^^^atever the causes, the fact remairis that the virulence of a disease 

 in one continent is iro safe guide to its destructiveness if introduced into 

 another. 



The preparation of lists of diseases, which it is desired to exclude from 

 each country, was accepted as an essential part of the scheme for seeming 

 international co-operation in checking the spread of plant diseases and pests, 

 drawn up at the Intcinational Phytopathological Conference held at Rome in 

 Februarv, 1911. The object of the Conference was to franir a Convention 

 to control the inter-state circulation of horticultural produce. The meetings 



' Shear, C. L. " Soiiio observations on phytopathological problems in Europe and 

 America." Phytopathology III, 1913, p. 77. 



