22 DISEASES OF CULTIVATED PLANTS 



world-wide dispersion of diseased darnel, rye-grass, and 

 Italian rye-grass is ample. 



Many of the ' rusts' have permanent mycelium in the root 

 or rhizome of the host-plant, which ensures the continuation 

 of the fungus without reinfection by spores, and also ensures 

 the introduction of the disease into a new country, if roots or 

 rhizomes are introduced. 



It has recently been proved that the mycelium of the rust 

 fungi attacking cereals persists in the leaves as long as the 

 plant lives, and produces uredospores during warm periods 

 that occur throughout the winter months. In cold countries 

 a very few degrees above freezing-point is sufficient for the 

 formation of uredospores. In the spring there is usually a 

 final outbreak of rust on old plants that have survived the 

 winter, and the spores thus produced infect the spring crop, 

 and so commence the disease afresh. 



Many other examples could be given where a parasitic 

 fungus has perennial mycelium located in some part of its 

 host, but perhaps sufficient evidence has been furnished to 

 indicate the facility with which fungus diseases can be intro- 

 duced into a new district quite independent of fungus spores, 

 and in a manner that defies detection otherwise than by 

 microscopic examination. 



When plants are introduced into a new country they are 

 rarely attacked by the fungi indigenous to that country. This 

 at first sight appears somewhat remarkable, but when it is 

 remembered, as I have already explained, that most of our 

 destructive parasites have become so highly specialised as 

 only to be capable of infecting a single kind of plant, or at 

 most a few closely allied plants, the reason becomes apparent. 

 The following interesting fact illustrates this point. Pelar- 

 goniums were originally introduced into this country from 

 South Africa; during their stay with us they have not been 

 attacked by the rust so common on our wild geraniums. 

 Quite recently a selection of the best varieties was returned 

 to South Africa, where they were promptly attacked by 

 Puccinia granularis, the rust common on pelargoniums in 

 South Africa. 



One marked exception to this rule has occurred in the case 

 of the very destructive coffee disease. When this plant was 

 introduced into Ceylon it was attacked by an indigenous rust 

 (Hemileia vastatri%), that occurs on two or three plants 

 belonging to the coffee family. So destructive did the 



