234 Transactions British Mycological Society. 



the Weymouth pine, instead of relying on their own resources. 

 In those days there was no adequate system of plant disease 

 inspection at the ports, and it was subsequently found that 

 this European nursery stock introduced with it the deadly 

 blister rust fungus which began to spread to the indigenous 

 Weymouth pines before measures were taken to combat it. 

 This disease is now one of the most serious that American 

 foresters have to contend with, and its introduction shews the 

 folly of allowing cultivated plants to be distributed throughout 

 civilised countries without safeguards to prevent the transmis- 

 sion of disease. The blister rust of the Weymouth pine is un- 

 fortunately common in this country, and is the chief reason 

 why this tree is not used more extensively for afforestation. 

 Epidemic disease on these pines is here dependent upon the 

 presence of an alternate host, Ribcs spp., because the fungus 

 cannot propagate itself directly from pine to pine. On the 

 other hand the presence of pines (apart from an initial source 

 of infection) is not necessary for the continuation of an epidemic 

 on currant bushes. 



Again, when a new plant is introduced into a country, a para- 

 site often attacks it violently although the pest has only been 

 found hitherto sporadically upon related indigenous plants. 

 Coffee was introduced into Ceylon as a plantation crop about 

 1870, and an important industry sprang up in connection with 

 it. Soon, however, it began to be attacked by a fungus disease 

 {Hemileia vastatrix), which up to that time had only occurred 

 to a slight extent upon jungle plants closely related botanically 

 to coffee. With large numbers of plantations in close proximity 

 to each other, the disease spread rapidly and became so un- 

 controllable that coffee-growing in the island became un- 

 profitable. Heavy losses were incurred, and a few years after 

 the commencement of the epidemic, coffee ceased to be grown 

 on a commercial scale in that part of the tropics. That is an 

 illustration of the danger of disease breaking out in epidemic 

 form upon a newly introduced economic plant, the disease itself, 

 although indigenous, being of negligible importance until the 

 appearance of the specially susceptible plant. 



Epidemic diseases of plants are disseminated in various ways. 

 Most of the parasites produce spores which can retain their 

 vitality for considerable periods unless exposed to extremes of 

 temperature, insolation or desiccation. Many of these diseases, 

 therefore, can be distributed over relatively wide areas by means 

 of wind currents. With such a disease as potato blight this is 

 undoubtedly the chief means by which the fungus is spread in 

 an epidemic condition. Upon first development of the fungus in 

 an active or sporing state in any season, the disease in its initial 



