PLANTS AS DISEASE PRODUCERS 315 



smuts are carried by the single fruits of various grains. In the aecial 

 stage of the cedar-apple fungus, Gymno sporangium juniperi-virginiance, 

 the spores are set free during dry weather at a time when they are most 

 likely to be wind-carried.^ The spores of the water molds are carried 

 by currents of water and those of the cranberry gall due to Synchy- 

 trium vaccinii. The motile zoospores of the damping-off fungus need 

 water for their dissemination. The spores developed during the 

 Sphacelia stage of the ergot fungus on rye are carried by insects. The 

 formation of the conidiospores is accompanied by a sweet substance, 

 the so-called honey-dew, which is much relished. Birds, especially 

 woodpeckers, disseminate the spores of the chestnut-blight fungus, 

 Endothia parasitica, and in a great many different ways man is active. 



EPIPHYTOTISMS (EPIDEMICS) 



When a plant disease becomes virulent, rampant and aggressive, 

 spreading rapidly from place to place, it is said to be epiphytotic 

 (epidemic). A number of such epiphytotisms (epidemics) have oc- 

 curred and the destruction due to some particular plant disease has 

 been enormous. The potato crop in the British Isles during the 

 summer of 1845, owing to a high temperature and abundant rains, 

 suffered entire destruction in the short space of a fortnight. This was 

 due to the ravages of Phytophthora infestans, an oomycetous fungus, 

 whose spores in wet weather produce numerous infecting motile 

 zoospores. The destruction of the potato crop led to the repeal of the 

 corn laws of England, and as a sequence, the inauguration of a free trade 

 policy. The Irish famine was the direct result and thousands of the 

 natives of the Emerald Isle emigrated to America. With respect to the 

 disease known as peach yellows Dr. Erwin E. Smith writing in 1891^ 

 says: "Formerly this disease was confined to a small district on the At- 

 lantic Coast, but during the last twenty years it has invaded distant 

 regions hitherto free, and has entirely ruined the peach industry over 

 very considerable areas. Within ten years the disease has taken fresh 



1 Heald, V. D.: The Disseminations of Fungi Causing Disease. Trans. 

 American Microscopical Society, xxxiii: 5-29, June, 1913. 



2 Smith, Erwin F. : Additional Evidence on the Communicability of Peach 

 Yellows and Peach Rosette, Bull, i, Div. of Vegetable Pathology, U. S. Dept. 

 Agric, 1891. 



