REPORT OF THE SOCIETY 85 



surface. In this condition it is soft, easily crushed and pressure causes the 

 exudation of a brownish watery liquid. In advanced stag'^s the symptoms 

 might frequently be mistaken for those of tuber r^^ts caused by Fusarium 

 species. 



Life History of the Fungus 



Orton has shown that a similar rot may be produced by Rhizopus nigricans 

 Ehrenb but the work of Link demonstrates that "Leak" is usually caused by 

 Pythi'im deharyanum Hesse. 



This fungus is aggressively parasitic if humidity and temperature conditions 

 are satisfactory and is the common cause of ''damping off" in green houses and 

 nurseries. The myceUum is coenocytic, except with age, irregularly branched 

 and rather coarse: Conidia are produced terminally on branches of the myce- 

 lium or they may be intercalary. The}' are globose to elliptic and average 22 

 microns in diameter. They may germinate at once by one or more germ tubes. 

 Oospores are also produced which are smooth, spherical and thick-walled and 

 able to live over an adverse period. Growth is slow at 48 deg. F. and practi- 

 cally ceases at 41 deg. F. while it is best at about 86 deg. F. In an experimental 

 inoculation Link found that at 86 deg. F. the fungus penetrated to a depth 

 of 4 cm. (134 ins.) in 67 hours. 



Varietal Susceptibility 



Rurals and Burbanks dug during warm weather are especially susceptible 

 and inoculation experiments by Link tend to show that Triumph, Green ^Vlount- 

 ain, Early Ohio. Rural New Yorker and Irish Cobbler are susceptible. 



Control 



The disease can be controlled by care in harvesting and handling potatoes 

 and by sorting out wounded tubers. 



(c) Late Blight and Rot 



This disease, caused by Phytophthora infestans (Mont.) De Bary, is too 

 well known to need emphasizing as to its economic importance. It is not often 

 that a fungus can materially affect the poUcy of a country but this is what 

 Phytophthora infestans did in Great Britain. Late blight was so serious in 1845 

 in England and Ireland that the potato crop was a failure. So much was this 

 the case in Ireland that a famine occurred and many thousands of Irishmen left 

 Ireland for America. To relieve the distress the Corn Laws were repealed and 

 in a sense this initiated a Free Trade policy. 



