Oct. II, 191S Phytophthora infestans in Irish Potato 95 



tubers produced young plantlets when they were allowed to sprout in 

 comparatively dry soil. 



De Bary (4) describes a case which is interesting in this connection 

 and ser\'es to emphasize the importance of moisture conditions. A 

 potato plant was found which had become infected by P. infestans in the 

 parent tuber. Portions of the stem just above the surface of the soil 

 were infected and discolored, but dry weather prevented the fungus from 

 progressing farther in the tissues or sporulating. This was surely a, case 

 where moisture checked the fructification of the fungus. Two similar 

 cases, which are even more striking as showing the close relation of 

 moisture and development of the fungus, are described in this paper. In 

 these the fungus grew up the stem to the surface of the soil and infected 

 the foliage, but the hot, dry weather checked its further spread. 



It is not necessary that the optimum conditions for the growth of the 

 fungus should prevail continuously. This is clear from the author's 

 experiments where the tubers were started in dry soil and later trans- 

 ferred to wet soil and the fungus grew up the stem. Too much emphasis 

 can not be placed upon the importance of environmental factors and the 

 state of germination of the tuber in the production of diseased plants from 

 seed infected with P. infestans. A combination of these three conditions 

 is not always prevalent in the open nor in the ordinary greenhouse, which 

 may well account for the accumulation of negative data. In this con- 

 nection may be cited one of several experiments where over 300 tubers 

 were planted in a greenhouse, where the moisture and temperature could 

 not be readily controlled, and not a single infected plant was obtained. 

 Clinton (8), Pethybridge (24, 25), and many others have reported similar 

 results from extensive field trials. 



In closing this portion of the discussion it should be pointed out that 

 not all infected tubers give rise to infected shoots and become centers of 

 foliage infection. In fact, only a small proportion function in this way, 

 according to the studies of the author; nor has any method been worked 

 out whereby an infected tuber can be made to give rise to infected plants 

 such as are shown in PlatesVIand VII. Whether the progeny of a diseased 

 tuber will or will not become infected is determined by the response of the 

 fungus and host, coupled with environmental conditions. It is known 

 beyond all possibility of doubt, however, that a certain proportion of the 

 diseased tubers planted under field conditions may produce progeny 

 which becomes infected by the mycelium growing up the stem. Once 

 above the surface of the soil, the fungus may sporulate and cause foliage 

 infection on the initial and adjoining hills. Infection spreads rapidly 

 from such an infection center and is the forerunner of an epidemic. 

 Hecke (12) has also noted this early stage in the development of an 

 epidemic. It seems logical to assume that these infection centers start 

 from planted infected seed potatoes. 



