Oct. II, 1915 Phytophihora infestans in Irish Potato 87 



may have been that spores were present somewhere on the cut surfaces, 

 but they were not sufficiently abundant to be found even after long and 

 careful search. 



On June 20, 20 more of the 183 seed pieces were dug up and examined, 

 but again neither conidiophores nor spores of the fungus could be found. 

 The cut surfaces of the seed pieces in every case had either corked over 

 or started to decay. 



No mycelium could be found growing free in the soil about the diseased 

 tubers. No evidence was obtained showing that the fungus continues 

 to sporulate on the seed tubers in the soil. Spores are produced abun- 

 dantly on the cut surfaces of tubers recently planted in moist soil only, 

 but these disappear in the course of a week or 10 days. In an earlier 

 part of this paper it has been shown that spores may be borne in consid- 

 erable abundance on sprouts killed before they reach the surface of the 

 soil. Whether these spores ever function in infecting other potato tissue 

 below the surface of the soil has not been shown definitely by any of the 

 earlier workers or by any of the writer's experiments. 



There is still another possible source of conidial infection that should 

 be mentioned in this connection. A common practice in northern Maine 

 and other potato-growing sections is to feed the culls to hogs or to dump 

 them in some out-of-the-way place about the farm. In the culls there 

 are usually a considerable number of tubers infected with late-blight. 

 When the skin is ruptured on these, the fungus may fruit. Spores borne 

 in this way may reach potato foliage and lead to infection. Then again, 

 as observed by the writer in numerous cases, tubers infected with late- 

 blight are often dumped in some wet or swampy place on the farm. In 

 two such cases an infection of late-blight was found on the mass of 

 growing plants as early as July 25 and 29. It was impossible to deter- 

 mine how and where the infection originally started, but it was clear 

 that the disease had made a good beginning. It is, of course, needless 

 to say that if such cases developed near a potato field, it might readily 

 become infected. 



Whatever may be the possible relation of the conidia to the renewal 

 of epidemics of P. infestans, two points are perfectly clear: (i) That 

 spores are borne in the soil on the freshly cut surfaces of infected seed 

 and on sprouts when the soil is sufficiently moist and (2) that the spores 

 probably do not remain viable more than two to three weeks in the soil. 



rate; of spread of the; myce;uum of phytophthora infestans in 

 the potato stem 



The rate of spread of infection in the potato stem is of interest because 

 of its direct bearing on the growth of the mycelium from the diseased 

 tuber up through the stem. Healthy plants from 20 to 55 cm. high 

 were exposed to infection with P. infestans by spraying a spore suspen- 



