Oct. ii, 1915 
Phytophthora 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: (1) 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 MYCELIUM 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- 
