- 3 - 
D escription : • On - susceptible varieties of potato the disease is 
characterized by wart-like outgrowths' o-n the tubers or re- 
placing tubers at the end of stolons. These "warts" are 
more commonly developed from the eyes of tubers, less often 
on stolons or bases of stems. The warts are whitish at first 
but turn black and often become watery as they get older. 
When exposed to light at the ground surface, warts are green. 
Warts may be extremely small or may be larger than the tubers 
on which they are borne. 
On resistant varieties of potatoes infected cells may 
turn brown and fleck off without producing spores and when 
sori mature, they may be difficult to find even when ex- 
amined with extreme care. Such infections could not be found 
with certainty by inspection methods applicable to commercial 
shipments or even to small lots of potatoes. 
In structure S. endobioticu m is a very simple fungus, 
without mycelium. The first infections in the spring are 
produced by oval shaped one-ciliate zoospores about 2 mu 
in length which are formed when the resting spores or spo- 
rangia germinate. These first infections develop as sori 
each of which divides to form about five summer sporangia. 
Within a few days after infection takes place there may be 
an abnormal enlargement and multiplication of host cells to 
form a tumor or in highly resistant varieties there may be 
little enlargement of the host tissues. The summer sporangia 
germinate by the production of 200-300 oval shaped one-ciliate 
zoospores about 1.5 mu long. If these zoospores have been 
mature for two or three days before there is sufficient 
moisture to cause their release, a large part of them may 
act as gametes, gametes from different sporangia uniting in 
pairs to form zygotes. If the zoospores cause new infec- 
tions such infections develop as sori and produce summer spo- 
rangia, but infections produced by zygotes from resting 
sporangia. Resting sporangia become dark brown, thick walled, 
more or less globular and 50-70 mu in diameter. These rest- 
ing sporangia may remain viable for some time but usually 
germinate as soon after maturity as water is available whether 
that is within an hour or two or is delayed a week or more. 
Zoospores from resting spores may begin to disintegrate 
within a couple of hours or so after the sporangium ruptures 
if they are unable to infect a host and those from summer 
sporangia in less than an hour. Either zoospores or zygotes 
may infect wart tissue as well as normal host tissue. The 
enormous numbers of sporangia, particularly of resting spo- 
rangia, that may be produced during a season, and the ability 
of the resting sporangia to remain viable for years enable 
the fungus to survive ordinary crop rotations and cause heavy 
damage at times in spite of the fact that it is not wind 
borne, that the zoospores and zygotes which infect the host 
are able to travel very short distances by their own efforts 
and are short lived. 
