114 
ALBAN STEWART 
there, only two or three small knots were discovered upon the trees. 
The choke cherry and wild plum, P. americana Marsh., are infected 
in the vicinity of Madison, Wis., but no other species so far as was 
observed by the writer, although Professor Gilbert found an infection 
on the pin cherry in one instance. The wild black cherry grows 
here, sometimes in badly infected thickets of choke cherry bushes, 
but it is not infected. It is evident that there is either a great differ- 
ence in the susceptibility of species of Prunus in different localities, 
or there is a different physiological form of fungus for each species. 
Knots caused by this fungus arise from two kinds of infections^ : 
primary, those which result from the infection of the host plant by 
spores, and which reach their full development the year following the 
infection, and secondary, those which result from the spreading of the 
fungus through the tissues of the host from knots already formed. 
Knots arising from primary infections usually occur on young 
branches either of the current year's growth or on those not over two 
or three years old. They do, however, occur at times on much older 
branches, as instances were noticed where knots had been formed on 
branches a centimeter or more in diameter and which had six or seven 
rings of annual growth. If cross sections of young branches are 
examined, after they have begun to swell, it will usually be found that 
the xylem is greatly altered into, or nearly to, the pith at the distal 
end of the knot, while the proximal end will have quite a zone of 
normal xylem surrounding the pith. From this one would judge that 
most of the primary infections take place at about the time, or shortly 
before, the cambium begins its activity in the spring, which is about 
May I at Madison. 
The external modifications following secondary infections begin to 
appear two or three weeks after the first conidia are formed. These 
are shown first by a cracking open of the bark and a slight swelling of 
the stem beyond the knot, which spreads farther as the season ad- 
vances. The infection apparently extends beyond the swollen area 
at times but in such instances the extent of the disease can usually be 
determined by examining cross sections of suspected areas with a 
hand lens, since the tissue in such areas usually has a light brown 
iln this article the effect of the disease and not the cause is being considered. 
On this account the terms infection and infected will be used in a rather broad sense, 
and it is to be understood that the fungus was not always found in contact with the 
cells or tissues in question. 
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