Mar. 1, 1925 
PTiytopTithora Rot of Pears and Apples 
465 
showed the characteristic symptoms of 
Phytophthora. The fungus was recov¬ 
ered from two of these pears by cul¬ 
turing. All of the others were cut up 
and were found to show the character¬ 
istic vascular darkening. 
EXPERIMENT 3 
Some of the clay soil from an orchard 
at St. Joseph, Mich., was placed in a 
moist chamber and sterile water was 
poured over it until water stood on the 
soil to the depth of about 2 cm. Six 
sterilized pears, apparently free of skin 
punctures, were then placed in the 
moisk chamber on the soil. All of these 
pears developed Phytophthora rot, and 
pure cultures of the organism were 
obtained from every one of them. The 
first pear developed the rot on the 
thirteenth day, two more on the twenty- 
first day, and the remaining three on 
the twenty-second day. Similar re¬ 
sults were obtained in another experi¬ 
ment which duplicated No. 3, except 
that apples were used instead of pears. 
Controls provided in both experiments 
remained sterile. 
Nothing is known of how infection 
occurred in the moist chambers con¬ 
taining pears resting on soil covered 
by water. Quite evidently the fungus 
existed in the soil and continued to 
grow there even though entirely sub¬ 
merged. It is possible and indeed 
quite probable that the fungus grew 
out into the water; but whether the 
infection which occurred was brought 
about by zoospores or directly by the 
mycelium is a question to which the 
experiments and the observations made 
on them give no answer. 
The results obtained in these experi¬ 
ments are strong evidence that the 
Phytophthora isolated from apples 
and pears is able to penetrate the 
uninjured skin of those fruits. Evi¬ 
dence to the same effect was seen in 
the orchards visited in Michigan. 
MORPHOLOGICAL FEATURES 
Twenty of the isolations from dis¬ 
eased pears and apples, including the 
three from Washington and Idaho 
apples, were subjected to comparative 
study after they had grown for 9 to 70 
days on potato dextrose agar, on 
SherbakofTs oatmeal agar, and on 
oatmeal paste. On potato dextrose 
agar they all produce conidia (pi. 2, 
A and B) and oogonia (pi. 2, C to H) 
in abundance and also large numbers 
of bodies which are very similar to 
the “sphaero-conidia” (pi. 2, I to L) 
described and figured for Phytophthora 
19974-25f—6 
cactorum by Lafferty and Pethybridge 
(6, p. 37 and pi. II, fig. 13). On oat¬ 
meal agar they produced large numbers 
of conidia and oogonia, but very few 
“sphaero-conidia”; in some cultures 
there were none of these bodies. 
Measurements of the various spores 
forms were made for three cultures, 
which so far as could be told by exami¬ 
nation with the microscope were rep¬ 
resentative of the whole 20. One of 
these cultures had been isolated from 
the Delicious apple from Washington, 
one from a pear from Michigan, and 
one from an apple from Michigan. 
The measurements can be summarized 
as follows: Oospores, 15 to 43 p in 
diameter, average 26.6 p; conidia, 
length, 25 to 44 p, average 34.4 p; 
conidia, width 21 to 31 p, average 26.8 
p; “sphaero-conidia,” 25 to 45 p 
in diameter, average 34.6 p. All of 
these are in close agreement with the 
constants given for the spore forms 
of Phytophthora cactorum by other 
workers. (Rosenbaum (11) Beach (1) 
and measurements quoted by the 
latter from other papers.) 
The conidiophores of this fungus 
have been found as (1) short branches 
of the mycelium bearing a single 
conidium, or (2) longer branches 
bearing conidia in groups of 2 to 10 
or more (pi. 2, M). The largest 
number of cji.idia seen in a single 
group was 15. Both kinds of branches 
arise from the mycelium in the manner 
shown for one of the longer branches, 
in the lower portion of Plate 2, M. 
All three of the spore forms men¬ 
tioned above were found on the surface 
of both pears and apples collected on 
the market. They also appeared on 
the surface of pears and apples inocu¬ 
lated in the laboratory, usually within 
six or eight days from the time of inoc¬ 
ulation. Favorable conditions for this 
external growth seemed to be a moist 
atmosphere and a temperature of 20° 
to 25° C. or even slightly higher. A 
few oogonia were seen in mounts of 
diseased tissue from the inside of a 
number of fruits. 
Measurements of the various spore 
forms as found on diseased fruits can 
be summarized as follows: Oospores 
20 to 35 p in diameter, average 26.6 p; 
conidia, length 30 to 53 p, average 40.3 
p; conidia, width 19 to 32 p, average 
36.9 p; sphaero-conidia, for the few 
that were seen, 26 to 38 p in diameter 
average 32.3 p. 
It will be seen that the conidia are 
slightly longer on the host than on 
culture media but that the other two 
spore forms average about the same 
size no matter where they are produced. 
