PHYTOPHTHORA EOT OF PEAES AND APPLES 1 
By Dean H. Rose, Pathologist, and Carl C. Lindegren, Field Assistant, Office 
of Fruit Disease Investigations, Bureau of Plant Industry, United States Depart¬ 
ment of Agriculture 
INTRODUCTION 
On September 5, 1923, the junior 
author collected on the South Water 
Street Market, Chicago, Bartlett pears 
from Fennville, Mich., which showed 
black spots covering about half the 
pear. On the surface of these spots 
there was a scanty growth of mycelium 
(pi. 1, A). When the pears were cut 
through at these spots the flesh under¬ 
neath was found to be for the most part 
very light brown in color but markedly 
darker , brown in the vascular system 
(pi. 1, B). Mounts of the surface fun¬ 
gous growth under the microscope 
showed numerous conidia resembling 
those of Phytophthora, and plantings 
from the diseased flesh yielded a fungus 
which agreed in all important respects 
with published descriptions of Phytoph¬ 
thora cactorum. The pathogenicity of 
the fungus isolated from these pears 
and from apples and other pears col¬ 
lected later has been proved by inoc¬ 
ulation into healthy pears and apples. 
Further examination of fruits on the 
Chicago market showed that the dis¬ 
ease is more widely distributed in the 
United States than had hitherto been 
suspected; a study of the fungus in 
culture and on the fruit brought out 
a number of facts concerning its mor¬ 
phology which seem not to have been 
observed by other workers. The pres¬ 
ent paper summarizes the results of this 
work and gives in addition a fuller de¬ 
scription of the disease than is available 
in earlier publications. 
DISTRIBUTION AND FIELD OBSER¬ 
VATIONS 
Following the collection of Phytoph¬ 
thora rot on the Chicago market on 
September 5, other collections were 
made as follows: 
On September 11 a shipment of 
Clairgeau pears from St. Joseph, Mich., 
was seen on the Chicago market which 
showed a loss of about 30 per cent from 
a rot similar to that described above. 
It was learned later that the entire 
shipment consisted of windfalls. 
On September 24 Anjou pears and on 
September 25 Clairgeau pears were 
found on the Chicago market which 
showed the symptoms of Phytophthora 
rot. Nothing was learned as to the 
origin of either of these lots of pears. 
It is probable, however, that they also 
came from Michigan, since they had 
been shipped in bushel baskets and 
Michigan was at that time the only 
Middle Western State shipping Anjou 
and Clairgeau pears in baskets into 
Chicago. Both varieties, if shipped 
from California or the Northwest, 
would have been packed in boxes. 
On September 27 several orchards 
near St. Joseph, Mich., were visited 
and numerous cases of Phytophthora 
rot were found, on both pears and 
apples. The decay was found only on 
windfalls, never on fruit hanging on 
the tree. In some instances the wind¬ 
falls under Clairgeau pear trees showed 
100 per cent infection, particularly 
those under trees where there were in¬ 
dications that water had been standing 
on the soil. Moreover, pears which 
looked sound when viewed from above 
as they lay on the ground were usually 
found to have a black spot on the lower 
side if the ground was moist to the 
touch. 
The varieties of pears found to be 
affected with Phytophthora rot were 
Clairgeau, Kieffer, Bose, Howell, Bart¬ 
lett, Anjou, and Angouleme (Duchess). 
The disease was also found to be serious 
among the windfalls on the following 
varieties of apples: Twenty-Ounce 
Pippin, Northern Spy, Tolman, Bald¬ 
win, Winesap, Jersey Sweet, Wealthy, 
Gideon, Rhode Island Greening, and 
Tompkins King. Cultures of a Phy¬ 
tophthora were obtained from all the 
varieties and all the collections men¬ 
tioned above. 
The influence of soil moisture was 
noted in two Kieffer pear orchards, 
both of which had been kept in clean 
culture. -In one of these the soil was 
moist to the touch and practically all 
of the fruit on the ground showed the 
typical symptoms of Phytophthora rot. 
In the other orchard the surface soil 
i Received for publication June 16,1924; issued, May, 1925. 
Journal of Agricultural Research, 
Washington, D. C. 
( 463 ) 
Vol. XXX, No. 5 
Mar. 1, 1925. 
Key No. G-449 
