Hold-Over Blight in the Pear 
3 
Inasmuch as there was no first hand information available on this 
subject for the higher altitudes, based upon actual laboratory examina¬ 
tions, the writer has undertaken a study of Hold-over Blight as it oc¬ 
curs in the arid climates, with the hope of adding something to our 
present knowledge, as well as obtaining more exact data than the well 
meant but too often unreliable facts given by casual observers. It 
seemed to me that if unfavorable conditions for the survival of bacteria 
existed anywhere, they should be found here, in Colorado. The dry 
winter winds, which often blow for days at a time, are notorious for 
their destruction of vegetation; the sun shines almost continually 
throughout the winter months and the temperature often falls as low 
as —16° F. to —22° F. It would be difficult, indeed, to find natural 
agents more inimical to microbic life than drying and direct sunlight 
combined with the possible injury by low temperatures. 
The investigation was begun in the summer of 1909, and has been 
carried on through the winters of 1909-1910 and 1910-1911. In order 
that we might have hold-over material with which to work which we 
knew contained the living germs in an active condition during the 
growing season, we visited certain orchards in the summer time where 
the blight was in progress and tagged a number of small limbs and 
twigs varying in size from 1 m.m. to 40 m.m. (3-8 to 1 1-2 in.) in 
which the blight was unquestionably present. Material from five 
different orchards has been examined. These orchards, scattered over 
a wide area, present climatic conditions representative of practically 
2000 square miles of fruit land on the Western Slope. 
METHODS 
Since it has been shown that the bacteria in the hold-over cankers 
are usually found in the zone just next to the living tissue, we always 
made it a point, when collecting the specimens to take such sections of 
the limbs as would include the lower boundary of the blighted, discolor¬ 
ed area together with a few inches of the adjacent live wood. 
Immediately befor making our isolations, each specimen was wiped 
thoroughly with a 1-1000 solution of mercuric chloride which was al¬ 
lowed to dry on the surface. This precaution was taken in order to 
eliminate as far as possible exterior contaminations. 
The outer bark was removed with a scalpel, sterilized by boiling, 
and with a second sterilized scalpel, a diagonal cut was made just 
above the junction of the blighted and healthy areas, through the 
cambium into the wood so as to include portions of both the diseased 
and healthy cambium. Small pieces of both the blackened and nor¬ 
mal cambium were removed with aseptic precautions, and dilution 
plates were made from this material in nutrient agar, + 15 0 Fullers 
Scale. The plates were kept at 20° C. and after four days, the char¬ 
acteristic Bacillus amylovorus colonies were isolated onto agar slants. 
In addition to verifying the cultures as B. amylovorus by cultural and 
