88 
Journal of Agricultural Research 
Vol. XXIX, No. 2 
PROOF OF PATHOGENICITY 
The pathogenicity of the organism 
was established by its constant asso¬ 
ciation with and isolation from lesions 
and by numerous inoculation tests 
made with artichoke and other vege¬ 
tables. 
Vigorous fresh-appearing buds, free 
from blemishes, were used. They 
were prepared for the test by making 
a fresh cut of the stem; by thorough 
washing; by dipping into 1:1,000 
mercuric chloride solution for about 
a minute followed by thorough rinsing 
in sterile water. Longer immersion 
in corrosive sublimate solution was 
found to cause discoloration of the 
tender scales. Preliminary experiments 
and the controls proved the method 
adequate to remove or kill any spores 
on the buds. 
To duplicate the method of inocula¬ 
tion which seems to occur naturally, 
both spores and mixtures of spores and 
mycelium were used as inocula. The 
spores were suspended in water and 
the suspension either poured or sprayed 
on the buds. These methods, es¬ 
pecially the latter, approached as 
nearly as possible to the manner of 
inoculation in the field. Mycelium 
inoculation was made by inserting 
agar inoculum, or infected scales 
which generally had some mycelium 
on their surfaces, between the scales 
of sound buds. This duplicated the 
mode of inoculation involved when the 
disease spreads in transit. After in¬ 
oculation the buds were kept in moist 
chambers or covered battery jars. 
Inoculation experiments with in¬ 
fected scales demonstrated that the 
fungus can produce decay throughout 
its temperature range. As the buds 
freeze at —2° C., no tests were made 
below' this temperature. None of the 
cultures which had been grown on agar 
for some time produced appreciable 
decay at —2°, w r hereas strain No. 1610, 
recently isolated from ‘Artichokes, pro¬ 
duced decay. Four scales of a bud 
inoculated with this strain became in¬ 
fected to the extent of 6 sq. cm. after a 
week. The presence of the fungus in 
the advancing edge of the lesions was 
proved by microscopic examination 
and plate cultures. At —1° all strains 
produced decay. The percentage of 
successful inoculations increases stead¬ 
ily up to the optimum temperature of 
22° to 25° C. Failures were infrequent 
at this temperature but increased rap¬ 
idly above 24°; occurred in about 85 
per cent of the attempts at 26°; and 
lesions were obtained but rarely be¬ 
tween 26° and 32°. 
Although infection was more con¬ 
sistently and rapidly obtained through 
wounds when mycelium was used as 
inoculum, the experimental evidence 
shows that wounds are not a necessity. 
The inoculum was applied at the tips, 
the middle, and the bases of the scales, 
as well as at the cut surface of the 
stem, and consisted of mycelium taken 
from agar plates (PI. 1, C). Similar 
results were obtained by using infected 
scales as inoculum. Seventeen tests 
at 0° C. gave 12 lesions on wounded 
buds and 2 on unwounded buds. Fif¬ 
ty-one tests at 7° gave 46 lesions on 
wounded buds and 43 on unwounded 
buds. Fifty-one tests at 22° gave 48 
lesions on wounded buds, also 48 on 
unwounded buds. Eight tests at 30° 
gave five lesions on wounded buds, none 
on unwounded buds. It was difficult 
to keep the humidity high enough to 
produce infection at the higher temper¬ 
atures. Unfortunately, no apparatus 
was available for making controlled 
humidity tests. 
Except at very low temperatures, 
inoculation and infection by contact 
seem limited much more by moisture 
than by temperature or by the presence 
of wounds. With adequate moisture, 
infection took place and was quite 
independent of wounds, except at very 
low temperatures. A saturated or 
nearly saturated atmosphere induces 
very rank development of mycelium, 
which, however, as in plate cultures, 
seldom sporulates unless subjected to 
a change in humidity. It was necessary 
to wrap each bud in waxed paper for 
keeping in moist chambers in the 
laboratory, in high-temperature in¬ 
cubators, or in incubators cooled bv 
mechanical means which freeze out 
the air moisture. In low-temperature 
incubators, cooled by melting ice, 
wrapping was not necessary. 
Experiments with spore suspensions 
gave negative results except on 
wounded tissue. Moisture was again 
a limiting factor, but even more de¬ 
cisively. In early experiments no 
infection was obtained at any tempera¬ 
ture with spore suspensions sprayed 
or poured on wounded or unwounded 
scales. Wrapping was entirely inade¬ 
quate when spore suspensions were 
used, but the difficulty was finally 
solved by fogging the chambers con¬ 
taining the buds once every 24 hours 
during the first three or four days. 
This was done by spraying water into 
the chambers with an atomizer. It 
was a very unsatisfactory method be¬ 
cause there was no way of determining 
just what degree of humidity is essen¬ 
tial for spore inoculation; but it dupli¬ 
cated field conditions in that the air 
