618 
Journal o f Agricultural Research 
Vol. XXXI, No. 7 
arose from these tips. Similar observations have been made on 
other fungi. Brown ( 9) found that Botrytis cinerea did not produce 
a substance in the germ tube which was capable of dissolving cuticle. 
Blackman and Welsford (6) found later that in the early stages of 
infection, B . cinerea effected its entrance into bean leaves solely by 
mechanical pressure exerted by the germ tube. It should be noted 
in this connection, however, that although first penetration of the 
cuticle is made by mechanical pressure, Boyle (7) found that as soon 
as the infection hypha gets within the cellular tissues, a chemical 
actio q takes place and cells die some distance beyond the invading 
hyphae, thus suggesting the secretion of an enzyme. 
INFECTION BY MEANS OF ASCOSPORES 
It has heretofore been the general belief that it was necessary for 
ascospores of the genus Sclerotinia to fall upon some dead organic 
matter and first establish themselves saprophytically in order to gain 
enough vigor to attack living plants. The work of Stevens and Hall 
{29) on S. libertiana and that of Beach (3) on S . minor indicated that 
direct infection by ascospores seldom, if ever, occurred. Pethy- 
bridge, in his investigations of the stalk or sclerotium rot of potatoes 
produced by S. sclerotiorum , writes {22 ): 
Although these spores or, perhaps, more correctly speaking, their germ tubes, 
'are incapable of entering normally vigorous green potato foliage and stalks, 
they are evidently capable of causing infection of the foliage when it is yellowing 
and ripening off, and probably are also capable of infecting the stalks at the scars 
remaining when the older leaves have fallen. During the past season at Clifden 
circumstances were very favourable for observations on the mode of attack of this 
fungus, and in very numerous instances it was abundantly clear that infection 
occurred first on one of the older yellowing leaves from which the fungus passed 
directly to the stalk which bore it. 
The writer has been able to show in laboratory experiments that 
freshly cut slices of turnip, carrot, and lettuce leaves held oyer 
“shooting” apothecia will become so severely infected that the whole 
surface of the host will be covered with white, cottony mycelium, 
and the tissues reduced to a watery mass within three to five days. 
The rate of growth is comparable with that produced by a direct 
transfer of a vigorous mycelium from plate cultures. Successful 
inoculation has also been obtained by spraying an ascospore suspen¬ 
sion in distilled water upon individual leaves of head lettuce. In 
this experiment, however, the growth of the mycelium was so general 
over the surface of the leaves that it was impossible to locate the 
original points of infection. These experiments show that ascospores 
can at least infect freshly-wounded tissues if not living cells. 
CROSS INbCULATIONS y 
As just pointed out, a study of the cultural characteristics and 
morphology of the various Sclerotinia cultures obtained from market 
produce indicated that the same strain of Sclerotinia attacks a wide 
range of hosts. Cross-inoculation tests were made to determine 
whether there were any physiological races or strains. In general, 
the experiments were made at room temperature, in diffused light. 
Fresh host or host tissues were obtained from the market and sterilized 
in mercuric chloride (1 to 1,000) for a few minutes, and then rinsed in 
sterile water. Inoculations were made in wounds and on unwounded 
