Nov. 2 * 9 . 1915 
Varietal Resistance of Plums to Brown-Rot 
379 
due probably to the greater number of spores being produced. This is 
not of considerable importance, however, except under extremely fav¬ 
orable weather conditions, when it may be the cause of a great deal of 
damage to fruits (Smith, 1889). A source of infection, common in com¬ 
pletely ripened fruits and not common to green fruits, is through wounds 
caused by the cracking of the plums. This cracking is due either to 
excessive rainfall after a dry period, causing a rapid increase in turgor 
with the consequent splitting of the fruit, or to water remaining between 
plums which are in contact. This effect was also noted when ripe plums 
kept in a moist chamber cracked where they were, in contact with the 
glass if water was present. 
The second reason for the ripe-rot effect is the fact that the ripe fruit 
of some varieties is much more susceptible to rot after infection takes 
place than the green ones (see p. 388). 
varietal resistance of plums to the fungus 
That plums and peaches vary in their resistance to brown-rot has been 
noted from time to time. This power of resistance has been ascribed to 
various causes, such as a thick skin in certain varieties of resistant plums, 
a small amount of down on resistant peaches, and late ripening of some 
varieties, with consequent avoidance of the disease because of temperature 
conditions. 
During the summer of 1913 attempts were made to determine whether 
definite differences in resistance to the brown-rot fungus really exist in 
plum varieties. Inoculation tests were started as early as June 14, 
when the plums were about one-third grown, and carried through on 
some varieties until maturity. Infection was brought about at first by 
spraying the plums with distilled water containing the spores. Later, a 
more effective method was found to be that of placing the plums in con¬ 
tact with moistened mummies well covered with spores. In both'cases 
the experiments were carried on under bell jars in the laboratory. 
RELATIVE RESISTANCE OF VARIETIES 
Table IV shows the relative resistance of varieties as determined by 
the inoculation of 262 plums through uninjured skin and the subsequent 
rotting of the fruits. 
The skin and flesh descriptions, except where indicated, were taken 
from a table prepared by Dr. M. J. Dorsey, of the Minnesota Experiment 
Station, in a study of “fruit characters'’ in hybrid plums, prepared inde¬ 
pendently of the investigations on resistance. The descriptions of vari¬ 
eties indicated by an asterisk (*) were made by the writer. 
