Nov. 29. 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). 



VARIETAI^ RESISTANCE OF PLUMS TO THB 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 191 3 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. 



