378 Journal of Agricultural Research voi. v, no. 9 



well covered with germinating spores, no instances were found in which 

 the germ tubes gained entrance directly through the cuticle. 



Further evidence that the germ tubes do not usually penetrate the 

 cuticle was obtained when two green plums of B X W 15, a very resistant 

 variety, were scraped lightly with a sharp knife, thereby removing the 

 cuticle without otherwise injuring the epidermis, and were then inocu- 

 lated. These, with seven others of the same variety which had not been 

 so treated, were sprayed with distilled water containing chlamydospores 

 and put under a bell jar. At the end of 58 hours the two plums which had 

 been scraped showed 10 and 13 spots, respectively, but rotted very 

 slowly from the infection points. The seven unscraped plu'ms were at 

 this time without infection spots, but eventually three of these showed 

 evidences of infection. 



Because of this method of infection, resistance can not be attributed 

 entirely to morphological differences in the epidermis of the varieties. 

 There are however, certain morphological differences in the stomata and 

 lenticels which contribute to resistance, the nature of which will be dis- 

 cussed later. When once the fungus has gained entrance the plums 

 always rot more or less rapidly, depending upon the variety. 



FIELD OBSERVATIONS 



It is apparent from the facts given that the small amount of rot found 

 in the orchard on green plums is not due to any greater resistance to 

 infection which the green fruit may possess over ripe fruit. Neverthe- 

 less, the brown-rot in the orchard causes the greatest damage as a ripe- 

 rot rather than as a green-rot. 



It is a fact of considerable importance that it is not until the plums 

 are ripe and begin to soften slightly that the fungus does its greatest 

 damage as a ripe-rot. This is due probably to two reasons. The first 

 is that there are greater possibilities of infection at this time. Field 

 observations show that green plums will rot on the trees, owing usually 

 to infection through curculio or other wounds, and that the rot will 

 spread from one to another where they are in contact. Thus the number 

 of rotted fruits and hence of infection sources to the ripe fruit is grad- 

 ually increasing. Although there are other methods of infection, the 

 largest number in ripe fruit is due directly or indirectly to contact with 

 rotten green plums. It is very common in the field to find large groups 

 of plums on a tree completely rotted, while other groups on the same 

 tree are entirely free from rot. In these groups it is nearly always 

 possible to trace the original source of infection back to one plum which 

 has in most cases been infected through a wound of some kind while still 

 green. 



Another source of infection, more common in ripe or nearly ripe fruits 

 than in green fruits, is direct infection from spore suspensions in water, 



