Varietal Resistance of Plums to Brown-Rot 



381 



were inoculated by contact with mummies in a moist chamber, the 

 following results were noted after 27 hours: 



Another trial with BXW15, directly following this and carried on 

 under the same conditions, showed a few infection spots in three out of 

 five contact points, indicating that in some cases the fungus can enter 

 these resistant plums. A number of other experiments, comparing the 

 relative resistance to infection of BXW15 with that of other varieties, 

 showed results comparable to those given above. 



Soon after infection takes place a small decayed spot appears on 

 the surface of the plum. These spots increase in size rapidly in the 

 susceptible varieties and soon completely cover the plum. This often 

 requires not longer than 24 hours after infection has taken place. On 

 the resistant forms, however, the spots increase in size slowly, some- 

 times taking several days before they entirely cover the plum. The 

 rapidly rotting plums take on the characteristic brown color of rotten 

 fruit; but the slower rotting varieties often become dark blue and when 

 completely rotted become black. 



Usually when the susceptible varieties are one-half to three-quarters 

 rotted, they begin producing tufts of chlamydospores over the rotted 

 area. On the sand cherry and some of the sand-cherry hybrids, which 

 are very susceptible, the spore tufts are usually large and numerous 

 (PI. XXXVIII, fig. 9). Varieties such as B X W21 , which appear interme- 

 diate in the rapidity with which they rot, usually produce spore tufts, but 

 they are nearly always smaller and less numerous than those on the 

 susceptible varieties (PI. XXXVIII, fig. 7 and 8). In the case of the 

 most resistant varieties it is seldom that spores are produced if the skin 

 has not been broken. If the plum has been wounded, spores are usually 

 produced through the wound (PI. XXXVIII, fig. 6). Under particularly 

 favorable conditions pustules may appear through the uninjured skin, 

 in which case they are usually small, and few in number. 



RELATION OP SKIN TmCKNESS TO RESISTANCE 



In order to detertnine the part played by thickness of skin in resistance, 

 inoculations were made by cutting off a small piece of skin and planting 



