Nov. 2 9 , 1915 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: 
Variety. 
B X W16 
B X W2 . 
Burbank. 
B X W15 
Topa. 
Opata.... 
Number 
of 
plums. 
Points 
contact. 
Number of 
infection 
spots. 
1 
I 
Many. 
I 
I 
Do. 
I 
I 
20. 
4 
6 
None. 
1 
z 
1. 
1 
1 
Many. 
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 OF SKIN THICKNESS TO RESISTANCE 
In order to determine the part played by thickness of skin in resistance, 
inoculations were made by cutting off a small piece of skin and planting 
