Nov. 29, 1915 
Varietal Resistance of Plums to Brown-Rot 
39i 
CONCLUSIONS 
(1) The brown-rot fungus in Minnesota seems to be identical with that 
found in other parts of this country and with Sclerotinia cinerea of Europe. 
Chlamydospore tufts vary in color from gray to bright ocher. For the 
production of the ascus stage the sclerotium apparently must be buried in 
the ground for two winters. Mummies which have hung on the trees for 
one year are still capable of producing apothecia. 
(2) Infection may take place through the uninjured skin at any time 
during the development of the plum fruit. The hyphse enter through the 
stomata and lenticels. Varieties show great differences in resistance to 
infection, owing to the production of parenchymatous plugs which fill the 
stomatal cavity and to lenticels made up of layers of corky cells through 
which the hyphse are unable to penetrate. Corky cells lining the stom¬ 
atal cavity merely delay infection. 
(3) Varieties show variations in resistance to rot after the hyphse have 
gained entrance. Resistance is apparently correlated with (a) a thick 
skin; (b) the production of parenchymatous plugs which fill the stomatal 
cavity; (c) the production of corky walls in the lining cells of the stomatal 
cavity; and (d) firmness of fruit after ripening. There seems to be no 
relationship between oxidase content of the fruit and resistance or be¬ 
tween tannin content and resistance. 
(4) Brown-rot is essentially a ripe-rot, affecting the plums most notice¬ 
ably as soon as they begin to soften slightly as a result of ripening. 
Varieties which are resistant remain firm on ripening. Softening during 
ripening is due to the solution of the middle lamella. 
(5) The hyphae of S. cinerea in the tissue of plum and apple fruit are 
entirely intercellular. The middle lamella is dissolved slightly in advance 
of the penetration of the hyphae. The absence of the middle lamella in 
fruits which have softened owing to ripening explains the greatly increased 
spread of the disease at ripening time. Attempts to demonstrate the 
presence of the middle-lamella-dissolving enzym, pectinase, in rotting 
fruits or to extract it from a culture of the brown-rot fungus on apple 
cider proved futile. 
(6) The rot caused by S. cinerea is a firm-rot due to the mechanical 
support of the hyphae which completely fill the intercellular spaces left by 
the collapse of the host cell walls. Penicillium expansum produces a soft- 
rot, because of the fact that few hyphse are produced and, therefore, little 
mechanical support is given to the rotted tissue, which as a consequence 
collapses as the rot proceeds. The hyphae of P. expansum are intercel¬ 
lular and produce a substance which dissolves the middle lamella even in 
the absence of the fungus hyphse. 
