Nov. 29, 1915 Varietal Resistance of Plums to Brown-Rot 39 1 



CONCLUSIONS 



(i) The brown-rot fungus in Minnesota seems to be identical with that 

 found in other parts of this country and with Sclerotinia cinerca 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. IVIummies 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 hyphae 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 hyphae are unable to penetrate. Corky cells lining the stom- 

 atal cavity merely delay infection. 



(3) Varieties show variations in resistance to rot after the hyphae 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 ijiotice- 

 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 5. 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 5. 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 hyphae 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 hyphae. 



