1893.] PUBLIC DOCUMENT — No. 33. 235 



during winter and spring. That this practice is exceedingly 

 weakening to the plants, and renders them more susceptible 

 to the attacks of disease-producing organisms, cannot be 

 doubted. The more rational practice of some growers, of 

 giving the plants a rest in the cold frame during a part of 

 the year, cannot fail to produce more sturdy plants ; and it 

 may be doubted if it is not in the long run more profitable, 

 when all factors are taken into account. 



III. The Black Knot of the Plum. — Plowrightia morhosa 



(Sz.) Sacc. (Plate V.) 



Work on other diseases has crowded out much which has 

 been planned in continuation of that already reported * on 

 this important trouble, but a few words may be added, 

 especially in regard to the practical treatment of the dis- 

 ease. During the past year cultures with the ascospores 

 and pycnospores of the fungus have been carefully repeated, 

 with results in every case identical with those reported in 

 our previous account. On bread saturated with an infusion 

 of prunes, pycnospores, taken from pycnidia developed from 

 ascospores on prune gelatine, produce a very luxuriant 

 mycelium. This mycelium, at first white, soon assumes a 

 salmon-red tint, and later becomes black. The first pycnidia 

 on such a culture were ripe in five days from the sowing of 

 the spores ; but others continued to form until the entire 

 surface of the substratum was covered by a compact pycnidial 

 crust. This pycnidial form seems to be very rarely devel- 

 oped under natural conditions, but there can be no doubt 

 that it belongs to the life cycle of the " black-knot" fungus. 

 It seems very probable that some relation exists between its 

 suppression and the facts next to be detailed, but this is not 

 the place for the discussion of such probabilities. Careful 

 experiments have been made as to the power of the pycno- 

 spores to infect the living tissues of the host-plant. It is 

 clear that one or more of the spore forms of the fungus must 

 possess such power, and it was hoped that experiments with 

 each of the known forms might before now have been made. 

 But this has not been possible. In case of the pycnospores 



* Eighth Report Massachusetts Experiment Station, pp. 200-210, and PI. I. 



