[Vol. 10 



24 ANNALS OF THE MISSOURI BOTANICAL GARDEN 



of rot lesions in the tubers varied considerably under the differ- 

 ent conditions of the artificial inoculation experiments. Lateral 

 spread from the points of inoculation seldom took place to the 

 extent it does in tubers invaded at the stem end and under nat- 

 ural conditions. The depth to which the lesions extended was 

 usually to a point considerably beyond the depth of the needle 

 prick, occasionally to the opposite side of the tuber. The char- 

 acter of the rot developed in the tubers inoculated artificially 

 varied somewhat. Sometimes the affected tissues were quite 

 moist and soft. Again, the tissues attacked were quite dry, be- 

 ing more or less spongy or cheesy in texture. Diseased material 

 exhibited a more or less putrescent odor but in no case did inoc- 

 ulation with pure cultures effect a profound, gray, slimy, obnox- 

 ious, gaseous, soft rot. Variations, such as those mentioned, ap- 

 peared to be associated to a considerable degree with (1) the 

 available sugar content of the tuber, (2) the moisture content 

 of the tuber, and (3) with certain environmental factors, nota- 

 bly temperature and humidity. The lesions in immature pota- 

 toes were generally more profound and the affected tissues softer 

 than those in ripe tubers, particularly of a late, hard, starch- 

 abundant variety A drop of inky black liquid was exuded at 

 the points of inoculation in many cases. Occasionally a slight 

 bulging of the tissues occurred at these points. When affected 

 tubers were cut or broken open the diseased tissues were charac- 

 teristically dark-colored, becoming brown or black upon exposure 

 to the air. 



Additional observations. — To extend the above observations 

 on pathogenicity of the various strains at hand, the artificially 

 inoculated tubers were cut into seed pieces and planted in pots in 

 the greenhouse. In many instances the seed piece was entirely 

 consumed by the blackleg rot before sprouts started. In a few 

 cases, however, sprouts developed, only to be rapidly invaded 

 by the pathogen and killed. The case of culture 160.1 is typical. 

 Four stalks emerged from the ground but by the time they had 

 grown to be from 3 to 6 inches tall (30 days after planting the 

 seed pieces), the parasite invaded the stem tissues. The infec- 

 tion extended rapidly upward. At first the diseased tissues (cor- 

 tical) appeared water-soaked. The tissues became dark-colored 

 over night, and in a day or two the plants were prostrated (pi. 

 1, fig. 3). What proved to be a pure culture (plates) was recov- 

 ered from this material. This was later used for inoculation work 

 and found to be pathogenic in tubers. 



