Oct 8, 1941 Blackleg Potato Tuber-Rot under Irrigation 87 



INOCUIrATlON OF HEALTHY TUBERS IN THE LABORATORY 



Over 60 tubers both new and old of the Netted Gem as well as of the 

 Irish Cobbler varieties were inoculated in small lots at dififerent times 

 with three strains of the western decay bacillus. When inoculations 

 were made in wounds of any kind, whether on the side of the tuber or at 

 either end of it, the results were invariably positive. The progress of 

 the decay was much slower when uninjured potatoes were inoculated. 

 In these latter cases the organism penetrated either through the eyes or 

 through the young growing sprouts. If the infected potatoes are re- 

 moved from the moist chamber after the decay has made considerable 

 headway and are exposed to the dry air of the laboratory, the diseased 

 tissues become shriveled and folded, resembling very closely the original 

 specimens of natural infection (PI. 14, A, C). Ordinarily if the infected 

 material is kept in moist chambers the decay is soft, mushy, spreading 

 either equally throughout the tissue or sometimes more on the surface 

 of the tubers, and is not confined to their piths as is usual in typical 

 cases of blackleg. The color of the decaying areas ranges from that of 

 the normal flesh to light or dark brown, often with blackish streaks or 

 stripes in younger portions nearer to healthy tissues, but never black 

 throughout. The margin is usually well defined, and there is no gradual 

 transition from dead to sound tissues. This internal appearance changes 

 considerably when tubers are taken from the moist chamber and are 

 exposed to drying. The decay of the bark is then more or less arrested, 

 and the disintegration centers mainly in the pith, so that a more or less 

 sound shell surrounds the centers of the active decay. The diseased tis- 

 sue is brown to black, the older regions becoming slimy (Pi. 14, B, D). 

 In all cases the decay gives off a very strong putrefactive odor. 



PLANTING OF ARTIFICIALLY INOCULATED TUBERS IN THE GREENHOUSE 



Seven Irish Cobbler tubers inoculated with the western bacterial or- 

 ganisms and partly decayed were planted in sterilized soil in pots. Four 

 showed subsequently a stem decay and three remained apparently un- 

 affected. One plant became girdled and died early. The disease ap- 

 peared first on the remaining three plants in the form of black streaks in 

 various positions on the stems, particularly at the leaf petioles. Later 

 on in some instances the entire stalk became black at the base. Tuber- 

 rot did not appear except on one tuber in one of the diseased pots. In 

 this case it was a soft, watery decay, light in color, not typical for black- 

 leg. The causal organism, identical with the original strains, was, how- 

 ever, recovered from this area. Healthy sprouting Irish Cobbler tubers 

 were immediately planted in the same pots in which these specimens 

 were grown. The new plants were very vigorous, and none of them con- 

 tracted the disease. 



