3? 



Dry Scab of Potatoes. 



[APRIL, 



mitted to Kew for examination. Two of these came from 

 different parts of Scotland, the other from England. The 

 disease in each instance proved to be "Dry Scab,' 5 which up 

 to then had been believed to be absent from Britain. 



The injury is confined to the tubers, and no indication 

 of its presence is suggested by the foliage, which remains 

 unaffected. It causes disfigurement of the surface of the 

 tuber, followed by local patches of a dry rot. Its presence 

 is revealed by the occurrence of blackish-olive or blackish- 

 violet patches, which soon become depressed below the 

 general surface of the tuber, due to the drying and breaking 

 up of the tissue. Very frequently only one or two such 

 sunken areas, which vary from half an inch to an inch across, 

 are present on a tuber. As a rule, numerous very minute 

 black sclerotia are present in the epidermal cells, or on the 

 surface of the tuber, in the region of the sunken areas. In 

 some instances numerous minute sclerotia, scattered over 

 the surface of the tuber, are alone present. During a certain 

 period in the development of the disease, the depressed 

 patches and the sclerotia are covered with the fruit of the 

 fungus, which under a pocket-lens resembles myriads of 

 very minute, upright black bristles. As the dark coloured 

 mycelium permeates the tuber, the tissue becomes dry and 

 somewhat powdery, and crumbles away, leaving a shallow 

 cavity. Those portions of the skin bearing sclerotia also 

 readily peel off in flakes, which remain in the soil and 

 endanger future crops. If a tuber bearing sclerotia or 

 sunken areas is placed in a damp, warm place, the fruit of 

 the fungus is produced in abundance within a few days. 

 vSuch fruit is not confined to the obviously diseased portions, 

 but occurs scattered over the entire surface of the tuber, 

 proving that mycelium migrates from the primary diseased 

 areas, and that portions of the tuber apparently free from 

 disease are in reality infected; consequently, it is not 

 advisable to use as "sets " any tubers showing signs of the 

 disease, as is too frequently done, after the obviously diseased 

 portion has been cut away. 



An illustration of an affected tuber and of the fungus, 

 together with some other particulars, is given in the 

 Bulletin. 



