478 DISEASES OF CULTIVATED PLANTS 



SPONDYLOCLADIUM (Martius) 



Vegetative hyphae creeping, septate, fertile erect, simple, 

 rather rigid, coloured; conidia fusoid, 2-many- septate, 

 coloured, in superposed whorls. 



Dry scab of potatoes. This disease is due to Spondylo- 

 cladium atrovirens (Harz.). It causes disfigurement of the 

 surface of the tubers, followed by a dry rot. Its presence is 

 usually indicated by the occurrence of blackish-olive or 

 blackish-violet patches, which soon become depressed below 

 the general surface of the tuber, due to the breaking up of 

 the tissue. Very frequently only one or two such sunken 

 areas, which vary in size from half to three-quarters of an 

 inch across, are present on a tuber. In other cases there is 

 a thickish layer of dingy olive mycelium present everywhere 

 just under the skin, and the surface of the potato is more or 

 less covered with very small warts. As a rule, numbers of 

 very minute, black sclerotia are formed in the epidermal cells 

 or on the surface, in the neighbourhood of the diseased areas, 

 or in some instances, minute sclerotia are alone present. 

 During a certain period in the development of the disease, 

 the patches are covered with the fruit of the fungus, which 

 under a pocket-lens appears under the form of numerous, 

 very minute, upright, black bristles. As the mycelium 

 permeates the tuber, the tissue becomes dry and somewhat 

 powdery, and breaks away in patches. Those portions of the 

 skin bearing s< lerotia also break away in flakes, which remain 

 in the soil and endanger future crops. The dark-coloured 

 mycelium of the fungus spreads from diseased areas along 

 the epidermal (ells of the tuber, and if a portion of such an 

 infected tuber is placed in a damp, warm situation for a few 

 days, a plentiful crop of the fruit of the fungus appears on 

 the surface of the tuber. In like manner, if a portion of skin 

 bearing sclerotia is placed under favourable conditions for 

 growth, similar fruit springs from the sclerotia. 



The sclerotia, in the absence of fruit, were described by 

 Frank, under the name Phellomyces sclerotiophorus, as an inde- 

 pendent fungus. At a later date. Appel and I.aubert suc- 

 ceeded in obtaining the fruit of Spondylocladium atrovirens 

 (Harz.) from these sclerotia, consequently Phellomyces sclero- 

 tiophorus disappears as an entity, and as a specific parasite 

 attacking potatoes. 



