Potato Scurf arid Potato Scab. 
175 
in the laboratory have been accidental and have never produced 
scurf when healthy potatoes were infected with them. 
The true scab on potatoes is shown in Fig 3, B. The 
appearance of the tubers is very like potato scurf ; but under 
the pocket lens one at once detects the absence of the glossy 
appearance, due to the new cells of the epidermis in the inter- 
spaces. Here the interspaces show a rough irregular surface, 
consisting of broken cells of the epidermis, which can easily 
be detected under the microscope. On the other hand, while 
the patches found in scurf exhibit, as a rule, a circular or oval 
shape, the patches in scab are rarely curved, but show more or 
less straight border lines. In addition, the tubers attacked by 
true scab exhibit the presence of minute dark spots, which may 
easily be taken for particles of soil adhering to the surface, but 
the washing that would remove the soil does not affect them. 
Under the microscope these dark spots are found to consist of 
compact hardened mycelium. Though thoroughly soaked for 
hours they offer great resistance to the dissecting needles and 
are with difficulty Separated. The mycelium is of a reddish- 
brown colour, septate, and has enlarged oval cells, approaching 
in appearance hyphee of sclerotia forming fungi. 
Potato tubers from a farm in Essex served as material for 
the investigation of this scab. They were covered with a dull 
reddish-brown webbing, which was raised into numerous 
points as if grains of sand were below it. They had thus 
excited the farmer’s curiosity. On removing the reddish- 
brown mycelium it was found that the granular elevations 
were caused by small masses of mycelium which maintained a 
firm hold on the potato (Fig. 3, C), and that the more super- 
ficial, spreading mycelium left a net work of shallow furrows 
on the surface of the potato somewhat like those seen in scurf, 
but forming more or less straight channels. Under the micro- 
scope the covering was found to consist of septate, branching 
mycelium, of dull carmine-brown colour (see Fig. 3, D). The 
small dark bodies were composed of the same mycelium, 
though clearly modified to form sclerotia. Thus the connection 
between the dark bodies observed on the tubers of the potatoes 
and the net-like furrows was determined. The mycelium 
was ascertained to belong to the fungus Rhizoctonia , which, 
Explanation of Fig. 3. 
A, Portion of the surface of healthy potato, showing scurf. 
B, Potatoes with the scab disease. 
C, Portion from the surface of scabbed potatoes enlarged, one half showing 
strings of Rhizoctonia molacea Tul., the lower half showing sclerotia. 
D, Mycelium of R. violacea enlarged. 
E, Section through sclerotium. 
F, Starch grains pierced by the mycelium. 
G, Severe fissures in the development of scab. 
