344 
Report of the Botanist of the 
Summary of botanist's reports for 1884-7. 
Publication of reports for 1884-7. 
Index to botanist's reports for 1884-7. 
The Potato Scab. 
The potato crop is much deteriorated each season by roughened 
spots on the surface of the tubers, varying from shallow to more or 
less deep-seated concavities filled with effete matter. This affection is 
so common and uniform in its features that every reader will at once 
recognize it under the name of scab. 
The scab makes its appearance upon the tubers in the hill when they 
are no larger than hazel-nuts or even smaller. It first shows as 
irregular, superficial, very small spots, which lose the pale, smooth 
look of the healthy skin, and turn brown and rough. These spots 
continue to increase in diameter and depth until the potatoes are 
removed from the ground at harvest time. Other centers of scab arise 
as the tubers grow, and when sufficiently numerous coalesce, and in 
extreme cases envelope the whole tuber. As the spots enlarge they do 
so at the expense of the tissues of the potato lying beneath therm 
which pass over into a spongy brown substance filling the cavity of 
the scabbed spot. The change in the tissues is not a truly putrefac- 
tive one, but a gradual loss of life in the cells of a well-defined area, 
followed by a very slow decomposition. Beneath the spots the 
substance of the tuber remains unaffected. 
The cause of potato scab has been long in dispute. The explana- 
tions which have recently found most favor in American journals, 
ascribe it on one hand to the attack of parasitic fungi, and on the 
other hand to the depredations of worms, grubs, niyriapods, or other 
small animals in the soil. It is not the present purpose of the writer 
to attempt a refutation of either view, but attention is called to the 
significant fact that mycologists and entomologists who have looked 
into the matter find no good grounds for such explanations. One of 
the strong arguments, moreover, against these views, although by no 
means an unanswerable one, is that no particular kind of fungus or 
small animal, in truth no fungus or animal at all, can be found to 
invariably accompany the injury at any particular stage of its develop- 
ment. 
A study of the minute structure and growth of the spots throws 
some light upon the matter. The potato tuber is enveloped by a thin 
skin, readily separable as a semi-transparent delicate membrane when 
the tuber is cooked by boiling. This membrane is nearly or quite 
impervious to moisture and gases. It thus protects the interior tissues 
