47 



POTATO SCAB. 1 



This disease, which is known to practical gardeners as scab or sc:rf, 

 may be recognized by the brown, irregular concavities, with ragged, 

 often upturned edges, that make their appearance in the tuber. These 

 concave places spread gradually and become filled with brown, decayed 

 remains of cells, excrements of mites, mycelium threads, etc. 



The deeper these places eat into the flesh of the healthy tuber the 

 greater is its loss of sound tissue, and consequently of food value; but 

 even if the scabby portions are not very deep and the decrease in food 

 value unimportant, the potatoes affected with scab spots lose consider- 

 ably in market value on account of their diseased appearance. 



If the small projections on the edge of the scab are examined they 

 will be found to consist of parenchyma cells, which once formed a part 

 of the healthy tissue of the tuber, but are now dark colored, and mostly 

 without starch grains, in place of which the other cell contents are 

 massed together in a spherical shape. From these dead, suberified 

 cells there is either a gradual transition into the healthy, starch bearing 

 tissue, or they are separated from the latter by a layer of true tabular 

 cork cells which have arisen, at the expense of the starch, from the cell 

 layers bordering on the scab spot. 



The danger to the potato lies in the fact that as long as it remains in 

 the ground the edges of the scab very often begin to degenerate again 

 and the spot becomes deeper. If ihe potatoes are once dug, no further 

 X)enetration of the scab spot can be discerned, even when buried for the 

 winter. In case of early varieties that are gathered at different times, 

 a section of the scab spots shows that they are no deeper in those po- 

 tatoes dug at the end of the season than in the ones gathered some 

 weeks earlier. I conclude from this circumstance that the extension of 

 scab spots is not continuous, even while the potatoes remain in the 

 ground, but is confined to different periods, and perhaps in many cases 

 to one. 



I consider the times when hard showers follow a long continued drought 

 as such scab forming periods. This view is founded upon the chauges 

 which occur when a potato is brought into a very moist atmosphere or 

 part of it into direct contact with dripping water. By this means the 

 development of the leuticels becomes so increased from their normal 

 condition that they are easily recognized by the naked eye as small, 

 white, woolly warts. ^Nearly all parts of plants which are covered by a 

 persistent cork layer possess interruptions on the surface in form of leu- 

 ticels or bark-pores, in which there is a loose, corky tissue provided with 

 air conducting intercellular spaces. This tissue usually arises beneath 

 the stomata aud shares in their physiological work, i e.,the exchange 

 of atmospheric air between the interior and exterior of the plant. In 



1 Translation of paper by P. Sorauer, Handbuch der Pflanzen-Krankheiten, 1886, 

 p. 227. 



