246 
Journal of Agricultural Research 
Vol. VII, No. 5 
and form plasmodia, which move about in the immediate vicinity of the 
sorus and destroy the host cells. This type of dryrot has recently been 
described by Kunkel (6). The destruction of the cells is followed by a 
brownish discoloration and shrinkage, which in the early stages is often 
most conspicuous on one side of the sorus, but later surrounds the pustule 
and a small depressed area appears (PI. 11, fig. D). The plasmodium, which 
has never been found except in the parenchyma, works largely in the 
superficial tissues, producing a dry, hard spot from % to 1 cm. in diameter 
and % to X cm. in depth. Isolations made from this type of rot often 
give a variety of fungi, most of which are saprophytes; but in some of 
these isolations wound parasites exist and probably find such injured 
spots excellent points at which to begin the complete destruction of the 
tuber. 
The entrance of wound parasites through the open sori marks the 
beginning of the third and most destructive type of powdery-scab 
dryrot (PI. 11, fig. B, C, E). In the bottom of many of the sori there is 
little or no wound cork (PI. 12, fig. A, B), and the pit is filled with dead 
tissues and numerous spore balls in the same way as open cavities in the 
tuber are filled with small masses of culture media on which fungi 
naturally flourish. 
The most common of the wound parasites found associated with the 
early stages of rot is a species of Phoma, producing brownish to gray 
lesions (PI. A and PI. 11, fig. A). As these lesions progress they become 
more sunken, darker, and often hard and bony; and when removed, 
which can often be easily done, they leave a clean and smooth cavity 
in the tissues of the tuber. The shape and texture of the spots removed 
give the impression of a button; hence the name “ button-rot,” by which 
the trouble is known among farmers. Eater stages of the lesions (PI. 
13, fig. A, B) vary from 2 mm. to 5 cm. in diameter and often reach a 
depth of 2 to 4 cm. (PL 11, fig. C). The diseased tissues are sharply 
defined on the surface, where the pustules are numerous and where 
infection may take place through each of the pustules; in this way 
large areas may become discolored and later depressed (PL 11, fig. E). 
Often, after the above-described lesions have formed, other wound-rot 
and decay organisms enter, in which case the symptoms are somewhat 
confusing (Pl. 11, fig. B, C, E). The more common of these organisms 
are Fusarium coertdeum (Lib.) Sacc., F. discolor , var. sulphureum Schlect., 
and various bacteria. The symptoms of each of these are typical of 
the particular species, as described by Wollenweber (16). These fungi 
have been repeatedly isolated and identified from single-spore cultures 
as well as from typical rots produced in potato tubers artificially inocu¬ 
lated. Generally when any of these wound-rot organisms are present, 
the lesion is soggy and less firm to the touch, and its surface is often 
cracked and broken. 
