163 
illustration specimens provided; Maine Bul. 227, fig. 44; and U. S. Agr. 
Dept. Bul. 82, pl. III. osserve:— 
; __9. The large open cankers, extending deeply into the tuber and 
lined with the dead and decaying tissues of the host. pRaw a tuber 
showing this form of lesion. 
On the roots and stolons. Study the specimens provided. oBSERVE:— 
: 10. That here the lesions are gall-like in appearance; form and 
size; relation to the root or stolon, i.e. on the side only or involving the 
entire diameter? DRAW. 
Comparative studies. There are a number of scab-like diseases of 
potato tubers, the lesions of which are more or less alike. A brief examina- 
tion of the more common tuber diseases, in comparison with the Spongo- 
spora scab-lesions, is important. Examine, so far as available, the speci- 
mens and illustrations of the FOLLOWING :— 
11. The black-wart disease. In addition to specimens and 
illustrations provided, study Pl. Ind. Bur. Circ. 52, pl. I-II. sxetcu 
a tuber to show the character of the lesions of this disease. ; 
12. The common scab. Note the slightly raised irregular corky 
scab-spot. If this disease is serious, the tuber may be much checked and 
cracked. DRAW. 
13. Silver-scurf. This disease does not cause a distinct scab; 
at most there is but a wrinkling or sloughing-off of the epidermis. (See 
Schultz, Jour. Agr. Research 6:345-346.) 
14. Black scurf or Rhizoctonia scab and canker. ' This disease 
is very common but not often directly injurious to the tubers. Note 
the small dark-brown or black sclerotial bodies of the fungus scattered 
over the tuber. It is held by some that checking of the surface and even 
the formation of deep cankers are sometimes symptoms of this disease. 
(See specimens or Maine Bul. 230, fig. 71-73.) Make prawincs to show 
the tuber-symptoms of this disease. 
ETIOLOGY 
The Spongospora scab is caused by the myxomycete, Spongospora 
subterranea (Wallroth) Johnson. It is generally distributed throughout 
northern Europe, Canada and northeastern United States. It is a native 
of South America where it occurs as a pathogene of the potato in its native 
home. 
Life-history. This slime-mold does not differ materially from the 
saprophytic myxomycetes in its structure and life-history, except in so far 
as it has been modified by its parasitic habit. 
Primary Cycle. Spore-balls in the soil constitute the source of 
inoculum for the primary infections. The spores may apparently remain 
dormant in the soil for several years. Remove some of the spore-mass from 
a sorus on the tuber provided; mount in potassium hydroxide and 
OBSERVE :— 
15. The rather large dark spore-balls; color and structure. 
Each cell of the spore-ball is a spore. DRAW. 
Pathogenesis. The spores in a single spore-ball all germinate at 
about the same time. There emerges from each cell of the spore-ball, 
a single, usually uninucleate ameba. (See Kunkel, Jour. Agr. Research 4, 
pl. XX XIX, fig. 1-2. copy). 
