METAPLASTIC DISEASES 
SPONGOSPORA SCAB OF POTATOES 
This slime-mold disease has come into considerable prominence in 
recent years because of its discovery in the seed-growing potato-sections 
of northern United States and Canada. It appears to be confined to the 
potato. Soil and climatic conditions seem to greatly limit the geographical 
range of the disease. It is evidently a disease of cool, temperate climates 
or high table-lands and valleys. 
SYMPTOMS 
The lesions of this disease are confined to the underground parts of the 
host; tubers, roots and stolons (rhizomes). 
On the tubers. Three types of lesions are to be found on the tubers; 
the scab (the most common form), the wart-form and the canker-form. 
Scab-form. Examine the specimens labeled ‘‘young sori.’ OB- 
SERVE -— 
1. The small brownish spots (ringed with India ink). These 
are the first evidences of the disease. 
2. The smooth, brown mounds (also ringed with ink),—the 
covered sori of the pathogene. 
3. The crater-like openings of some of the sori; they are mature 
and are opening for spore-dispersal; form, size and color of the sori. 
Examine the specimens labeled ‘‘mature sori.’”” OBSERVE :— 
4. The large open brown sori. Note the torn and upraised 
cuticle about the margin. 
5. The brown powder within the sorus. Pick it out with the 
needle. 
6. The empty depressions left after this powder is disseminated. 
i Make a DRAWING of the entire tuber to show the sori and their distri- 
ution. A 
Make a series of enlarged prawincs (at least four), showing the ap- 
pearance of the sori in different stages of development. 
Wart-form. This form is not common but may develop in severe 
cases in moist soils. Read Maine Bul. 227:94, and study figure 45; or 
study specimens if available. OBSERVE :— 
7. That the lesions are more or less confined to the seed- or eye- 
end of the tuber. 
8. That they are much more enlarged and gall-like than the 
ordinary scab-form. How does the location of the lesions help to explain 
their warty character? 
SKETCH a tuber showing the wart-form of the disease. 
Canker-form. Where the lesions are numerous and coalesce, the 
tubers may show, in an advanced stage of the disease, large deep 
cankers. This is held by some to be the result of secondary activities of 
the pathogene. (See Kunkel, Jour. Agr. Research 4:237.) Study the 
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