POTATO DISEASES IX HAWAII. 27 
encroaching fungus. This disease is caused by a bacterial-like or- 
ganism until recently known under the name Oospora scabies, but 
its relationship to another group of fungi having been shown, it is 
now called Actinomyces ehromogenus. 
The corky scab disease not only reduces the value of the crop but 
also appreciably lessens the yield. The losses through decreased 
yield have been estimated variously. For example, Goff 1 planted 
equal amounts of very scabby seed and seed free from scab on a soil 
which had been in clover sod. The scabby seed yielded 199f pounds 
and the scab-free seed 477§ pounds. The decreased yield is brought 
about through the failure of the seed to germinate and the lowered 
vitality of the plants. The value of the crop is reduced in several 
ways. The unsightliness of scabby potatoes and their changed taste 
and odor lower their market value. The increased liability to tuber 
decay and the the extra thick paring necessary in preparing the 
potato for the table result in considerable loss to the consumer. 
This disease is not very prevalent in the islands, but at times on 
poorly drained, hard-packed soils or soils which have been limed 
it is more or less troublesome and probably the source of considerable 
loss. 
Control. — Seed selection and disinfection, certain types of ferti- 
lization and soil treatment, and crop rotation have been found more 
or less successful in preventing the common or corky scab. The 
organism causing the disease is distributed for the most part by the 
use of scabby seed potatoes, and only scab-free tubers should be used 
for seed. To reduce the probability that scab organisms are carried 
to the soil by the seed, the latter should always be dipped in one of 
the potato-seed disinfecting solutions (p. 9). 
Scabby and refuse potatoes should not be fed to farm animals if 
the manure from these animals is to be scattered on the potato land, 
as it has been demonstrated that the organism can survive passage 
through the alimentary tract of animals. 
The development of the scab is favored by a high humus content of 
the soil and also by an alkaline reaction. To the latter fact may be 
attributed the unfavorable results of adding lime or wood ashes to 
potato soils. Halsted 2 obtained favorable results by fertilizing with 
kainit (Stassfurt salt containing 20 per cent potash). According to 
Beckwith 3 and Taft 4 muriate and sulphate of potash also reduce scab 
injury. The plowing in of green manure and, in fact, any cultural 
practice which tends to make the soil acid will reduce the common 
or corky scab on badly infected fields. Crop rotation has been sug- 
1 Goff, E. S. Experiments in potato culture. Wisconsin Sta. Rpt. 1892. pp. 278-280. 
- Halstead. B. D. Field experiments with potatoes, I. New Jersey Sta. Bui. 112 (1S95), 
pp. 1-12; Rpt. 1S96. pp. 309-315. 
3 Beckwith, M. H. Potato scab. New York State Sta. Rpt. 1SS7, pp. 307-315. 
4 Taft, L. R. [Potato scab.] Michigan Sta. Bui. 57 (1890), pp. 23-25. 
