DISEASES OF CULTIVATED PLANTS 565 



treatment is made. The potatoes, however, should not be wet, as the 

 disinfection is more thorough if the surfaces are dry. 



Small quantities of potatoes may be disinfected by soaking in a 

 solution of 1 pint of formaldehyde to 30 gallons of water for two 

 hours. Either the gas or the solution treatment may be applied some 

 time previous to planting, provided the potatoes are not exposed to 

 reinfection by being put into receptacles that have previously held 

 scabby potatoes. The treatment should also be made before the 

 potatoes are cut for seed. 



Wart Disease of the Potato. The disease, which has been known 

 as warty disease, black scab, canker, and cauliflower, is one which at- 

 tacks the tuber principally, and consequently is not observed until 

 harvesting time. In a bad attack of the disease big, dark, warty ex- 

 crescences, sometimes as large as the tuber itself, appear on its sides 

 or ends. The growth consists here of a mass of coral-like or more or 

 less scabby excrescences or nodules, similar in appearance to the well- 

 known crown or root gall of apples. The adherent earth can be 

 easily washed off when the character of the growth becomes more ap- 

 parent. It is not spongy and not detachable from the tuber. It is 

 of a somewhat lighter color at the base and dotted with minute rusty- 

 brown spots over the surface. In an advanced stage the tubers are 

 wholly covered by this growth, having lost every resemblance to pota- 

 toes. They are lumps of irregular outline, never sypherical or ob- 

 long, but simply a mass of ragged and edged excrescences. A still 

 more advanced stage occurs when the fungus has utilized every par- 

 ticle of food stored in the tuber and has reduced it to a brownish- 

 black soft mass giving off a very unpleasant putrefactive odor. This 

 is the most dangerous stage of the disease, and the tubers which have 

 reached it can not be harvested whole. They break in pieces, and 

 thus the brownish, pulpy mass, consisting almost entirely of spores 

 of the fungus and remains of the cell walls of the potato, is broken 

 up, the spores are liberated in millions, and the land is badly infected 

 for years. 



In a mild attack the eyes first appear grayish, then turn brown, 

 and finally black, while in a healthy tuber these are whitish or pur- 

 plish in color. The tuber is only slightly disfigured and its keeping 

 qualities do not seem to be impaired. While the tuber is the part of 

 the plant chiefly affected, infection may take place in all the young 

 tissues of the plant, the roots, stolons, stems, and even the leaves. 



Threatening Nature of the Disease. All reports indicate that 

 the potato wart is one of the most serious of all known diseases of the 

 potato. It converts the tuber into an ugly, irregular, and utterly un- 

 salable growth. When established in a field it may affect the entire 

 crop, and the land remains so infected that potatoes can not be suc- 

 cessfully grown for six or more years. When once established in the 

 land it is useless to grow potatoes again until the pest has been 

 starved out or otherwise destroyed ; but so far as is known no other 

 crops are liable to be attacked. Quite the worst case seen in Cheshire 

 occurred on land that had not borne potatoes for six years ; seed from 

 the same source as that employed on this land yielded satisfactory re- 



