MICHIGAN EXPERIMENT STATION BULLETINS. 195 



POTATO SCAB. 



This is the name applied to a diseased condition of the tubers in which 

 the skin has a distended appearance similar to a scab, or where the skin 

 and a part of the tuber is destroyed leaving a cavity in the tuber. 



Wire-worms, grubs, mites, etc., have been found eating the decaying 

 matter in the cavities, or it may be possible they add to the injury, but it 

 is not true that they are the cause of the disease. 



The disease is caused by a parasite.* There may.be more than one 

 form of the disease; one forming the deep scab or the cavities, and the 

 other the surface scab, although this point is not settled at present. Prof. 

 Bolley, from his experiments of 1891, made the following summary:' 



" 1. Scabby or disease-bearing seed tubers can and will under ordinary 

 circumstances produce a diseased crop. 



" 2. Seed tubers free from the disease germs will in any soil — sand, clay, 

 or muck — raise an undiseased product, provided only that the soils them- 

 selves are free from the disease, 



" 8. The seed germs can remain from crop to crop in the ground. 



" 4. By soaking the seed tubers before planting in certain chemical 

 solutions I have been enabled to raise an undiseased product, whenever 

 the ground was known to be free from disease." 



From these conclusions, it is seen that, if the germs of this disease on 

 the seed potatoes are destroyed and the potatoes are planted on soil free 

 from the germs, the resulting crop will be free from the scab. Soils that 

 contain the germs are those on which diseased potatoes have been raised, 

 but the length of time that the germs will remain dormant in the soil is 

 not known, so it is impossible yet to tell how long before the soil once 

 affected is again free from the disease. The disease is carried by manure 

 containing potato tops of affected tubers, or litter from around potato pits, 

 and by tools in cultivating growing potatoes where one end of the rows is 

 free from the disease and the other end affected. 



The best method of destroying the g'^rms of the disease is by soaking 

 the seed in corrosive sublimate (bichloride of mercury). Purchase two 

 ounces of this substance at a drug store, pour it into a vessel containing 

 two gallons of hot water and stir frequently until it is dissolved; to this 

 add thirteen gallons of water. After freeing the seed potatoes that are to 

 to be treated, from the surplus dirt, plunge them into this solution and 

 leave for one and one half hours. 



If a person is to treat the seed very extensively, gasoline barrels sawed 

 in two will make convenient receptacles. The seed can be immersed by 

 setting wicker baskets containing the tubers in the solution; by using 

 three of these receptacles a person can soak the seed as fast as the cutting 

 is done. It is immaterial whether the seed is soaked before or after 

 cutting. All seed soaked should be planted or destroyed. The solution 

 should not be placed in metallic vessels. 



Prof. Bolley 's * experiments of 1892 have more than confirmed the 

 value of this treatment. Treated seed gave a crop, of which 99.33 per cent, 

 were free from the scab, while of the product from the untreated seed less 

 than one per cent, were free from the scab. He says, "that not only are 

 the tubejs affected, but the base of the vine as well, which results in a 



' North Dakota bnlletin No. 4, from which the material of this eubject is largely gleaned. 

 * Science, Dec. 23, 1892, p. 355. 



