Part II. 



POTATO DISEASES. 



129 



FiQ. 6. — Scab. 



Scab. — This disease appears as rough, scabby areas on the 

 tuber surface (Fig. 6). It is caused by one of the higher bacteria 

 which live in the soih The disease does not penetrate the flesh of 

 the potato, and its chief 

 harm is the unsightly ap- 

 pearance which it gives to 

 the tubers, thus reducing 

 their market value. Shrink- 

 ing and loss of weight in 

 storage result from rapid 

 evaporation of moisture 

 through the scabby surface. 



Control. — Do not plant 

 on land from which scabby 

 potatoes were harvested the previous year. Avoid the use of 

 lime, wood ashes and basic slag, as scab develops more freely 

 in alkaline soils. Stable manure also favors development of the 

 disease. 



Treatment of " seed " tubers to destroy the scab organism should 

 always precede planting. It is advisable to use corrosive sublimate 

 solution recommended for scurf, as this is equally effective against 

 scab. This should be done before sprouts start and before cutting; 

 otherwise injury may result. Soaking in formaldehyde solution 

 (1 pint to 30 gallons of water) for two hours will kill the scab 

 organism, but this is not efficient against scurf, which is becoming 

 more common and troublesome than scab in Massachusetts. 



Blackleg. — This is a bacterial disease. It causes early rotting 

 of seed tubers after planting. From these it spreads upward 

 through the young shoots, which turn black and rot below the 

 ground (Fig. 7). Diseased plants are stunted, turn yellow and die 

 early without setting tubers. The disease, which is usually most 

 troublesome in heavy, wet soils, is now of frequent occurrence in 

 Massachusetts. 



Control. — Control measures should begin with the selection of 

 clean, sound seed tubers. Disinfection of seed tubers for scurf and 

 scab will kill any of the blackleg bacteria which may be on the sur- 

 face, but this will be of no avail if the tubers are in the least decayed, 

 as the solution will not reach the interior. 



