112 AGRICULTURAL EXPERIMENT STATION. 



POTATO SCAB. 



This important disease of the potato has been studied somewhat 

 the past season to determine if possible whether it is due to a fun- 

 gus parasite. Quite extended microscopical examination failed to 

 show any evidence of mycelium or spores in the tissues about the 

 scabs, that would account for the effect. The withered tissue in the 

 scabs in some cases was mouldy- but this was secondary not primary. 

 The only organic form observed excepting mould was animal, viz : 

 Nematode worms, not unlike the vinegar eel. These were not always 

 present and when found were probably there to feed upon the fer- 

 menting starch The disease is certainly local in the scabs, the ad- 

 joining tissue being perfectly normal and not even discolored. The 

 disease is confined to the cortical and sub-cortical cells and has all 

 the appearance of having resulted from mechanical injury, which 

 has ruptured the skin, the wound being healed by the shriveling of 

 the adjacent tissue or the formation of a layer of cork cells under 

 the scabs. Though variable in size the scabs are so characteristic 

 they must be due to a common cause. P^xtensive experiments con- 

 ducted at the Massachusetts Experiment Station seem to show that 

 the disease is not propagated by the seed, scabby seed producing 

 healthy tubers and vice versa. 



Those who have made this disease a careful study believe it is not 

 due to animal or plant parasites, but is caused by the conditions of 

 growth. Experiments conducted at the N. Y. Experiment Station 

 to test the effects of soil, excessive moisture, chemical fertilizers and 

 fresh stable manure upon the production of scab, seem to show that 

 fresh manures and excessive moisture increase the disease. It 

 appears that conditions favoring rapid growth also favor the pro- 

 duction of scab. Much moisture in the spring followed by drought 

 is believed to increase the disease, also a dry spring followed b}' a 

 wet fall has the same effect. Uniform conditions during the season 

 should decrease the disease, if the above is correct. It has been 

 observed that potatoes grown in a moist atmosphere will have formed 

 on their surface warty prominences. The skin of the potato is made 

 of what botanists call cork tissue. It does not cover the tuber but 

 at intervals there are minute structures called lenticels through which 

 gases pass in and out of the potato. "When the potato is exposed 

 to excessive moisture these lenticels increase in size and the skin 

 thickens near them, causing the warts spoken of above. If the 



