I'OTATO DISEASES AND THEIR KliMEDlIiS. I9 



did SO, and offered on behalf of our Experiment Station to pay 

 him for possible loss if he would cut back the tops by one-half 

 in alternate rows in his field and report the outcome to the Sta- 

 tion. The result was a yield of 152 pounds where pruned as 

 compared with 221 pounds where unmolested. 



But the greatest enemies of the New England potato crop in 

 this latter stage of its development are the diseases due to fungi 

 or bacteria — the blights and the rots. 



In addition to the " tip-burn " already mentioned two forms 

 of leaf blights annually occur. First, the Early Blight appears 

 in July, a fungus disease causing well defined black spots with 

 minute target-board like rings as markings on them. This dis- 

 ease is worst on dry, sandy soils and in the dryer parts of the 

 season. Fortunately this early blight rarely destroys all the 

 leaves and it never spreads to the tuber or causes rot. The 

 fungus which causes it lives over winter probably on. the dead 

 stalks and leaves. From July first on one sees — at least in Ver- 

 mont fields — much spotting of the leaves closely resembling the 

 early blight, but of wholly different cause. This is the " burn- 

 ing " or spotting by Paris green or other insecticides. 



The Late Blight, as indicated by its name, is usually delayed 

 four weeks or more, generally appearing in late August. Once 

 underway it spreads through the field with alarming rapidity, 

 as all know too well, providing it is favored with warm, moist 

 weather, often blasting all within a week. Moreover, its 

 damage does not cease here, for the same fungus passes to the 

 tubers, thus starting the dreaded rot. This is started by the 

 germs or spores which wash from the blighting leaves through 

 the soil until they reach the tuber. 



If conditions are favorable the fungus upon gaining entrance 

 to the tuber spreads rapidly through it, killing parts or all of its 

 tissue and leading to either the "dry" rot or the '"Wet" rot 

 according as more or less water is present in the soil. In wet 

 soil the fungus not only destroys the tuber rapidly, but spreads 

 to other tubers. If conditions are different the development of 

 the fungus in the tubers stops without causing visible injury 

 and it may remain dormant in the tuber over winter. We are 

 without positive evidence that this fungus can live over winter 

 in the dead plants or in the soil and it is believed that it is only 

 through the use of infected tubers for seed that the disease is 

 perpetuated year by year. Unfortunately there is no practical 



