314 MAINi: AGRICUI^TURAI, e;xPE:riME:NT STATION. I909. 



more common and more severely attacked on the lower or more 

 moist portions of the field. If one stalk from a given seed-piece 

 was diseased any others coming from the same piece were 

 invariably found to be affected to a greater or less degree also. 



As a rule the plants first begin to show signs of disease when 

 they are 6 to 8 inches high and growing rapidly, i. e., in northern 

 Maine at or soon following the first of July. The progress of 

 the disease is markedly influenced by weather conditions. Very 

 moist, cloudy weather may tend to favor rapid progress, result- 

 ing in the early death of the young plants, so that only the dead 

 stalks remain scattered among the healthy plants, within a 

 month or six weeks, or even less time after its first appearance. 

 A period of dry weather coming on after the disease is well 

 started below ground may check its progress, but cause the death 

 of the plant at an equally early period on account of its inability 

 to withstand the lessened water supply. Again conditions 

 between these extremes, such as existed during the summer of 

 1909, may prolong the attack well into August. 



In brief there is no evidence that blackleg under ordinary field 

 conditions in Maine spreads from plant to plant in the field. 

 The number of diseased plants appears to be determined by the 

 number of infected seed-pieces planted, modified by conditions 

 of the soil wet or dry. Infection of the growing plant always, 

 so far as observed, begins below ground, usually at the junction 

 of the stem with the seed piece, which probably decays or begins 

 to decay before the stem is attacked. The rapidity of the pro- 

 gress of the disease and its severity varies with the weather 

 conditions, or amount of moisture in the soil, but a plant once 

 attacked never recovers sufficiently to produce merchantable 

 tubers. 



Mjeans of Distribution. 



As suggested in the preceding paragraph there is every reason 

 to believe that blackleg is largely, if not wholly, distributed by 

 means of infected seed tubers. As yet this statement is not 

 backed up by sufficient experimental data, but observations so 

 far made all point to this conclusion. Some of these observa- 

 tions are as follows : — 



The first case in Maine seen by the writer was on new land 

 recently cleared of forest and never before planted to any agri- 



