AGRICULTURAL EXPERIMENT STATION. 175 



always present in plants suffering from the Potato Rot. There 

 are other rots, wet and dry, of various kinds, not due to this fun- 

 gus which are sometimes confounded with it. The true Potato 

 Rot may under different conditions be wet or dry.) 



Secondary Causes. 

 There are several secondary causes which modify the disease 

 when present. Moisture and heat favor the growth of the fungus. 

 The disease is worse on heavy and poorly drained lands. The 

 atmospheric conditions, the nature of the soil, the methods of cul- 

 ture, vitality of the plant, and the variety of the potato, have no 

 power to originate the disease. They can only favor or impetle 

 its progress. 



Ck)NDiTiONS OF Growth. 



Warm weather and moisture at the time the summer spores are 

 ripe and dropping, will cause them to germinate, therefore the 

 disease is usually worse in hot, moist seasons. Should it be dry 

 at the time the spores are ripe they soon die and the disease does 

 not spread. The fungus seems to require a suflflcient time to per- 

 fect itself before producing spores. It would produce spores 

 earlier in early plantings and probably mature faster in early varie- 

 ties. We often see early potatoes affected, while later varieties 

 contiguous are exempt. Also early varieties exempt when 

 later in the season the slower growing varieties alongside become 

 diseased. 



The fungus is most active when the temperature is between 60° 

 and 70° F. At lower temperatures than 40° its developement is 

 arrested, but it can survive a much lower degree. At 80° F and 

 upward the spores are gradually killed. Extremely hot weather 

 would be unfavorable to the germination of the spores. The 

 disease does not appear in Maine until about Aug. Ist, and some- 

 times later, and often not before early varieties are ripe and har- 

 vested. If the source of infection is in the early potatoes when 

 planted the disease appears earlier. If in later varieties it 

 developes apparently later. The disease might spread from an 

 early infected variety to a contiguous, uninfected field of later 

 varieties, though there is reason for believing that the proper stage 

 of developement of the potato plant is necessary for infection. 



Description. 

 Mycelium. — The plant body of this fungus is composed of 

 slender, jointless, transparent threads, which permeate the stem, 



3 F 



