l68 MAINE AGRICULTURAI, EXPPERIMENT STATION. 



tube. If the swarm spore is lodged upon a potato leaf the 

 tube usually enters the leaf through the breathing pores, 

 branches and permeates the tissues, killing them as it goes. 

 Thus it produces the characteristic blotches on the leaves as 

 shown by Fig. i6, page 171. The spores may be washed down 

 into the soil also and infect the tubers in the soil, or if the crop 

 is dug while the tops are still partly green, but blighting badly, 

 the tubers may become infected at this time and the rot develop 

 in storage as in the season of 1909. 



The remaining illustrations of Fig. 15, page 169, show the 

 stages in the .formation of swarm spores and how they finally 

 germinate and enter the leaf. After a small dead area is pro- 

 duced the fungus throws out the fruiting organs as illustrated 

 on the large section of the leaf and the process already described 

 is repeated over again. This makes plain why it is absolutely 

 necessary to cover each and every potato leaflet with a thorough 

 protective coating of bordeaux mixture, and why spraying 

 partially or improperly done may be practically useless. 



When potatoes are blighting badly, if the margins of blight- 

 ing spots are examined on the under sides of the leaves a deli- 

 cate white fringe may be seen. (See Fig. 16.) This is made 

 up of hundreds of little fruiting organs each bearing from one 

 to several fruiting bodies, each of which will divide into from 

 6 to 16, usually about 10, swarm spores and each of these 

 swarm spores capable, under proper conditions, of producing 

 another blighting leaf or capable of causing the destruction of 

 a merchantable tuber. It is thus easily seen how that one 

 blighting leaf may produce sufficient spores to infect every hill 

 of potatoes on an acre of land, provided all the spores produced 

 could come in contact with these plants. This explains why a 

 field, apparently free from blight, may be found to be badly 

 affected only a few days later, and how potatoes showing a 

 comparatively small amount of blight on the foliage before 

 being killed by frost or before being dug, may develop a large 

 amount of rot in the field or in storage as the case may be. In 

 the first instance the blight existed for some days on the lower 

 and more shaded leaves (where it would not be noticed by the 

 ordinary observer) till a day or two of favorable weather 

 occurred. Then a large crop of spores were produced which 

 infected the plants on the whole field. In the case of tuber 



