178 MAINE STATE COLLEGE 



When the plant or fungus or both have sufficiently grown, which 

 occurs during the summer and fall months, fruiting branches are 

 developed on the threads. These find their way through the 

 breathing pores of the leaves, and bear the summer spores, which 

 are developed in great numbers and blown by the wind on other 

 plants. These spores germinate, with favorable conditions, and 

 develope new threads, or they produce several oval shaped spores 

 which have two vibrating hairs, by means of which they move 

 about in water, and finally coming to rest, also produce threads. 

 Tliese threads enter new plants and spread the disease. The 

 spores develope rapidly and germinate quickly, so that from a few 

 diseased plants a large patch, under favorable conditions of mois- 

 ture and heat, ma}' be destroyed in a single day. The fruiting 

 stalks and the mycelium from the germinating summer spores, stop 

 the breathing pores of the plant, preventing a proper circulation. 

 This causes rapid decay of the tissues of the plant, which turn 

 black, becoming slimy and ill-scented. The developement of the 

 Slimmer spores, causes the leaves on the under side, to present a 

 grayish mildewy appearance. Some of the summer spores fall 

 upon the ground and are washed into the soil by rains, reach the 

 tubers and infest them. It is supposed that the threads of the 

 fungus go down the stems after killing the leaves, and finally 

 reach the tubers. It is known that "potatoes covered with from 

 1 to 3 inches of earth are pretty certain to become diseased if 

 the fungus is on the tops, while those planted four inches deep 

 are more rarely infested." Prof. Scribner argues from this, that 

 if the spores reach the tubeis through the soil, those near the 

 surface would be most afftcted.. Would not the tubers near the 

 surface be affected first, by mycelium descending the stem? The 

 roots bearing them come off higher up. Are not the conditions 

 near the surface more favorable for the rapid growth of the 

 fungus, and possibly the infection near the surface greater from 

 this cause ? Unless the tubers are harvested before they are 

 infected, they begin to rot in the ground, and the loss may amount 

 to 50 per cent, of the crop. If any of the harvested potatoes 

 contain the fungus, the disease will spread in the pits or cellar, 

 from potato to potato. Potatoes harvested and stored damp, are 

 likely to become infected from the germination of summer spores 

 which fall upon them while being harvested. This completes the 

 round of life. 



For cuts and explanations see pp. 172-3. 



