Rural School Leaflet 



921 



A germinating swarm 

 spore showing germ 

 tube entering leaf 



enough of the leaf surface to allow the passage of the germ tube within. 

 It makes its way between the leaf cells where it branches in several direc- 

 tions, forming the mycelium. Other enzymes are 

 secreted that kill the protoplasm of the leaf 

 cells, and the nutritive cell sap contained within 

 passes into the spaces between the cells where it is 

 absorbed by the mycelium. Little side branches 

 (haustoria) also extend directly into the cells and 

 absorb the nutritive substances found there. As 

 cell after cell is invaded in this manner, they 

 present a watery appearance which later turns brown and dry upon the 

 death of the cells. Usually only the lower leaves of a few plants here 

 and there show these spots, so that they are easily overlooked and a 

 casual observer would say there was no blight present. No real loss 

 has occurred and if the weather continues dry thereafter there will be 

 no further development of the disease. But if another storm period 

 occurs a few weeks later, these spots serve as sources of infection from 

 which spores are produced by the millions and distributed by wind or 

 other agencies to neighboring plants. 



These spores are produced on the extremities of fruiting stalks identical 

 with those formed on the cut surface of the potato. They are produced 

 by the mycelium within the leaf and extend through the underside of the 

 diseased area of the leaf. They are usually produced during rainy or 

 moist weather extending over a period 

 of several days. The spores borne on 

 them are short-lived under air-dry con- 

 ditions, and must soon germinate and 

 gain entrance to the host plant if 

 they are to reproduce the fungus. 

 They may be blown to healthy leaves, 

 but if moisture is not present within 

 twenty-four hours they will die. If 

 moisture is present, however, they 

 will germinate within an hour and 

 produce swarm spores, which in their 



turn germinate by the production of Below from right to left are shown four 

 an infection tube. This enters the stages in the formation of the swarm 

 leaf in the manner previously describ- 

 ed and a spot is developed. 



Many of the spores fall to the ground 

 and germinate there. The swarm spores make their way through the soil 

 to the surface of the young tubers, which they likewise penetrate, and a 



spores. The way in which the swarm 

 spores germinate is shown above 



(After U. S. Dept. Agr.) 



