494 Journal of Agricultural Research voi. iii.no. e 



and suggests that under natural conditions infection may be spread short distances 

 by the wind. 



The second possibility is brought out by j\letcalf and Collins (ii), as 

 may be noted in the following quotation: 



As both kinds of spores appear to be sticky, there is no evidence that they are trans- 

 mitted by wind except where they may be washed down into the dust and so blown 

 about with the dust. 



While it has not yet been demonstrated that pycnospores are carried 

 in this way, the tests of Heald and Gardner (7) on the longevity of pycno- 

 spores in soil give added plausibility to the theory, since these spores were 

 found to persist in the soil between periods of rain and were able to 

 withstand complete desiccation in the laboratory for months. 



Attention was first directed to the strong probabihty of wind dissem- 

 ination of ascopores by Rankin (14), who reported their forcible ex- 

 pulsion. In a later report the same writer (15) makes the following 

 statement : 



Under moist conditions the ascospores are shot forcibly out in tlie air where they 

 can be caught up by the wind and carried for a considerable distance. The speaker 

 found ascopores being shot from mature pustules during every rainy period last sum- 

 mer. * * * The question at once arises, Wliy could not these ascospores once shot 

 into the air be carried long distances and, owing to their abundance, cause a large 

 majority of the infection? 



After carrying out field experiments during the summer of 191 2, 

 Rankin (16), referring to ascospores, says: 



They are shot out in vast numbers with every rain during the summer and are 

 carried by the mnd. 



Detailed field work on dissemination was carried out by Anderson (i) 

 and his assistants for the Pennsylvania Chestnut Tree Blight Commission 

 (2). These publications confirm the statement of Rankin that expulsion 

 of spores takes place only when the pustules are moist. The seasonal 

 duration of shooting under natural conditions was not determined, as the 

 field tests were confined to the month of August. Under artificial con- 

 ditions in the laboratory, the time required for moistened bark bearing 

 perithecia to begin the expulsion of spores was determined, the shortest 

 time recorded being three minutes. 



The duration of the shooting period following a rain was determined 

 by artificial tests in either the field or laboratory, performed by soaking 

 the specimens or drenching cankers with water. The maximum dura- 

 tion recorded was five hours and two miniutes. While these tests under 

 artificial conditions gave suggestive results, they were not necessarily a 

 reliable indication of what would happen under natural conditions. 



It was also determined that bark kept constantly moistened continued 

 to expel spores for a maximum period of 25 days, and the point was 

 emphasized that no continuous rainy weather would be longer. The 

 fact that ascospores expelled during a rain would be washed down to 



