Mar. 25, 1915 Dissemination of Chestnut-Blight Fungus 



509 



it should also be noted that on September 20 (Table II), when foggy 

 weather followed the rain, spores were prevalent in the air during at least 

 five hours after the rain had ceased. 



A glance at the figures representing the number of spores falling each 

 minute on a surface equal to i square foot shows that during periods of 

 one to four or more hours after a rain — in other words, during such time 

 as expulsion continues — healthy trees among diseased ones would be sub- 

 ject to infection, since some of the ascospores would find lodgment upon 

 exposed parts of trunks and branches. 



The results obtained in the early morning of September 20 by making 

 exposures in an open field at varying distances from the principal source 

 of spores (figs. 2 and 3) are presented in Table VIII. 



Table "VIII. — Relation of distance from source of spores to number of spores falling on 

 an area of I square foot per minute in IQI3 at West Chester, Pa.<* 



a Plates exposed on Sept. 20, 1913. 



These exposures, all made within about half an hour and in the same 

 general direction from the plot of diseased trees — i. e., the direction 

 toward which the wind was blowing — show that in a general way the 

 number of spores falling upon equal surfaces in equal intervals of time 

 decreases as the distance from the source of spores is increased. The fact 

 alone that in an open field at rather long distances from diseased trees 

 ascospores were prevalent in the air to such an extent that every minute 

 from 6 to 24 spores were settling upon a surface equal to i square foot 

 (PI. LXIII, figs. I and 2) indicates that at such a time many opportunities 

 would be offered for exposed parts of undiseased trees at considera- 

 ble distances from diseased ones to become infected by wind-borne 

 ascospores. 



Furthermore, these results show that the maximum distance over 

 which ascospores might be transported by the wind was by no means 

 obtained, and the large numbers found at the longest distances in this 

 experiment, given in Table VIII, when a light wind prevailed, indicate 

 that even with a relatively light wind ascospores are probably conveyed 

 distances far greater than these. 



In a consideration of the exposure plates yielding the high numbers of 

 colonies of Endothia parasitica it is interesting to note the relatively 

 large proportion of the spore content of the air fomied by ascospores of 

 this fungus at certain times (Table IX). 



