512 Journal of Agricultural Research voi. iii, no. s 



Obviously no exposures could be made during a rain, but ascospore- 

 trap examinations have shown that abundant spore expulsion may 

 occur during the actual fall of the rain. It is evident, however, that at 

 such a time wind dissemination would be reduced to a minimum, because 

 the spores upon expulsion would soon be washed to the ground or to 

 near-by bark or fohage. 



Therefore, under the conditions of our tests it can be said that, with the 

 exception of the few stray ascospores loosened from temporary lodgment, 

 wind dissemination of Endothia parasitica occurs only during certain 

 periods after rains, when ascospore expulsion is in progress. 



ASCOSPORE-TRAP TESTS 



In order to detect ascospore expulsion whenever it occurred, use was 

 made of what we have termed "ascospore traps." An ascospore trap 

 consisted of a glass object slide held in place over perithecial pustules 

 on the bark of a diseased tree by means of a wooden bracket either above 

 or below the slide (PI. LXIV, figs, i and 2). The slide was wedged firmly 

 into a slot in the bracket so as to be suspended about one-eighth of an 

 inch or less from the papillae underneath. These traps were placed on 

 lesions of various ages on trees more or less scattered throughout the 

 experimental plot (fig. i). 



As the ascospores of Endothia parasitica are expelled they adhere to 

 the glass, and the spores expelled from each ostiole usually form a definite 

 "spot," so that the number of spots on the slide represents the number 

 of perithecia in the area underneath which have exjjelled spores. 



During the progress of the work on wind dissemination, it was found 

 possible by means of these traps not only to detect the occurrence of 

 ascospore expulsion but to determine even with some degree of accuracy 

 the exact duration of perithecial activity. 



As has been brought out in the discussion of the exposure-plate tests, 

 the occurrence of ascospores in the air in numbers is directly dependent 

 upon the continuation of their expulsion after a rain has ceased. The 

 duration of expulsion becomes, therefore, an essential factor in deter- 

 mining the period during which wind dissemination may occur. 



In making this determination the method of procedure was as follows: 

 Out of the total number of 69 ascospore traps usually about 20 were 

 selected, representing areas of vigorous perithecia where previous expe- 

 rience indicated that abundant expulsion was most likely to occur. The 

 sUdes from these traps were collected as soon as possible after the rain 

 and were replaced with clean shdes. Then, after a convenient interval, 

 this second set of slides was collected and replaced with clean ones. This 

 operation was repeated at intervals of several minutes to several hours 

 until none of the slides bore spots of expelled ascospores. 



Several series of trap collections were usually made after each rain, 

 and a subsequent examination of each slide revealed whether or not any 

 expulsion had occurred under that trap in the period during which that 

 particular slide had been in place on the tree. Although usually visible 

 to the unaided eye, an examination with a hand lens was often necessary 

 to detect very faint or very diffuse spots of ascospores on the slides. 



The detailed results for September 20 are given to show the behavior 

 of individual traps (Table X). The results given in the summary for 

 the other dates were obtained in a similar manner and the individual 

 records will therefore be omitted. 



