Mar. 25. I9IS Dissemination of Chestnut-Blight Fungus 511 



occurs only when the perithecia-bearing bark has been wet by rains, the 

 following facts are presented to show that wind dissemination is directly 

 dependent upon weather conditions causing spore expulsion. 



Of the total number of 756 plates exposed during these tests 95 were 

 exposed while ascospore expulsion was known to have been in progress, 

 and of these, 41 yielded colonies of Endolhia parasitica. Of the remaining 

 661 plates exposed at other times than those noted above, but 23 yielded 

 colonies of E. parasitica, and 14 of these were exposed within 12 hours 

 after expulsion was known to have occurred. 



To bring out in a more striking manner the relation of rain to wind 

 dissemination, it is worthy of note that out of a total of 427 ascospores of 

 Endothia parasitica caught in the exposed plates 402, or 94 per cent, were 

 caught in plates exposed while spore expulsion was known to have been 

 in progress, and of the remaining 25 spores 3 were caught within 5 hours 

 after the cessation of a rain (Sept. 8) and 12 more were caught within 12 

 hours after ascospore expulsion was known to have occurred. This 

 leaves but 10 out of 427 spores, or 2.3 per cent, seeming to be stray 

 ascospores bearing no relation to a rain. 



As to the origin of the 22 colonies of Endothia parasitica appearing in 

 plates exposed when spore expulsion was known not to be in progress 

 (see Table II), the following points are cited to prove that they originated 

 from stray ascospores which, after expulsion, lodged on near-by or, per- 

 haps, distant trunks, limbs, or leaves and were subsequently loosened 

 by the mechanical action of some agency. 



1. All but one of the 21 plates containing these colonies yielded only 

 a single colony of Endothia parasitica each. 



2. In one colony a fragment of bark was visible at its center. 



3. All except one of these spores were caught at stations more or less 

 overhung by branches of diseased trees, and all except three were caught 

 on stumps surrounded by sprouts. 



4. Only I out of 192 plates exposed at unsheltered stations when 

 expulsion was not in progress yielded a colony of Endothia parasitica. 



If these had been stray spores that were still floating in the air since 

 expulsion, they would have fallen just as frequently into plates exposed 

 out in the open at unsheltered stations. During a period of ascospore 

 expulsion following a rain it seems probable that the spores would not 

 all be swept away by air currents but that some few would find lodgment 

 upon near-by leaves and branches. Such lodgment is especially hkely 

 to take place Lf there is no noticeable wind when expulsion is in progress. 

 Thus, it seems quite probable that the colonies obtained when perithccia 

 were not active originated from spores dislodged from either healthy or 

 diseased parts of trees more or less overhanging the plates. 



Unless attached to a bark fragment, the path of these spores in falling 

 would not necessarily approach the vertical, and such spores might be 

 transported by the wind just as readily as though they were freshly 

 expelled. This explains, perhaps, why one spore was caught in plate 

 No. 5037, exposed 27 feet from the nearest chestnut tree. The probable 

 reason, then, why, with this exception, such stray spores were caught 

 only under trees is that the rareness of their occurrence in the air pre- 

 vented their detection elsewhere than in very close proximity to their 

 place of temporary lodgment, since with the exposure-plate method the 

 chance of detecting these spores decreases very rapidly as the distance 

 from their source is increased. 



