Pflanzenkrankheiten. 257 



rains; when there is a rapid drying on the bark, this period is 

 Short, but when drying is retarded, this period is correspondingly 

 extended. The tests indicate a general prevalence of ascospores 

 within the first 5 hours following the cessation of rains, with less 

 abundance during later hours. The longest period for the entire 

 series was 14 hours. 



During periods of dry weather ascospores, although not gene- 

 rally prevalent, may occasionally be detected by the exposure-plate 

 method. These are apparently stray ascospores expelled during 

 some previous period of rain and now loosened from lodgment on 

 sonie near-by objects. 



In and near badly diseased chestnut groves or forests the num- 

 ber of ascospores falling on each square foot of exposed surface fol- 

 lowing a period of rain, as indicated by exposure plates, is very 

 large and is sufficient to offer abundant opportunity for new infections. 



Ascospores are forcibly expelled in large numbers from the 

 perithecia during and after each warm rain in case the amount is 

 sufficient to soak up the pustules. Following a dry period a rain of 

 0.18 to 0.25 inch has been observed to cause copious expulsion of 

 ascospores, while rains of O.Ol to 0.10 inch, if iramediately preceded 

 by a copious rainfall, have been sufficient to cause the resumption 

 of Spore expulsion. 



As determined by the ascospore traps, the duration of expul- 

 sion depends on the rapidity with which the bark dries and only 

 continues when the stromata are moist. Under natural conditions 

 in the tield the period of expulsion for eight rains varied from 45 

 minutes to 13 hours and 14 minutes. 



In some cases at least the maximum of ascospore expulsion 

 occurs after the cessation of rain. 



The fact that the period of ascospore expulsion as determined 

 by the ascospore traps coincides in general with the period during 

 which spores were obtained by exposure plates points to these 

 forcibly expelled spores as the ones prevalent following periods of 

 rain. This is definitely substantiated by the development of colonies 

 in the exposure plates from ascospores only 



It is possible to determine the presence of ascospores of the 

 chestnut-blight fungus in the air under natural conditions in the 

 field by the Standard aspirator method of bacteriological analysis. 

 By this method positive results were obtained following four diffe- 

 rent rainy periods, but only when the period of aspiration included 

 a period of copious ascospore expulsion. 



By the use of water spore traps stationed at varying distances 

 from diseased trees it was possible' to determine that ascospores 

 are prevalent in the air and fall upon exposed surfaces in conside- 

 rable numbers, the number diminishing with the distance from the 

 source of supply. 



By making possible long exposures the water spore traps offered 

 some advantages over the exposure-plate and aspirator methods. 

 The presence of spores of the chestnut-blight fungus, however, was 

 never shown by this method unless the period of exposure included 

 a period of ascospore expulsion. 



The failure lo obtain colonies of Endothia parasitica from the 

 water spore traps exposed during dry periods, as well as the fact 

 that only ascospore colonies were indicated in the aspirator and 

 exposure-plate tests, points to the conclusion that pycnospores are 

 not generally prevalent in the air at any time. If present they 



Botan. Centralblatt. Band 129. 191.5 1"^ 



