Pathogenicity of the Chestnut Bark Disease 51 



under natural conditions, infection may be spread short distances 

 by the wind. 



To determine the probability of spores of the chestnut tree 

 blight fungus being in the air, seven agar plates were exposed 

 under infected trees for twenty minutes. Rain had fallen twelve 

 hours previous. In these plates many colonies appeared. These 

 were isolated on agar slants and identified as the true blight fun- 

 gus. Agar plates placed on the ground at a distance of 50 feet 

 from moistened pustules also caught spores. 



The aspirator method was used as a check on the above 

 experiment. The apparatus for this experiment consisted of an 

 aspirator bottle having a capacity of fifteen liters. This bottle 

 was filled with water. A glass tube drawn out to a point at one 

 end in the flame of a gas jet was inverted in the top of the neck 

 of the bottle. The tube was partially filled with sugar on top of 

 which was placed a cotton plug. The point of the glass tube 

 was broken ofT and the water allowed to run out thus drawing 

 fifteen liters of air through the sugar. The spores were deposited 

 on the sugar which was dissolved in sterilized water and centri- 

 fuged. The sediment was drawn out with a sterile pipette and 

 cultured on agar plates. From fifty liters of air examined by 

 this method three colonies were successfully isolated. 



An experiment was instituted to determine the length of time 

 which spores are capable of remaining suspended in the air. A 

 glass tube one inch in diameter and five feet long was placed in 

 a perpendicular position. A piece of bark containing perithecia 

 which were discharging ascospores at the rate of 3,000 spores per 

 minute was placed at the top of the tube so that the spores would 

 shoot into the tube. It was left in this position for ten minutes. 

 Clean slides were placed at the bottom of the tube to catch the 

 spores. The first spores fell in two minutes and the last spores 

 fell in four hours and sixteen minutes. 



It is not yet definitely determined what agency is most con- 

 cerned in the dissemination of the spores, but many investiga- 

 tors claim that insects are closely related to the progress of the 

 disease. The spores are easily carried upon the legs and bodies 

 of beetles and moths. Spores of the fungus were successfully 

 transferred from a culture to other media by causing insects to 

 walk over the cultures. Masses of spores were found adhering 

 to the hairs of the legs of an adult chestnut borer after the insect 

 had walked over conidial threads. 



In order to cause an infection, the spores must enter through 

 a wound or an abrasion in the bark. This leads many to believe 

 that the boring insects are especially harmful on account of the 



