416 F. M. Rolfs 



From the results it was evident that a moist lower surface is essential 

 for inoculation and spread of the pathogen on the leaves. Observations 

 show that dew plays quite as important a part in the dissemination of 

 the pathogen as does rain. Rain and clew are not only unportant fac- 

 tors for inoculation, but they also carry the bacteria to the healthy leaves, 

 twigs, and fruit, and thus frequently serve as agents of transportation. 



The bacteria are no doubt carried from tree to tree and disseminated on 

 the young leaves, fi'uit, and growing twigs by insects of various kinds. 

 Waite (1898), Stewart (1913), and others have proved that bees, wasps, 

 flies, snails, aphids, and beetles carry the pear blight organism from oozing 

 cankers to open blossoms and j^oung shoots, and thus spread the l3lossom 

 and twig bhght of apples and pears throughout the orchard. From the 

 large numbers of various insects often found working on the diseased 

 foliage and fruit of trees affected with Bacterium Pruni, and from the 

 nature of the bacterial ooze, there appears little doubt as to the importance 

 of insects in the clissemination of this organism. 



In spring, when the weather is cold and unfavorable for the develop- 

 ment of the organism and the insects are sluggish, only a few primary 

 inoculations occur and only a few scattered spots develop on the foliage. 

 As the season advances and the weather becomes warmer, the bacteria 

 multiply and escape from the spots of the early infections, the secondary 

 infections gradually become abundant, and the leaves that contained 

 only a few spots at first soon l^ecome dotted with them. Insects also 

 become more active, and secondary spots of infection multiply rapidly. 



In the inoculation experiments on plum leaves in 1913, snails and 

 aphids were undoubtedly the chief agents in convejdng the organism from 

 the inoculated diseased leaves to the young leaves as they mifolded. The 

 new leaves of the trees on which there were no snails nor aphids remained 

 free from infection. Poured plate cultures were made from each of eleven 

 snails and eighteen aphids collected from these trees. Three of the 

 snail cultures and two of the aphid cultures yielded colonies of the organism. 



The great frequency v\^ith which infection occurs on the leaves along the 

 ])ath taken by ants leaves little doul^t that these msects also aid more or 

 less in spreading the disease. Nevertheless, cultures from more than one 

 hundred ants gave negative results. 



As a rule, insects naerely carry the virus from leaf to leaf or from tree to 

 tree, and seldom introduce the bacteria directly into the tissue of the host. 



