A Bacterial Disease of Stone Fruits 393 



fall. The older, first infected leaves are usually, although not always, 

 the first to go. One leaf after another falls, leaving finally only a tuft 

 of young leaves at the tip of each twig and giving the tree a characteristic 

 appearance (Fig. 60). 



On the twigs 



In some seasons black cankers form on the twigs in early spring, but 

 as a rule these do not appear in abundance until May or June. The 

 first visible indication of the canker is the appearance on the young shoots 

 of a water-soaked spot surrounding a lenticel. In some cases the diseased 

 area bulges out, more or less, in the beginning. As the spot enlarges 

 it soon elongates. Single spots measure from one to four centimeters in 

 length and often extend from one-half to two-thirds of the way around the 

 shoot. The area becomes more or less sunken in age, soon becomes brown 

 or purplish brown, then dark brown, and finally purplish black or black 

 (Fig. 61). It frequently happens, especially on the nectarine and the 

 peach, that several of the spots become confluent, forming areas which 

 extend from two to three inches along the stem and in some cases com- 

 pletely girdle it. In favorable seasons the tips of many of the shoots are 

 entirely blackened and killed (Fig. 62), this condition being most prominent 

 on the nectarine and the peach. In most cases the injury is overcome 

 gradually, and as a rule all traces of these black cankerous spots disappear 

 in the second year. 



The black spots on apricot twigs also disappear for the most part, but 

 on plum twigs many of them persist from year to year, forming perennial 

 cankers. As many as twenty-two cankers in different stages of develop- 

 ment have been counted on a single plum twig (a water sprout). Open 

 cankers also form on the apricot, the nectarine, and the peach, but they 

 are far less abundant on these trees than on the plum. 



The disease is usually more aggressive during the first two or three years 

 after planting than it is later. It is not only likely to defoliate the young 

 trees prematurely, but in many instances it kills back the twigs and 

 frequently stunts the trees. A case of this sort was under observation, 

 in which two hundred Elberta peach trees from an infected nursery were 

 mixed with a shipment of twenty-three thousand healthy trees. Within 

 three years from the time of planting, every tree in the orchard was more 

 or less diseased, and many of the trees remained small and produced 

 small, inferior fruit. 



