THE MONTHLY BULLETIN. 



23] 



shout. II' the disease is virulent and conditions favorable, it may attack 

 small branches of the previous season's growth, killing them hack to a 

 distance of several feet. The dead area about the base of the infected 

 leaf or shoot varies greatly in size and is ordinarily more or less irregu- 

 lar in shape, frequently extending so listance along the stem. (See 



Pig. 58.) In the case of younger growth, girdling is apt to occur and 

 the shoot dies hack to this point. The rapidity with which the disease 

 spreads is remarkable and only a few days of the proper weather con- 

 ditions are required for serious damage. 



As the dry season approaches, activity is retarded and in midsummer 

 the disease is apparently quiescent. The wilted shoots and leaves 

 shrivel and dry. At this season there may appear a veritable herbaria 

 of various fungi, among which are the common withertip, Cladosporium 



• 4 V 





Fio. 59. — Orange tree affected with Citrus blast. Bacterium citrarefaciens. 



The photograph was taken in the late spring. Note the shriveled leaves 



still hanging on t lie liviv 



ami gray mold. The line of separation between healthy and infected 

 tissue becomes very clear-cut and definite. Frequently in early summer. 

 a pinkish, resinous gum is exuded from the lesions. These, as well as 

 all the dying or dead tissues, take on a characteristic dark color and 

 are sunk below the surface of the healthy tissue. This dead wood, as 

 il dries, becomes very hard and tough. The healing process begins, and 

 in the wounds callous material is developed which raises the dead tissue 

 op in the form of scabs. These are sloughed off in the course of time, 

 leaving very characteristic scars which may be visible for several years. 

 A peculiarity noted is that the infection frequently does not seem to 

 penetrate through the bark into the wood. Indeed, in some the cam- 

 bium layer is not killed, which is evidenced by the formation of callous 

 material in the center of the lesion as well as about the edges. Where 

 trees containing a large amount of weak, twiggy, interior brush are 



