PATHOLOGICAL CONDITIONS 91 



Dieback and Rosette. — Dieback in its early stages appears usually in the 

 spring. Some or all of the buds toward the ends of the shoots remain dormant, 

 while lower buds start. The shoot ends that do not vegetate may remain alive 

 all season or they may dry out and die earlier. "The appearance of one of these 

 dieback shoots the following summer was that of a completely dead tip from 

 6 inches to 1 foot long, often with a distinct marginal crack between it and the 

 living part below. From some point back of this tip a healthy lateral developed 

 to renew the branch."'^ 



The early stages of dieback may be observed in cross sections of dieback 

 twigs of the current season's growth. "Such a twig usually shows entirely dead 

 tissue near its tip and a discoloration in the cambial area running back for a 

 variable distance. Under the microscope this discolored zone shows, if the 

 sections are taken near the tip, a large number of cells with browned contents in 

 the cambium, phloem and pericycle. If sections are made from parts of the 

 twig a short distance below, it will be seen that growth has been made subsequent 

 to the injury. The injured cambium has produced a quantity of the so-called 

 parenchyma wood, the browned cells of the phloem and pericycle being pushed 

 outward. Finally, the parenchj^ma zone becomes buried by a layer of new 

 xylem, outside of which are found normal bark and cambium." . . . Often 

 some of the buds on the lower part of such dieback shoots "developed clusters 

 of very small, lanceolate leaves with shortened petioles. In some cases the twigs 

 made a very short terminal growth, resulting in a thickened, shortened axis an 

 inch or so long, bearing a cluster of leaves, some normal and some short lanceolate, 

 the general effect being that of a long bare twig capped by a rosette of leaves. "^^ 



In commenting on these diseases Mix remarks: "It is evident that we have 

 under consideration, not two distinct apple diseases, but at the most, two types 

 of the same disease: (a) Drouth spot, with which are associated abnormalities 

 of the foliage, called drouth dieback and drouth rosette; and (6) cork, which 

 may occur in association with drouth spot, but which often occurs independently, 

 and is then not associated, except rarely, with any disease of the foliage. 



" The writer's observations show that these diseases may occur in both wet and 

 dry seasons. There is, however, a marked relation of weather conditions to the 

 disease. They tend to disappear during wet weather and are much more serious 

 during a dry period, especially dry weather occurring early in the season. 



"Since, however, in a wet season, and under conditions where there seems to 

 be no deficiency of moisture, these diseases may occur in trees that have been 

 previously diseased year after year, insufficient soil moisture cannot be looked 

 upon as the sole cause. . . . 



"It is suggested that the exact manner of occurrence of the injury may be 

 by the leaves robbing the fruit of water during a critical period of low root supply 

 and high transpiration. Rapid wilting of the fruits can be brought about by 

 excessive transpiration from the leaves. It has been seen that this wilting may 

 result in the death of certain cells near the vascular bundles, forming lesions 

 resembling those of drouth spot, and occasionally, of cork. Chandler has pre- 

 sented evidence that transpiration from the leaves may bring about a scarcity of 

 water in the fruit under field conditions. It is not impossible that this is at least 

 one of the ways in which the disease may be caused. 



"This seems more likely than that injury is due to an excessive transpira- 



