88 FUNDAMENTALS OF FRUIT PRODUCTION 



"Considering the facts at hand it would seem that a lack of available soil 

 moisture, at critical periods in the vine's growth, or a lack of root aeration as a 

 result of the impervious subsoil together with the shallow depth of surface soil, 

 are the principal contributing factors to the affection. With this soil type the 

 sickness is at its height in seasons of drought as well as in those of excessive 

 rainfall. Soils such as the yellow silt are generally deficient in organic matter, 

 and hence in their water-holding capacity. With them the affection is worst in 

 seasons of drouth and least in those of normal rainfall. During early summer the 

 vine makes a rapid growth of succulent shoots and leaves which require large 

 amounts of water to develop."^" Newly planted vineyards, where the vines 

 do not yet have extensive root systems, are more Ukely to be affected. 



Dieback. — From the form of drought injury described in the grape, 

 it is but a step to more serious conditions resulting in the death of 

 some of the twigs, shoots or branches of the tree. This may occur in 

 trees of almost any kind, the symptoms varying somewhat in different 

 species. However, there is no mistaking the disease when it is present. 

 Without doubt dieback may be due to any one of a number of factors. 

 Chief among these is an inadequate water supply, not necessarily at the 

 time the symptoms are first noticed, but perhaps many months earlier. 

 Batchelor and Reed^^ have described dieback as it occurs on the English 

 walnut. Since its appearance there is fairly typical of its occurrence on 

 many other fruit trees the following account is taken from their report: 



"We have very convincing evidence to show that trees which enter the 

 dormant period in the fall in a perfectly normal and healthy condition may 

 suffer from dieback due primarily to a lack of sufficient soil moisture during the 

 winter months. During the winter, trees give off moisture through the Umbs 

 and twigs. If for a prolonged period there is not enough soil moisture available 

 to the roots, the trees are unable to obtain sufficient water to offset the loss by 

 evaporation from the branches. In that case young branches, the thin bark of 

 which permits rapid loss of water from the wood, may die as a result of desic- 

 cation. This injury is first evident when such branches fail to produce new growth 

 the following spring. . . . Frost injury is usually confined to 1-or 2-year old 

 wood, but winter drought may kill back limbs 8 years old. 



"Another condition which is equally critical and apt to injure bearing trees, 

 as well as young ones, is the occurrence of a fluctuating water-table. The sudden 

 rise of a fluctuating water-table kills that portion of the root system which is 

 located in the saturated stratum. In severe cases where the major portion of the 

 root system is killed the twigs and young hmbs of the tree later exhibit typical 

 cases of 'dieback.' It might seem paradoxical that the top of the tree should dry 

 out and die when the roots stand in an excessively wet soil, but there is nothing 

 contradictory in the situation when it is seen that the death of the major portion 

 of the roots makes it impossible for the top to receive the necessary moisture to 

 sustain life." 



Though much of the dieback or exanthema found in citrus trees is 

 due to disturbed conditions of nutrition there seems to be no doubt that 



