PATHOLOGICAL CONDITIONS 95 



Silver Leaf. — Sorauer^^'' describes one type of silver leaf occurring 

 on apricots, plums, cherries and apples. The immediate cause of the 

 silvery or milky appearance of the leaves is the partial separation of the 

 epidermal cells from one another and from the palisade cells, the inter- 

 cellular spaces becoming greatly enlarged. The older leaves are more 

 subject than the younger. This disease is usually associated with some 

 gummosis of the limbs and in aggravated cases the affected branches die. 

 Aderhold suggests that the failure of the middle lamella to cement 

 adjoining cells is due to a lack of calcium, which permits the pectin to 

 become soluble. As the disease generally occurs locally in the plant, 

 the lack of calcium is not the result of a deficiency in the soil but is due 

 to a local disturbance in the conducting system. 



Some other forms of silver leaf occasionally appearing in the orchard 

 and affecting entire trees or entire orchards may be due to quite different 

 causes. 



Lithiasis. — Drought at or shortly before the riaaturing season of pears 

 has been noted often to cause increased grittiness of the flesh, the stony 

 aggregations around the core becoming larger. Sorauer^^^ describes an 

 aggravated form of this trouble under the name lithiasis. In this drought 

 disease sclerotic tissue develops near the surface of the fruit, particularly 

 on the sunny side. Ordinarily it is found only in cases of extreme 

 drought. 



Summary. — Either an excess or a deficiency in soil moisture is likely 

 to be accompanied by a disturbed condition within the plant and often 

 by the appearance of some pathological symptom. Among those brought 

 on by excesses in the moisture supply are fruit splitting, fasciation, phyl- 

 lody, oedema, chlorosis, scaly bark and water core. High atmospheric 

 humidity is an important contributing factor in oedema; fruit splitting 

 is due to an irregular soil moisture supply as much as to an excess. 

 Measures against all of these troubles should be preventive rather than 

 remedial. They include provision for adequate drainage and caution 

 in the use of irrigation water. Premature defoliation and the attend- 

 ant ripening of the wood is one of the more serious results of a moisture 

 deficiency. It is likely to be followed by decreased vegetative growth, 

 lessened yields and in extreme cases, dieback. The earlier entrance 

 into the rest period and the poorer maturity of the wood both tend 

 toward susceptibility to winter injury. Dieback, rosette and little- 

 leaf are closely related disorders of the tree due in many cases to summer 

 drought. Often associated with these tree diseases, but sometimes more 

 or less independent of them, are a number of closely related diseases of the 

 fruit itself that have been described under the names: fruit-pit, cork, 

 drought spot, bitter-pit, Baldwin-spot, Jonathan-spot and black-end. 

 It is probable that some of these terms as commonly used refer to one and 

 the same trouble, or at least they overlap. This group of disorders, 



