76 FUNDAMENTALS OF FRUIT PRODUCTION 



bacterial and fungous diseases. A series of dry seasons is almost certain 

 to be accompanied by an increase in the virulence of the Illinois blister 

 canker in those regions where that disease is prevalent. ^^ The influence 

 of soil moisture on certain physiological disturbances is discussed later. 



Residual Effects of Soil Moisture. — The influence of precipitation or 

 of irrigation early during the growing season is more or less immediate. 

 On the other hand water falling or applied late during the growing season 

 may have less of an immediate effect on the plant and a correspondingly 

 greater effect at a later period, or even the following year. Particularly 

 is this true of late fall or early winter rains or irrigation. This is due 

 partly to the fact that some of the water is stored in the soil for later use 

 and partly to the fact that the benefit that the plant derives from absorb- 

 ing some of it immediately may not be apparent until considerably later. 

 It is thus proper to speak of the residual effects of soil moisture. 



On Vegetative Growth. — It is a common observation that trees suffer- 

 ing from drought in late summer and early fall shed their foliage early. 

 This is particularly true of species and varieties ripening their fruit 

 comparatively early. The function of the foliage during late summer 

 and fall is to manufacture food materials which, for the most part, 

 are stored through the winter for use in tissue building in the spring. A 

 large part of the new growth (roots, shoots, leaves and flowers) in early 

 spring is at the expense of stored foods. Premature defoliation, from 

 drought or any other cause, therefore, is likely to result in a check to 

 growth the following spring through cutting down the available reserves. 

 Though exact experimental data in support of this line of reasoning are 

 not available there is abundant circumstantial evidence and the record 

 of numerous observations is very suggestive. 



Whitten^^^ has assembled some data bearing on this question for the 

 years 1894-1898 (see Table 36). In commenting on these he says: "It will be 

 observed that the last part of the years 1894 and 1897 were marked by severe 

 drouths, and that the average growth of uncultivated trees fell off to a marked 

 degree during the next year or two after each of these dry seasons. Where 

 trees were well cultivated, to conserve the moisture in the soil, this falling off of 

 growth was not noticeable. . . . The unfavorable effects of drouth upon 

 uncultivated trees may not be so apparent during the dry year itself as it is 1 or 

 even 2 years later." Though unfortunately data are not available as to the 

 exact moisture content of the soils in these plots during the 5-year period in 

 question, there is little doubt about soil moisture being mainly responsible for the 

 differences in growth recorded. 



On Yields. — The residual effects of soil moisture are not limited to 

 vegetative growth. In all probability they have rather general influence 

 and affect yield. This is indicated by investigation of the olive industry 

 near Sfax in Northern Africa.''^ 



