Page 32 



BETTER FRUIT 



May, 1920 



Winter Injury or Die-Back, Etc. 



Continued from page 10. 



for the welfare of the tree during the 

 winter. The regular practice of fall 

 irrigation in walnut groves may be con- 

 sidered as a profitable form of insur- 

 ance since it is impossble to predict 

 the occurrence or the volume of winter 

 rains. We know that frequently the 

 rains are too late and insufficient for 

 the welfare of walnut groves. It goes 

 without saying that trees on a light 

 sandy type of soil are in greater danger 

 of injury from winter drought than are 

 trees on a heavier type of soil. In addi- 

 tion to soil types there are other factors 

 involved in the production of winter 

 injury, such as, the character of the 

 rainfall, the rate and amount of evap- 

 oration, temperature, etc. 



The growth of a winter inter-crop 

 such as barley, during a normal season 

 may bring about the same adverse soil 

 moisture condition that the dry winter 

 does, unless great care is taken to apply 

 enough water for the needs of both 

 trees and inter-crop. For example, if 

 the barley is considered, the soil may 

 become excessively dry about the time 

 that the hay is maturing, which is the 

 time the walnuts should be starting into 

 active growth. 



Unless fall irrigation is practiced the 

 growth of summer inter-crops between 

 the walnut trees may be conducive to 

 winter injury of the trees. This is due 

 to the fact that the moisture is reduced 

 to such an extent at the end of the 

 growing season that the trees are in- 

 jured before the winter rains occur. 

 Such a case is well shown by an or- 

 chard of three-year-old walnuts in 

 Tustin which was interplanted to pep- 

 pers. The peppers and trees were last 

 irrigated August 28, 1918. There were 

 no early fall frosts to complicate the 

 case, as shown by the fact that peppers 

 were harvested as late as November 

 15 in this grove. In the spring of 1919 

 the majority of the trees showed serious 

 winter injury throughout the tops. 

 Without question, the soil moisture dur- 

 ing practically the entire dormant per- 

 iod of the trees was considerably be- 

 low the wilting point, except in the 

 surface fool of soil. The winter rains 

 of 6.97 inches which fell in small inter- 

 mittent amounts, penetrated the soil in 

 nearby bean fields to the extent of only 

 twelve to eighteen inches at the end of 

 the rainy season. 



The same results are often found 

 where beans have been grown as an 

 intercrop. Water is withheld from 

 such groves during the latter part of 

 the summer in order to ripen the beans. 

 After the beans are harvested the soil 

 of the grove is usually very dry. This 

 condition may be of benefit to the wal- 

 nut trees because it insures a state of 

 maturity which will bring them through 

 the winter in good condition. In such 

 cases, however, winter irrigation is 

 usually necessary and should be ap- 

 plied soon after the leaves fall from 

 the walnut trees. 



3. High Water-Table— A high water- 

 table may be a contributing factor to 

 the die-back of walnuts. When such a 

 condition exists it is usually impossible 



to bring about the early maturity of the 

 walnut trees by withholding the late 

 summer and fall irrigations. 



In the presence of a high, more or 

 less stationary water table, the twigs 

 and foliage, especially on young trees, 

 usually remain green and succulent 

 until December, unless killed earlier by 

 frosts. 



Another condition which is equally 

 critical and as apt to injure bearing 

 trees, as well as young ones, is the oc- 

 currence of a fluctuating water-table. 

 The sudden rise of a fluctuating water- 

 table kills that portion of the root sys- 

 tem which is located in the saturated 

 stratum. In severe cases where the 

 major portion of the root system is 

 killed the twigs and young limbs of the 

 tree later exhibit typical cases of "die- 

 back." It might seem paradoxical that 

 that top of the tree should dry out and 

 die when the roots stand in an exces- 

 sively wet soil, but there is nothing 

 contradictory in the situation when it 

 is seen that the death of the major por- 

 tion of the roots makes it impossible 

 for the top to receive the necessary 

 moisture to sustain life. 



4. Alkali Injury — Alkali injury is 

 characteristic and may result in the 

 death of the walnut tree. On a soil 

 which contains alkali in harmful 

 amounts, the edges of the leaves of 

 walnut trees are often burned. This 

 condition may often be seen as early as 

 June or July and is followed by the 

 shedding of the leaves in August and 

 early September. The sudden and pre- 

 mature exposure of the twigs and limbs 

 to the hot fall sunlight through the pre- 

 mature shedding of the leaves may 

 cause severe sunburning. In such cases 

 also the terminal portion of the twigs 

 put forth a new crop of leaves In Octo- 

 ber which grow until killed by winter 

 frosts. As in the typical frost injury, 

 the twigs sunburn during the winter 

 and much of the current year's growth 

 dies back. In extreme cases of alkali 

 injury, the whole tree dies after having 

 suffered increasing injury for several 

 years. 



Summary — Winter injury or die-back 

 of walnuts is characterized by a sudden 

 death of the tops of the trees. Such 

 injury is usually first noticeable during 

 the early spring following the dormant 

 period. 



The most common causes of the win- 

 ter injury or die-back are: 



1. Early autumn frosts kill the im- 

 mature, growing shoots. Young walnut 

 trees are more subject to injury from 

 this cause than older trees, because the 

 former are usually later in maturing 

 their new wood. Such frosts cause the 

 foliage to drop prematurely and injure 

 the growing tips of the twigs. The de- 

 nuded twigs are subject to further in- 

 jury from subsequent fall and winter 

 sunburn. The presence or extent of 

 this injury is usually overlooked until 

 the following spring. 



To reduce the danger from autumn 

 frosts it is advisable to withhold the 

 late summer irrigation in order to pro- 

 mote the early maturity of the trees. 



2. Winter drought causes 5ie-back in 

 either young or bearing walnut groves. 

 Trees suffering from this condition fail 



to make new growth in the spring, ex- 

 cept from the trunks or main limbs. 

 The new growth on such trees has lost 

 so much water during the winter that 

 the buds are unable to develop in the 

 spring. The cause of the die-back has 

 been found to be due to an extremely 

 low moisture content of the soil during 

 a large part of the winter. Under such 

 conditions the water lost from the 

 young shoots during the winter cannot 

 be replenished by the root system. 

 Under such conditions the shoots die 

 from desiccation. 



Fall and winter irrigation of the wal- 

 nut groves has been found to eliminate 

 the injury from winter drought. The 

 amount of irrigation will depend upon 

 the type of soil, the amount of soil 

 moisture present at the end of the har- 

 vest season, and the system of inter- 

 cropping used. 



3. A high water-table may be a con- 

 tributing factor in killing walnut trees. 

 A permanently high water-table causes 

 the trees to prolong their growing sea- 

 son, with the result that they are killed 

 by frosts. The sudden rise of a fluctu- 

 ating water-table may kill a large part 

 of the root system and produce a typi- 

 cal die-back in the tops, even though 

 the wood is mature. 



4. Alkali soils containing such a high 

 salt content as to injure the root sys- 

 tems of walnut trees, also cause the 

 tops of the trees to die back in response 

 to the root injury. In the initial stages 

 of alkali injury the leaves turn brown 

 at the margin and fall prematurely. 

 The denuded shoots sometimes put out 

 a new set of leaves in the fall. The 

 top of the trees gradually die back until 

 the entire tree is lost. 



The recovery of alkali-injured trees 

 has never been noted by the writers. If 

 injury is being caused by the use of 

 irrigation water containing too much 

 alkali evidently the use of such water 

 should be discontinued. 



The wi'itei'S are pleased to acknowledge the 

 assistance given by Mr. D. C. Wylie of the 

 Field department of the California Walnut 

 Growers, Association and by Mr. E. E. Thomas 

 of the Citrus Experiment Station. 



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