rage lo 



BETTER FRUIT 



May, ig20 



nier following the inilinl injury. Such 

 growth in turn is less likelj- to mature 

 early and more likely to he injured by 

 autumn frosts than the normal tree. 

 Wiien the cycle of events just described 

 becomes established, it thus tends to 

 perpetuate itself. 



If the trees are kept thoroughly cov- 

 ered with whitewash during the winter 

 months, the injury resulting from sun- 

 burn may be somewhat reduced. Such 

 a treatment lends to maintain a lower 

 temperature within the twigs. Too 

 nnich reliance should not be placed 

 upon this treatment, for it has been ob- 

 served that at best it afl'ords only partial 

 l)rotection. As a general rule it has been 

 observed that frost injury to the foliage 

 of immature twigs, is practically certain 

 to be followed by die-back to some ex- 

 tent, in spite of any remedy known to 

 the writers at the present time. 



The coincidence of frosted foliage 

 and subsequent die-back in a five-year- 

 old grove is shown by Chart I. The 

 new wood on these trees was nearly 

 mature when the first frost occurred in 

 November, 1918. The observations on 

 die-back were made in May, 1919. The 

 majority of the trees that had been 

 frosted showed die-back injury; none 

 were frosted without subsequent die- 

 back. Two trees which were not af- 

 fected by the first frost still showed a 

 slight injury from die-back. In a few 

 cases, however, it was very apparent 

 from the blackened condition of the 

 twigs, especially the tips, that the wood 

 had been actually killed by the frost 

 at the time of the foliage injury. In 

 such instances as the last it is clear that 

 remedial measures will be of no avail. 



CHART I.— THE RELATION BETWEEN AU- 

 TUMN FROST INJURY AND DIE-BACK IN 

 A FIVE-YEAR-OLD WALNUT GROVE. 



10 9 8 7 fi n d q ■> 1 



Die-back trees, a; normal trees, o; trees 

 showing both frost injury and die-back, x. 



Injury from frost may be prevented 

 if it is possible to mature the young 

 walnut trees early in the autumn so 

 that the leaves will turn yellow and 

 normally fall from the tree before the 

 first early frosts which usually occur 

 from the 1st to the 15th of November, 

 in many of the lowlands in Southern 

 California. Early maturity can be pro- 

 moted by withholding irrigation water 

 in the late summer and early fall. Trees 

 which are not irrigated after August 

 15th may be expected to mature earlier 

 than trees watered in late August and 

 September. Whenever the soil mois- 

 ture is abundant in the autumn, due 

 either to heavy summer irrigation or to 



a high water-table, Ihc trees will cdu- 

 tinuc their growth so late as to be in- 

 jured by autumn frosts. A high water- 

 table and a frosty orchard site ntake 

 winter injui-.\ a piaclical certainty. It 

 is doubtfid if w.ilnuts can ever be suc- 

 cessfully grown on such lands. 



2. Winter Di-ought — 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 die-back due 

 primaiily to a lack of sullicient soil 

 moisture during the winter months. 

 Dui'ing the winter, trees give off mois- 

 ture through the limbs 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 dessi- 

 cation. This injury is first evident 

 when such branches fail to produce 

 new grow'th the following spring. 



It is usually not difficult to distin- 

 guish between the injuries caused by 

 autumn frosts and by winter drought. 

 Frost injury is usually confined to one 

 or two-year-old wood, but winter 

 drought may kill back limbs eight years 

 old. Winter sunburning is more com- 

 mon on shoots killed by autumn frosts 

 than on those killed by winter drought, 

 though the sunburned areas may not 

 be clearly defined until January. 



The principal contributing factors 

 which bring about such a condition are 

 dry winters and the lack of winter irri- 

 gation. Under such conditions the soil 

 may have little or no moisture available 

 for the tree roots below the surface foot 

 and even this zone does not contain an 

 optimum amount of moisture until raid- 

 season of the winter rainy period. 



The winter of 1917-18 was followed 

 by a great deal of die-back injury to 

 bearing walnut trees. An examination 

 of walnut orchards during this winter 

 showed that most of the soils were very 

 dry. In several cases in orchards 

 which subsequently showed die-back 

 injury, the moisture in the first four 

 feet of soil was below the wilting point 

 until late winter. Thus the roots were 

 unable to acquire sufficient moisture to 

 replenish the loss from the trees and 

 consequently the tips were killed by 

 excessive desiccation. 



We have found that winter-injured 

 trees are more susceptible to die-back 

 the year following the initial injury, 

 than are normal trees. The increased 

 susceptibility of injured trees is inevit- 

 able because of the succulent nature of 

 much of the young growth produced. 

 In the summer following the first "die- 

 back," the amount of living wood is 

 small in comparison with the root sys- 

 tem, and is forced into very active 

 growth. The type of growth produced 

 is therefore similar to that following a 

 severe pruning of the tree. 



The relation between soil moisture 

 and winter injury can be seen at a 

 glance in Chart 1. For convenience in 

 observing the winter-injury in this or- 

 chard, we have divided the trees into 



three classes: first, trees not winter- 

 killed; second, trees slightly winter- 

 killed, and third, trees badly winter- 

 killed. Reference to Chart 2 shows that 

 the winter-injury was greatest in the 

 lower section of the grove, less in the 

 middle section, and least in the upper 

 section. The two tree rows in this or- 

 chard below the lower section shows 

 less injuiy than the section just above 

 them, due apparently to the fact that no 

 waste water is allowed to run off this 

 orchard, but is held by a dike and thus 

 the lower two rows get an extra amount 

 of water, 'i'his orchard has been nor- 

 mally irrigated at the rate of only 1.8 

 acre inches per irrigation per month 

 fi-om May to October. 



The actual number of trees injured 

 in each section was as follows: 



No Slight Badly 



injury injury injured 



Upper section 48 4 



Middle section 12 26 14 



Lower section* 1 16 34 



•One tree is missing in this section. 



This distribution and the fact that it 

 cannot be due to chance, is clearly set 

 forth by the diagram. A soil survey 

 of this orchard shows no appreciable 

 difference in the surface soil or subsoil 

 types within seven feet of the surface. 

 This and other examples which have 

 been studied, point clearly to a lack of 

 soil moisture as the important factor in 

 causing winter injury of the walnut 

 trees in this and many other orchards. 



CHART 2. — DISTRIRUTION OF WINTER- 

 KILLED TREES, O. W. MORROW GROVE, 

 HEMET, MAY 15, 1919. 



UPPER SECTION 



ooooooooooooo 

 ooooooooooooo 

 ooooooooooooo 

 oxoooxoxxoooo 



MIDDLE SECTION 



oxxxxoxxxoxxo 

 ooxxoxxxooxxx 

 oaxxxaxaooaaa 

 xaaaaxxaaxxaa 



LOWER SECTION 



xxax axaaaoaa 

 xxaxaaaaxxxaa 

 xaxxaaaaaxaaa 

 aaxxaaaaaaaaa 

 oxxxaaxaaxaax 

 ooaxxaooxoxao 



o, trees not winter-killed; x, trees slightly 

 winter-killed; a, trees badly winter-killed. 



In view of results such as those pre- 

 sented in the foregoing pages it seems 

 evident that it is not difficult to prevent 

 this type of die-back, or winter injury, 

 due to winter drought. The application 

 of sufficient irrgation water to reach 

 the tree roots soon after the harvest 

 season will tend to prevent such in- 

 juries. The grower should use a soil 

 tube, or a soil auger, in the autumn to 

 determine whether the soil to a depth 

 of five feet contains enough moisture 



Continued on page 32. 



