BULLETIN 231] 



WALNUT CULTURE IN CALIFORNIA. 



375 



On the tree itself the trunk is frequently affected by sunburn, 

 causing on large trunks a condition frequently called by the growers 

 "black sap." This consists of a considerable area on the south or 

 southwest side of the trunk where the inner bark is dead and from 

 which the sap of the tree oozes out, oxidizing and turning black, as is 

 the habit of walnut sap whenever exposed to the air. This form of 

 sunburn occurs more commonly in the winter rather than in the 

 summer, and takes place at times when the days are hot and the 

 nights are cold. These conditions produce an irregular activity of 

 the sap, which causes the trunk to be especially susceptible to the 

 heat of the sun. The cambium is killed, usually on the southwest 

 side of the trunk, and the sap then escapes, oxidizes and ferments in 

 the affected area and a large wound may result in which ordinary 



FIG. 92. Sunburn on nuts. 



decay sets in. It is on account of this form of sunburn that high 

 grafting on black walnut stock is advocated in order to obtain the rough- 

 barked trunk of the black walnut. We have seen cases, however, when 

 even the latter was affected by this form of sunburn. Young trees 

 during their first year after planting in the orchard often become 

 badly sunburned in the trunk and on this account should be protected 

 after planting, as discussed on page 253. For the same reason it is 

 better to cut the trees well back before planting and grow a new stem 

 from one of the lower buds, as described on page 251. The whole 

 matter of sunburn, either on the fruit or the wood, is influenced to a 

 very large extent by the moisture conditions of the soil. In all cases 

 trees with an abundant moisture supply, and especially if in fairly 

 heavy soil of good water-retaining properties, are much less affected 

 by any form of sunburn, either summer or winter, than those which 

 are suffering for water. 



