WALNUT DISEASES 



2013 



regular concentric manner. Experience 

 has amply demonstrated that our native 

 black walnut roots are immune to this 

 fungus and we have seen cases where 

 English walnut, prune, almond and other 

 roots have been picked out and killed, 

 while northern California black walnuts 

 interplanted with these trees remained en- 

 tirely unaffected. It is altogether prob- 

 able that all of our black walnut species 

 and hybrids are highly resistant if not 

 totally immune to this disease. 



Fig. 1. Walnut Killed by Root Rot. 

 Armillaria mellea. 



Seedling Root Rot— Wilt 



Nursery seedlings of the Southern Cal- 

 ifornia black walnut occasionally wilt and 

 die rather suddenly, and show, on exam- 

 ination, a black rot of the main root just 

 below ground. This may appear either 

 before or after grafting, and, ordinarily, 

 at the worst, picks out only a tree or two 

 here and there, even in a large nursery. 

 We have known of only one case where 

 serious loss was experienced from this 

 source, and this on poorly drained land 

 where the trees were injured by an excess 

 of water. The trouble is caused by a soil 

 fungus, and seems to be confined to the 

 Southern California black. 



Sliriveled Meat 



Much trouble is experienced with wal- 

 nuts in certain seasons on account of the 



meat being shriveled and poorly devel- 

 oped. This affects some varieties and 

 some individual trees more than others, 

 and is often much worse the same year in 

 certain localities than in others. The 

 trouble is more apt to affect varieties 

 which come out late in the spring than 

 those which develop early. The cause of 

 this poor development or shriveling of 

 the meat has been ascribed to various in- 

 fluences, none of which have been posi- 

 tively proven to bring about this troublOc 

 The influences suspected have been the 

 same as those to which perforation or 

 non-development of the shell have been 

 laid, namely, poor pollination, lack of soil 

 moisture, and the attacks of the walnut 

 aphis. Which of these actually causes the 

 trouble has not been positively deter- 

 mined. 



While this trouble cannot be attacked 

 specifically, it can doubtless be alleviated 

 by sufficient irrigation, cultivation and 

 aphis control. 



California Experiment Station Bulletin 231. 



Sun Burn 



Serious damage is sometimes caused 

 from this source, both on the fruit and 

 the tree. In the latter case the trunk is 

 usually affected, dead areas developing in 

 the bark on the sunny side. This origi- 

 nates mostly in winter when the nights 

 are cold and the days bright and sunny. 

 Not usually serious on thrifty trees grow 

 ing in good soil. Black walnut trunks 

 with rough bark are not affected by sun 

 burn, so that trees grafted high on such 

 trunks are immune. 



On the fruit the sun sometimes has a 

 disastrous effect, causing a blackening 

 and burning of the husk on one side dur- 

 ing extremely hot weather in summer. 

 The meat is also blackened, and the husk 

 sticks to the shell so that separation is 

 difficult and a black spot is left. Thrifty 

 trees are less affected, particularly when 

 supplied with an abundance of moisture 

 at the root. Individual trees or varieties 

 vary in susceptibility to this trouble, and 

 this quality should be considered in plant- 

 ing in localities where trouble from sun 

 burn is likely to occur, j^ g. Smith, 



California Experiment Station Bulletin 218. 



