648 INSECTS AFFECTING VEGETATION 



particularly during the winter and in the fall after the crop is har- 

 vested. 



Trunk Rot. The trunk or main branches rot, with a produc- 

 tion of small, white bracket toadstools on the surface of the affected 

 wood. This fungus does not affect sound trunks, but follows deep 

 wounds, bruises, heavy cutting, sunburn or other injuries. The wood 

 of the English walnut is particularly susceptible to this form of 

 decay. All severe cuts or wounds should be carefully covered with 

 grafting wax to allow healing over without decay. In top-grafting 

 walnut trees this should be given special attention or serious decay 

 may follow. 



Sunburn. 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 originates mostly in winter when the nights are cold and 

 the days bright and sunny. Not usually serious on thrifty trees 

 growing 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 during 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 susceptiblity to this trouble, and this quality should be con- 

 sidered in planting in localities where trouble from sunburn is likely 

 to occur. 



Perforation. The shell fails to develop properly, being only 

 partially formed with numerous openings and thin places. This ap- 

 pears to be simply a lack of development due to climatic conditions 

 and occurs in season with a dry spring and a dry, hot summer. 

 Seems worse on trees attacked by aphis. 



Erinose. Blister-like swellings appear on the leaves, sometimes 

 becoming very abundant. This trouble is caused by an insect, so 

 minute that it is not visible to the eye. Not serious or requiring 

 treatment. 



Die-Back. The limbs die back from the ends, sometimes for 

 only a short distance and sometimes down to the main forks of the 

 tree. All the limbs or only a part of them may be affected. Two 

 general types of die-back may be distinguished, one on the old trees, 

 particularly "hard shells," which die back slowly all over the top, 

 and another in young trees from two to ten years old, which die back 

 suddenly during a single winter in part or all of the limbs, or some- 

 times the whole tree dies back clear to the ground. 



The former type of die-back affecting old trees occurs mostly on 

 light soils where the trees are getting old and the roots find insuffi- 

 cient moisture. Such dying back is also hastened by lack of cultiva- 

 tion, irrigation and plant food, which lack becomes more pronounced 

 as the trees grow older, and the tops and roots more crowded. The 



