2008 



ENCYCLOPEDIA OK I'KACTICAl- HORTICULTURE 



Fig. 1. 



Walnut Blight. 

 Walnut Jiiuli'ilosis uu Twigs, Showing Lesion on Older Wood Healing Over. 



blight infe(-tion sprecad in the nuiseiw iuid 

 infected the leaves of a California black 

 walnut seedling. Several different spe- 

 cies of juglans have been artificially in- 

 fected from pure cultures of the organism. 



Cliaracleristics of Disease 



The disease may be found on any of the 

 tender new growing jiarts of the tree, such 

 as young nuts, branches, nursery stock, 

 and the vascular system and parenchyma 

 of the leaves. The affected parts have a 

 characteristic blackish color, and often 

 pronounced lesions are produced. The 

 disease never kills back the branches to 

 any great extent, neither does it cause de- 

 foliation of the tree, and were it not for 

 its occurrence on the nuts, would be of 

 little economic importance, except some- 

 times on nursery stock, where the disease 

 is especially severe in the succulent 

 growth of the graft. 



The disease is especially virulent and 

 destructive on the young nuts. Many of 

 these become diseased and fall when from 

 one-eighth to one-half an inch in diameter. 

 These infections on the small nuts may 

 take place at any point, but are more fre- 

 quent at the blossom end, and readily 

 spread to the kernel or meat of the nut, 



which becomes black and unfit for use. 

 The most destructive period of infection 

 is early in the season, during April and 

 May in Southern California. Later during 

 the summer, under favorable conditions, a 

 superficial infection may occur, which 

 shows as small, dark-colored areas scat- 

 tered over the surface of the nut. This 

 infection is usually of little importance, 

 as the climatic conditions are unfavorable 

 and the outer tissue of the nut has com- 

 menced to harden, and so is unfavorable 

 for the deep development of the disease. 

 The infection first appears as a small 

 discolored or water-soaked area. This 

 gradually increases in size and becomes 

 blackish in color, with a surrounding 

 water-soaked zone of affected tissue. The 

 diseased tissue at length becomes black 

 and may have a shrunken, dried out and 

 cracked appearance. The diseased condi- 

 tion in the branches does not continue to 

 increase after the first year and eventu- 

 ally dies out, when the in.1ury a.gain heals. 



Cause 



The disease hn.j been jiroven to be pro- 

 duced by a species of bacteria {Pscudo- 

 mnnas juglandis) living in the diseased 

 tissue. Professor Newton B. Pierce first 



