. 410 WALNUT BACTERIOSIS. 



economic importance if it did not attack the nuts. These may 

 become diseased at ahnost any period df their development, but 

 become infected most readily while they are very small and while 

 the tissues are tender and growing rapidly. Frequently a large 

 number of the nuts become afifected when they are from one- 

 eighth to half an inch in diameter; the organism may attack them 

 at any point on the surface, but infection is by far the most com- 

 mon at the blossom end near the stigma. As on the leaves and 

 shoots the first evidence of infection is the water-soaked appear- 

 ance of the tissues; the area of infection spreads rapidly, and the 

 tissues blacken ; and become somewhat sunken ; the organism also 

 penetrates inwards and frequently, especially in the case of 

 blossom end infections injures the kernel. 



Lateral infections very frequently occur where two nuts are 

 in contact. 



When the nuts are attacked at an earlv stage in their growth, 

 large blackened areas develop, and the nut sooner or later drops. 

 Later infections which occur when the nut has almost reached its 

 full size, usually take the form of small dark spots scattered over 

 the surface; the late infections do not penetrate very deep, as the 

 tissues have by this time begun to harden ; but under favourable 

 conditions severe cases of late infection have been known to 

 occur, causing the hull to become blackened and shrivel and U> 

 cling to the shell of the nut. 



It is obvious that the lesions on the twigs are chiefly instru- 

 mental in carrying the disease over from one season to the next. 

 P'ierce* states that the organism " winter in fallen nuts beneatli 

 the trees and probably upon fallen leaves and upon the soil." The 

 danger from fallen nuts and leaves should be small in an orchard 

 which is properly cared for. and ])robably the risk of soil in- 

 fection is small as compared with that of infection from lesions 

 on the twigs. The organism is carried from branch to branch 

 and from tree to tree chiefly by the splash of the raindrops which 

 run over the infected surfaces, and also by wind-blown rain. The 

 leaves which are attacked early in the season are probably one of 

 the chief sources of infection for the fruit. 



Conditions Favouring the Disease. 



Walnut bacteriosis depends for its rapid development very 

 largely on weather conditions. The amount of blight varies from 

 year to year, and it is only severe when there is a considerable 

 amount of rain and a large number of cloudy days while the nuts 

 are forming. The organism infects the voung tissues through 

 the stomata, and in the presence of moisture the motile organism 

 moves freely over the surface of the nuts and invades the tissues 

 through these natural openings. 



It is probable, therefore, that the bacteriosis will cause 

 serious trouble in this country in districts where there is a heavy 

 rainfall in the spring and early summer. In less humid districts 



* Pacific Rural Press, 57 [25]. 387. 



