328 UNIVERSITY OP CALIFORNIA EXPERIMENT STATION. 



the shoots and are more spread out over the surface. The disease does 

 most serious damage on the fruit and is worse in wet seasons and in 

 certain valleys. If the disease starts early the nuts remain small and 

 undeveloped and the amount of fruit is greatly reduced. The disease 

 is frequent in walnut-growing sections of France. Bordeaux mixture 

 has been tried for this disease and while it proved somewhat successful, 

 yet it was too costly and difficult to apply. 



Characteristics of Bacteriosis. Walnut bacteriosis may occur on all 

 the tender, new, growing parts of the tree, such as young nuts and 

 branches, the petioles of leaves, as well as midveins, the fine lateral veins 

 and adjoining parenchyma. On the affected parts characteristic blackish 

 colored areas or quite pronounced lesions are produced. The blackening 

 is, as Pierce suggests, brought about by oxidation of the tannic acid in 

 the tissue. At any point of injury in the tender growth of the walnut 

 a very similar black coloration will occur. 



On Branches. Bacteriosis is at first confined to small areas, but under 

 favorable conditions these increase in size to a lesion or diseased area 

 extending two or three inches in length on the green shoot. The disease 

 always has its beginning on the young succulent growth which may be 

 near the growing end or at any other point. When the disease infects a 

 branch near its end, that part may be killed back, but this seldom occurs 

 except when the diseased lesion is very near the end. In the worst dis- 

 eased lesions the tissue is killed inwardly to the pith, while in less severe 

 cases only the bark and wood are diseased. As the shoot becomes more 

 woody it is more and more difficult to infect and no tissue ever becomes 

 affected after the first few months of its growth. The disease after the 

 first year, even in well defined lesions, gradually dies out and the tissue 

 heals over the old lesion, although in some cases short lengths of the 

 worst diseased shoots may die back for a few inches. The diseased 

 portion on the twig at first forms a small, discolored or water-soaked 

 area which gradually increases in size, and at length the central portion 

 is black and is surrounded by a water-soaked margin or fermentation 

 zone. As the shoot becomes more and -more woody the active develop- 

 ment of the disease is checked and no further tissue is involved. Then 

 the whole diseased area becomes blackened in color. The diseased por- 

 tion in many cases comes to have a somewhat shrunken, dried-out, 

 deformed, cracked condition because of the killing and drying-out of 

 the tissue. 



During the spring when the flow of sap is active there is often quite an 

 accumulation of liquid in the pith cavity beneath very badly diseased 

 lesions. This liquid has been tested at different times to see if this was 



""'Diseases of Walnuts," Bailey's Encyclopedia of Horticulture. 



