15fi Journal of Afiriciilture, Victoria. [10 March, 1916. 



are i^roduced. Vegetable pathologists state that this blackening is 

 brought about by oxidation of the tannic acid in the tissue. 



Yery little having been done in Victoria with regard to the physio- 

 logical and pathological characteristics of this disease, the following 

 abridged extracts are from Bulletin ISTo. 231, an American work already 

 referred to, and are the results of exhaustive scientific experiments, both 

 in the laboratory and field. 



Cause of Disease. — This disease has been conclusively proved to be 

 produced by a species of bacteria growing in the diseased parts. A 

 microscopic examination of diseased tissue shows countless numbers of 

 these small, rod-shaped organisms to be present. By employing 

 bacteriological methods pure cultures of these germs were obtained, and 

 then these pure cultures were used in making artificial inoculations into 

 healthy nuts and shoots, thus again producing the disease by inoculations 

 from the culture if the tissue was in an active growing condition. This 

 disease causes very characteristic, comparatively small, sunken black 

 areas on the small shoots of the trees. It does not attack branches of 

 any size, and does not injure them to such an extent that they die back 

 for several feet, as in the case of a trouble termed die back, which is 

 principally caused through a dry subsoil, planting the trees too closely 

 together, deficiency of plant food, and other causes. Although bac- 

 teriosis attacks the leaves, it does not cause defoliation of the trees, and 

 if this disease did not attack the nuts would be of little economic import- 

 ance. Young trees are much more free from bacteriosis than are those 

 that have been in bearing for a longer time. 



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 2 or 3 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 diseased lesions the tissue is killed inwardly to the pith, Avhile in 

 less severe cases only the bark and wood are diseased. As the shoot 

 becomes more woody it is more and anore difiicult 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, discoloured, or water- 

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

 portion becomes black, and is surrounded by a water-soaked margin, or 

 fermentation zone. As the shoots become more and more woody, the 

 active development of the disease is checked, and no further tissue is 

 i^'volved. Then the whole diseased area becomes blackened in colour. 

 The diseased portion, in many cases, comes to have a somewhat shrunken, 

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

 out of the tissue. The diseased tAvigs of the previous year are without 

 doubt the chief source of the initial infection each spring. 



The catkins are probably not diseased by walnut bacteriosis; they 

 often turn black, but this is probably only due to the natural process of 



