SPRAIN OR INTERNAL RUST SPOT OF POTATO

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Abstract
Summary.: In the present investigation necrosis of the tubers was found to be due to two distinct diseases: (1) Sprain or Internal Rust Spot, and (2) a disease hitherto unrecorded, which has been named Corky Bacteriosis of the xylem. The former is much the more important and accounts for 98 per cent. of the tissue destruction. In Sprain the tissue attacked is the parenchyma both within and without the vascular ring. The lesions vary greatly in form and may consist of spots, arcs, streaks or irregular blotches. The larger lesions are generally hollow in the centre. The histological structure of the lesions of Sprain varies slightly according to whether these occur in the starch‐packed cortical tissue or in the pith where the cells are less densely filled with starch, but in either case, is characteristic. A feature of the spots is that they are more or less completely invested with a zone of cork the inner layers of which are suberised. The soil on which. Sprain occurs in a virulent form is a light sandy loam deficient in organic matter and one on which potatoes scab severely. The disease is apparently slight when the crop is lifted but develops rapidly and continuously during storage. Infection of the crop arises mainly from contaminated soil and not from affected seed. The causative organism of Sprain has been isolated and typical spots of the disease have been reproduced by inoculation with it. It consists of a very short Bacterium, which, being new to the literature, has been named Bacterium rubefaciens. There is little evidence that this organism enters the fully formed tuber through its skin, and isolated spots which have no apparent connection with the peridenn are a feature of the disease. It is suggested that the organisms enter at a very early stage through the stomata or lenticels of the tuber‐bearing stolon, remaining quiescent in the water‐filled intercellular spaces of the tuber and becoming active only when the latter ripens. In potatoes affected with Sprain a certain necrosis of the xylem vessels may occur but may easily be overlooked. This Tracheomycosis is essentially different from that of Corky Bacteriosis. Corky Bacteriosis produces a browning and lignification of the vascular ring macroscopically resembling the Ring Bacteriosis of B. solanacearum. It is essentially a disease of the xylem vessels which thereby become closely invested with a thick layer of suberised cork. Corky Bacteriosis is transmitted from the mother tuber to its progeny by way of the stolons. It is also contracted from infected soil apparently by infection of the stolon or the stolon end of the tuber. The causative organism of this disease has been isolated and the disease reproduced by inoculation. The organism being new to the literature has been named B. suberfaciens.

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