OBSERVATIONS ON PHLOEM NECROSIS OF POTATO TUBERS

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Abstract
The results of field tests with random lots of potato tubers showing phloem necrosis of similar type are summarized. The vines from tubers of five of these developed typical leaf-roll symptoms, but those of three others showed no sign of leaf roll, or other symptoms that suggested a virus as the causal agent. The net necrosis in one of the latter samples was known to result from vine infestation of Paratrioza cockerelli Sulc, and in this case the stock returned to full vigour during the second season. It was further confirmed that once the vines show definite symptoms of leaf roll, the phloem elements of their tubers henceforth do not exhibit necrosis.The tissue contiguous to the necrotic phloem elements in the affected tubers was uniformly fluorescent, but the unaffected portion of such tubers, as well as non-necrotic tubers from positive leaf-roll plants, reacted negatively to ultraviolet light.

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