197619b0Nature1974867196302096196200028-0836196310.1038/197619b0ukNatureNatureNATUREnatureNature is a weekly international journal publishing the finest peer-reviewed research in all fields of science and technology on the basis of its originality, importance, interdisciplinary interest, timeliness, accessibility, elegance and surprising conclusions. Nature also provides rapid, authoritative, insightful and arresting news and interpretation of topical and coming trends affecting science, scientists and the wider public./nature/journal/v197/n4867issueJournal homeArchiveCurrent issueAdvance online publicationPrivacy policySubscribeNature Publishing GroupCurrent issue197619b0Possible Presence of a Phytotoxin in Red-ring Infected Trees
AU  - GOBERDHAN, L. C.Trinidad and Tobago Coconut Research, Ltd., Eastern Main Road, Champs Fleurs, Trinidad.RED-RING disease of coconuts is caused by the nematode Rhadinaphelenchus cocophilus (Cobb 1919) Goodey 1960, which colonizes the stem in a ring 3-5 cm inside the periphery where the fibro-vascular strands are densest, and causes the formation of a distinctive red-orange band in this zone. Characteristic symptoms are yellowing and wilting of leaves.Although the causal agent and its method of spread have been examined1, little or no work has been reported on the damage mechanism. The symptoms of red-ring disease are almost identical with those of bronze leaf wilt, which has been clearly shown to be due to vascular failure2; it is therefore possible that the damage mechanism of red-ring disease is similar in nature. This might result from blockage of vessels in the infected zone, but injection experiments using dyes into healthy and into diseased trees do not support this hypothesis, since the dye is detectable in the stem right up into the bud and out into the leaves3. Moreover, growing, healthy coconut trees can suffer very extensive damage to the stem and therefore to the vascular strands without obvious deleterious effects.
It is also possible, however, that wilt symptoms can arise from other causes such as the production of toxins. This may result in decreased efficiency of leaf cells with a consequent reduction in maintenance of the full transpiration stream4-6. Gross observations indicate that this may be the case in coconut trees infected with red-ring.
Preliminary experiments on seedlings and cuttings of tomato confirm this latter hypothesis. Hot- and cold-water extracts of red-ring infected tissue induce rapid and severe wilting of these plants, the effect being more rapid in the case of cuttings. The effect appears to be independent of the age of the extract and of the presence or absence of tissue fragments or Rh. cocophilus in the extract. The behaviour of tomatoes in extracts of healthy tissue is different, in that freshly prepared cell-free extracts do not produce wilting unless the extracts are allowed to stand for several hours before removal of tissue fragments. If tissue fragments be removed, it is necessary to stand the cell-free extracts overnight before they can induce wilting. There is, therefore, reason for believing that the toxin is not produced directly by the nematodes but is the product of tissue disintegration as a result of nematode invasion.
Exposure of extracts to temperatures of 100 C for 30 min does not affect their ability to induce wilting, and it is justifiable to suppose that the toxin, if present, is relatively thermostable.
Other tests have been carried out using three palms, Ptychorapis augusta, Licula spinosa and Thrinax morissii, as indicator plants to show the presence of a possible vivo-toxin. The results of tests with these plants are similar to those using tomato, although wilting is less pronounced. Wilting in diseased tissue extract does not occur in less than 72 h after immersion, while wilting in cell-free, healthy tissue extracts, takes far longer. These palms are far too small to observe either the external symptoms, such as yellowing, or internal symptoms, such as discoloration. In tomato plants, however, a distinct pattern of yellowing is observed. Yellowing precedes wilting in these plants. The yellowing begins along the midrib, extends along the lateral veins, then the inter -veinal spaces yellow and finally necrosis of the leaf occurs.
It is suggested that these observations are consistent with the production of a toxin or toxins in coconut tissues, and that the toxin or toxins are produced indirectly as a result of tissue breakdown following nematode invasion.Fenwick, , D. W., and Maharaj, , S., Nature, 185, 259 (1960).ArticleBain, , F. M., Bronze Leaf Wilt Disease of the Coconut Palm (Govt. Printing Office, Trinidad, 1937).Goberdhan, , L. C., Ann. Rep., 1961, Trinidad and Tobago Coconut Research, Ltd. (1961).Kalyansundaram, , R., J. Indian Bot., 33, 329 (1954).Struckmeyer, , B. E., Beckman, , C. H., Kunts, , J. E., and Riker, , A. J., Phytopath., 44, 148 (1954).Talboys, , P. W., Trans. Brit. Mycol. Soc., 40, 415 (1957).