<abstract language="eng">The writers review previous concepts concerning the reaction of citrus plants to the tristeza disease. It is recognized that environmental factors such as temperature, humidity and light can influence plant reaction, but these are considered of relatively minor importance. Characteristics of the plant itself govern its reaction to the disease and among those, resistance to infection, ability to permit virus increase and tolerance or non-tolerance of tissues are considered most important. Tests using viruliferous oriental citrus aphids for inoculation of various citrus types budded over sour orange rootstocks (table 1 and 2) showed a correlation between susceptibility to infection and severity of symptoms. The sweet oranges proved to be the most susceptible to infection and showed severe disease symptoms. The Barão sweet orange appeared to be more susceptible than Valencia. The mandarins tended to show some resistance to infection, but when infected, severe symptoms developed. The tolerant tangelos (7) behaved similarly to sweet oranges : They were very susceptible to infection and showed severe symptoms. The non-tolerant tangelos, susceptible citranges (7) and grapefruits behaved more or less alike, and showed medium susceptibility to infection and moderately severe disease symptoms. Among the grapefruits, Leonardy was. observed to be relatively more susceptible and showed more severe symptoms than Duncan. The pummelos, shaddocks and sour oranges were comparatively very resistant to infection and showed only moderate symptoms when infected. It has been found that the sour oranges can be more easily infected by tissue union than by the aphid vector. Poncirus trifoliata, citrumelos and resistant citranges showed no symptoms and no virus could be recovered from the inoculated plants even after three inoculations. The nature of injury caused by tristeza in the non-tolerant graft and intergraft combinations is discussed. The observation of symptoms shown by plants consisting of a sour orange inter-stem-graft between sweet orange roots and foliage seems to indicate that phloem collapse of the sour intergraft does not entirely prevent food translocation, since under field conditions the sweet stem below the sour intergraft continued, for almost two years, to increase in size at about the same rate as the sweet stem above. The growth of the sour intergraft was observed to be constricted and its lack of developments indicates a possible injurious effect of the disease on tissues other than the phloem. Root tissues of sour oranges have been found to be sensitive ;to injury, sincejsweet orange tops grafted directly onto sour orange roots and subsequently inoculated showed the usual tristeza symptoms. Death of rootlets and roots was found to occur not only in infected plants with tolerant tops and non-tolerant rootstocks but also on infected tolerant plants having an inter-stem-graft of non-tolerant sour orange. These observations and those made of root reaction on mechanically ringed plants indicate that although the root tissues of non-tolerant stocks may be sensitive to injury the rootlet and root symptoms are mostly secondary reactions. Tests carried out showed that the tristeza virus was recovered from insect protected sprouts grown at the ends of severed roots of Caipira sweet orange stock thus indicating that the virus was definitely present in the roots of a tolerant rootstock. Previously tristeza has been recognized as being associated with various citrus stock-scion combinations. In the present paper 50 seedling types have been reported as showing tristeza symptoms following heavy aphid inoculation. The symptoms shown by infected seedlings are similar to those shown by non-tolerant stock-scion combinations. Small sweet orange seedlings have been observed to show some symptoms of tristeza following heavy inoculations, but citrus types possessing tolerant tissues have shown a tendency toward recovery from symptom expression. The writers, on the basis of observed plant responses, have discussed the mode of inheritance of the main characteristis involved in plant reaction to tristeza. Some seedling progenies of known crosses between Poncirus trifoliata and sweet oranges (citranges) have been found to permit virus increase as the sweet orange parent does, whereas others behave like P. trifoliata and apparently do not permit virus multiplication. No relationship has been found between the trifoliata leaf-shape and inability to permit virus increase, since some of the hybrids that possess the trifoliata leaves, permit virus multiplication. Backcrosses of citranges to sweet oranges show a tendency to behave like the sweet orange parent. Hybrids between P. trifoliata and grapefruits (citrumelos) have behaved in most cases as the parent P. trifoliata. This seems to indicate that tolerant tissues and inability to permit virus increase are dependent on dominant factors in this type of cross. Hybrids between Citrus reticulata and C. paradisi (tangelos) do not show a clear-cut type of inheritance. Some tangelos behave Uke the mandarin parent, possessing tolerant tissues ; others behave like the grapefruit parent indicating possession of non-tolerant tissues. The inability to permit virus increase is a characteristic of little value in a rootstock improvement since tops of most commercial citrus varieties permit virus multiplication. It is pointed out that improvement of citrus rootstock with respect to tristeza should be aimed at combining tolerant tissues such as found in P. trifoliata or hybrids, C. sinensis, C. reticulata, etc., with other favorable characters such as vigor, resistance to gummosis, etc. Field tests showed that the reaction of infective buds on different rootstocks varies not only according to the tolerance of non-tolerance of the rootstock tissues, but also according to whether the buds themselves belong to types that possess non-tolerant or tolerant tissues. Infective buds of tolerant types, as sweet oranges, when budded on non-tolerant stocks produce a first flush of growth that is apparently healthy, but which later shows disease symptoms. For a period of about two months from date of budding, there is little or no difference between sprouts from healthy or infective buds of tolerant types. In contrast, when infective buds of non-tolerant types are budded on non-tolerant stocks, disease symptoms appear practically as soon as the buds start to grow. Observations indicate that infective buds from tolerant citrus types when budded on tolerant stocks usually do not develop symptoms even though the plant is a carrier of the virus. On the other hand observations indicate that infective buds of non-tolerant citrus types when budded on tolerant stocks may at times produce some symptoms. A possible explanation for the above-mentioned reactions is presented and is based on the relation between virus movement and food translocation in the plant. In the study of the relationship of tristeza virus to host tissues, plants composed of a sweet top over sour orange rootstock were allowed to develop two sweet orange branches. One branch of each plant was then ringed and inoculated by the vector. The branch not inoculated did not develop symptoms and 16 months after inoculation, buds were taken from the inoculated and the non-inoculated branches and tested for presence of virus. All buds taken from the inoculated ringed branches were found to be carrying the virus, whereas the buds taken from the other branch of the same plant did not have any virus. This experiment shows that the tristeza virus was not able to move across the ringed portion of the stem during that period, thus indicating that the tristeza virus is probably a phloem virus. Results from extensive buddings of various citrus stocks in the field indicate that buds taken from old sweet orange plants that have been known to be infected for a long time have been found to carry the virus in all buds. On the other hand, tests with buds taken from young plants recently infected, of Duncan grapefruit, sour orange and in some cases of Dancy tangerine and Valencia sweet orange, showed that not all buds were carrying the virus. It is not yet known whether buds that are mature prior to infection may temporarily escape virus invasion or whether citrus types which have non-tolerant tissues may limit to a certain extent complete systemic invasion of the tristeza virus
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