Original Articles

Citrus tristeza virus cross-protection of ‘Palmer’ navel orange in the Eastern Cape Province of South Africa


Citrus tristeza virus (CTV) is endemic in southern Africa due to the abundance of the citrus brown aphid, To x -optera citricida Kirk., the main insect vector. Virus-free propagation material in the Southern African Citrus Improvement Scheme is pre-inoculated with mild CTV sources (natural mixtures of strains) to protect the trees from the introduction of severe strains of CTV by aphids. This method of CTV management is known as cross-protection. Success is mainly dependant on the mild CTV source used for pre-immunisation, the scion and root-stock cultivars to be protected, and the environment. In this study different CTV sources (LMS 6, SM 36, SM 41, SM 45, SOSS 2) were evaluated in ‘Palmer’ navel orange on various rootstocks (rough lemon, ‘Troyer’ cit-range, ‘Swingle’ citrumelo, ‘C35’ citrange) that are commercially used in the Eastern Cape region of South Africa, the objective being to identify a suitable CTV source for commercial use. Trees that were pre-immunised with CTV source LMS 6 grew significantly better over a 7-year period than trees with CTV sources SM 36, SM 41, the severe source SOSS 2 and those that were planted virus-free. These trees also yielded the best over a 3-year period and had a 47% higher cumulative yield than trees that were planted virus-free, and 38% higher than the average of all the other treatments. Growth and production of trees that were planted virus-free gave an indication of the effect of challenging CTV strains that were introduced by aphids.

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