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Disease Note

First Report of Lethal Stem Pitting of Duke 6 Avocado Rootstock in South Africa. J. N. Moll, N. M. Grech, and S. P. van Vuuren, Citrus and Subtropical Fruit Research Institute, Nelspruit, South Africa.  Plant Disease 69:727, 1985.  Accepted for publication 18 March 1983.  Copyright 1985 The American Phytopathological Society. DOI: 10.1094/PD-69-727f.

In an avocado (Persea americana Mill.) orchard of 4,000 trees where Hass scions were grafted to clonal Duke 6 and Guatemalan seedling rootstocks in alternate rows, all 2,000 trees on Duke 6 started defoliating and declining rapidly after 3 yr of vigorous growth.  This malady is now also occurring in about 1,200 trees on Guatemalan seedling rootstocks as well as on an adjacent ungrafted Duke 7 tree.  Thorough investigation did not indicate any association with fungi, bacteria, nematodes, or insects.  Wood from primary and tertiary roots, the rootstock (trunk), and the scion (trunk, primary branches, and twigs) was analyzed for nitrogen, phosphorus, potassium, calcium, magnesium, zinc, copper, manganese, iron, and boron, but none occurred at aberrant levels.  Severe pitting of the wood was observed on rootstocks of all affected trees but not on the Hass scions.  Symptom development was correlated with drought.  All Duke 6 trees investigated to date throughout South Africa are pitted, suggesting transmission of the pathogen to the original imported Duke 6 either from the native rootstock or from a contaminated source from California, where a survey of 10 Duke 6 trees revealed several with pitting (H. D. Ohr, personal communication).  The malady bears many similarities to black streak disease found in California, but viruses or viroids were not detected by polyacrylamide gel electrophoresis.

Reference: Hayward, A. C. Plant Dis. Rep. 56:904. 1972.