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Etiology

Anatomy of Citrus Fatal Yellows: Evidence for Strains of the Causal Agent and for Double Infections with a Sunken-Vein Disease Agent. Henry Schneider, Plant pathologist, Department of Plant Pathology, Citrus Research Center, University of California, Riverside 92521; Phytopathology 78:687-694. Accepted for publication 2 December 1987. Copyright 1988 The American Phytopathological Society. DOI: 10.1094/Phyto-78-687.

In orchards, lemon trees (Citrus limon) with alemow (C. macrophylla) rootstocks expressed symptoms of decline that resulted from necrosis of vascular tissues in the rootstocks. The causal agent was graft transmitted from rootstocks of declining orchard trees to alemow seedlings in the greenhouse, where serial passages of the isolates were made in alemow. A characteristic array of stem and leaf symptoms (shoot symptoms) occurred in some infected alemow seedlings, while yellowing and cessation of growth (decline) occurred in others. Both symptom types originated from a unique pathosis consisting of death and collapse of differentiating phloem, with the cambium and differentiating xylem also involved. Unaffected parenchyma cells divided, enlarged, and compressed the pathological tissue. In the resulting parenchymatous tissue, a cambium sometimes was reconstituted and formation of replacement vascular tissues ensued. Lasting remissions sometimes resulted, but relapses also occurred. The six isolates studied differed in virulence, ease of transmission, and perpetuation. Fatal yellows (FY) was difficult to study because of low rates of transmission, long incubation periods, remissions, uneven distribution of the agent in the tree, a complexity of strains, and double infections with a sunken vein (SuV) disease agent. Symptoms of SuV were: veins sunken abaxially and raised adaxially, episodes of formation of small yellow leaves, and twisting of stems. The disorder is apparently a separate entity because, during serial passages of two isolates, the FY syndrome was lost and the SuV syndrome maintained.