VIEW ARTICLE

Etiology

Seasonal Concentration of the Pierce's Disease Bacterium in Grapevine Stems, Petioles, and Leaf Veins. D. L. Hopkins, Professor of Plant Pathology, University of Florida, Agricultural Research Center, Leesburg 32748; Phytopathology 71:415-418. Accepted for publication 11 September 1980. Copyright 1981 The American Phytopathological Society. DOI: 10.1094/Phyto-71-415.

Stem, petiole, and leaf vein tissues were collected five times during the season from cultivar Schuyler grapevines infected with Pierce's disease and were examined by light microscopy for bacterial occlusions in the xylem. Bacteria were not found in the 29 March samples of current season tissues, but were present 21 April. The highest concentration of bacterial occlusions occurred 7 June and high concentrations also were present 28 July. By 10 October, bacterial concentrations were greatly reduced. The number of vessels occluded per cross section was not an accurate indication of the amount of plugging in the tissue. Values obtained by serial sectioning of 0.5 cm of tissue were 4–12 times those obtained per cross section. Leaf veins from leaves with marginal necrosis had 80% of their vessels completely plugged in 0.5 cm of leaf vein tissue. Bacterial infestations in leaf veins were highly correlated with leaf marginal necrosis. Seasonal concentrations of bacterial plugs were closely correlated with symptom development and with seasonal vector transmission data.