Seasonal Concentration of the Pierce's Disease Bacterium in Grapevine Stems, Petioles, and Leaf Veins

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Abstract
Stem, petiole, and leaf vein tissues were collected 5 times during the season from ''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 March 29 samples of current season tissues, but were present on April 21. The highest concentration of bacterial occlusions occurred on June 7 and high concentrations also were present on July 28. By Oct. 10, 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 [leafhopper] vector transmission data.