Distribution of Xylella fastidiosa in Blueberry Stem and Root Sections in Relation to Disease Severity in the Field

Xylella fastidiosa causes bacterial leaf scorch, a new disease of southern highbush blueberry in the southeastern United States. Infections occlude the xylem of affected plants, causing drought-like symptoms and, eventually, plant death. To assess the likelihood of mitigation of bacterial leaf scorch through cultural practices such as pruning or hedging of affected plants, we determined the localization and population density of X. fastidiosa in naturally infected blueberry plants with varying levels of bacterial leaf scorch severity. Stem segments were sampled from the current season's growth down to the base of the plant, as were root segments on plants that were either asymptomatic or had light, moderate, or severe symptoms in three plantings affected by the disease. Stem sap was extracted from each segment and population densities of X. fastidiosa were determined using real-time polymerase chain reaction with species-specific primers. Detection frequencies were lowest (but non-zero) in sap from asymptomatic plants and highest in plants with severe symptoms. In asymptomatic plants, detection was generally least frequent (0 to 20.0%) in top and root sections and highest (4.6 to 55.6%) in middle and base stem sections. As disease severity increased, detection frequencies in roots increased to >80% in two plantings and to 60% in the third planting. Overall, detection frequencies were highest (>80%) in middle and base stem sections of plants from the moderate and severe disease classes. The lowest bacterial titers (averaging 0 to 2.1 × 10¹ CFU per 50 μl of sap) were observed in top and root sections of asymptomatic plants, whereas the highest titers (generally between 10⁴ and 10⁵ CFU per 50 μl of sap) were obtained from middle, base, and root sections of plants from the moderate and severe classes. The presence of the bacterium in middle and base stem sections at low disease severity indicates rapid distribution of X. fastidiosa in affected plants. Because the pathogen accumulates in the roots at moderate and high disease severity levels, management strategies such as pruning and mowing are unlikely to be effective in curing affected plants from bacterial leaf scorch.