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.

Oxytetracycline Dynamics on Peach Leaves in Relation to Temperature, Sunlight, and Simulated Rain.

Oxytetracycline (OTC), a member of the tetracycline antibiotics, is used as a foliar spray to control Xanthomonas arboricola pv. pruni on stone fruits and Erwinia amylovora on pome fruits. We studied the dynamics of OTC residues on attached peach (Prunus persica) leaves treated with 300 ppm active ingredient of an agricultural OTC in relation to temperature, natural sunlight, and simulated rain. We further evaluated the potential of three ultraviolet (UV) protectants (lignin, titanium dioxide, and oxybenzone) and one sticker-extender (Nu Film-17) to prolong OTC longevity on the leaf surface. OTC residue was determined by high-pressure liquid chromatography (HPLC)-UV (C18 reversed-phase column). In controlled conditions in darkness, constant temperatures up to 40°C did not affect OTC degradation on leaves. In contrast, OTC residue decreased rapidly in natural sunlight in the absence of rain, declining, on average, by 43.8, 77.8, and 92.1% within 1, 2, and 4 days after application, respectively; 7 days after application, OTC levels were near the detection limit. Use of shade fabric with 10 and 40% sunlight transmittance, simulating overcast sky, reduced OTC degradation significantly but did not extend OTC persistence beyond 7 days. Areas under the OTC residue curve, summarizing OTC dynamics during the 7-day exposure period, were negatively and significantly correlated with solar radiation and UV radiation variables, but not with temperature. UV protectants and Nu Film-17 were ineffective in improving OTC persistence in outdoor conditions. Simulated rain at 44 mm h-1 drastically (by 67.2%) lowered OTC residue after 2 min, and levels were near the detection limit after 60 min of continuous rain, regardless of whether plants were exposed to rainfall 1 or 24 h after OTC application. In artificial inoculation experiments with X. arboricola pv. pruni on attached peach leaves, OTC concentrations ≥50 ppm active ingredient (corresponding to ≥0.06 µg OTC cm-2 leaf surface) were sufficient to suppress bacterial spot development. By extrapolation from our outdoor exposure experiments, similar OTC residues following application of labeled OTC rates would be reached after less than 2 days under full sunlight, after 4 days under overcast sky, or after 2 min of a heavy rainstorm.

Quantitative aspects of the spread of asian soybean rust in the southeastern United States, 2005 to 2006.

ABSTRACT The regional dynamics of soybean rust, caused by Phakopsora pachyrhizi, in six southeastern states (Florida, Georgia, Alabama, South Carolina, North Carolina, and Virginia) in 2005 and 2006 were analyzed based on disease records collected as part of U.S. Department of Agriculture's soybean rust surveillance and monitoring program. The season-long rate of temporal disease progress averaged approximately 0.5 new cases day(1) and was higher in nonsentinel soybean (Glycine max) plots than in sentinel soybean plots and kudzu (Pueraria lobata) plots. Despite the early detection of rust on kudzu in January and/or February each year (representing the final phase of the previous year's epidemic), the disease developed slowly during the spring and early summer on this host species and did not enter its exponential phase until late August, more than 1 month after it did so on soybean. On soybean, cases occurred very sporadically before the beginning of July, after which their number increased rapidly. Thus, while kudzu likely provides the initial inoculum for epidemics on soybean, the rapid increase in disease prevalence on kudzu toward the end of the season appears to be driven by inoculum produced on soybean. Of 112 soybean cases with growth stage data, only one occurred during vegetative crop development while approximately 75% occurred at stage R6 (full seed) or higher. The median nearest-neighbor distance of spread among cases was approximately 70 km in both years, with 10% of the distances each being below approximately 30 km and above approximately 200 km. Considering only the epidemic on soybean, the disease expanded at an average rate of 8.8 and 10.4 km day(1) in 2005 and 2006, respectively. These rates are at the lower range of those reported for the annual spread of tobacco blue mold from the Caribbean Basin through the southeastern United States. Regional spread of soybean rust may be limited by the slow disease progress on kudzu during the first half of the year combined with the short period available for disease establishment on soybean during the vulnerable phase of host reproductive development, although low inoculum availability in 2005 and dry conditions in 2006 also may have reduced epidemic potential.