Citrus canker, caused by Xanthomonas axonopodis Starr and Garces pv. citri (Hasse) Vauterin et al., is one of the main problems affecting citrus production. In order to obtain resistance to phytopathogenic bacteria, insect genes, coding for antimicrobial proteins, have been used in plant genetic transformation. In this study, transgenic Citrus sinensis (L.) Osb. `Hamlin' plants expressing the antimicrobial insect-derived attacin A gene (attA) were obtained by Agrobacterium tumefaciens (Smith and Towns.) Conn-mediated transformation. Initially, the cDNA clone was used to construct a binary plasmid vector (pCattA 2300). The construction included the native signal peptide (SP) responsible for directing the insect protein to the extracellular space where bacteria is supposed to accumulate in vivo. In order to investigate the native SP effectiveness in a plant model system, onion (Allium cepa L.) epidermal cells were transformed, via biobalistics, using plasmids containing the attA gene with or without SP, fused with the green fluorescent protein gene (pattA 1303 and pSPattA 1303). Fluorescence accumulation surrounding the cells was observed only in tissues transformed with the plasmid containing the gene with SP, indicating the protein secretion to the apoplast. Citrus transformation was confirmed by PCR and Southern blot hybridization analysis in 12 regenerated plants. Transcription of attA gene was detected by Northern blot analysis in all transgenic plants. Eight selected transgenic lines were propagated and inoculated with a 106 cfu/mL suspension of the pathogen X. axonopodis pv. citri. Compared to control (non-transformed plant), seven transgenic lines showed a significant reduction in susceptibility to citrus canker. The results obtained here indicate the potential use of antibacterial proteins to protect citrus from bacterial diseases.

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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.

Genetic transformation with genes that code for antimicrobial peptides has been an important strategy used to control bacterial diseases in fruit crops, including apples, pears, and citrus. Asian citrus canker (ACC) caused by Xanthomonas citri subsp. citri Schaad et al. (Xcc) is a very destructive disease, which affects the citrus industry in most citrus-producing areas of the world. Here, we report the production of genetically transformed Natal, Pera, and Valencia sweet orange cultivars (Citrus sinensis L. Osbeck) with the insect-derived attacin A (attA) gene and the evaluation of the transgenic plants for resistance to Xcc. Agrobacterium tumefaciens Smith and Towns-mediated genetic transformation experiments involving these cultivars led to the regeneration of 23 different lines. Genetically transformed plants were identified by polymerase chain reaction, and transgene integration was confirmed by Southern blot analyses. Transcription of attA gene was detected by Northern blot analysis in all plants, except for one Natal sweet orange transformation event. Transgenic lines were multiplied by grafting onto Rangpur lime rootstock plants (Citrus limonia Osbeck) and sprayinoculated with an Xcc suspension (10 6 cfu mL −1 ). Experiments were repeated three times in a completely randomized design with seven to ten replicates. Disease severity was determined in all transgenic lines and in the control (nontransgenic) plants 30 days after inoculation. Four transgenic lines of Valencia sweet orange showed a significant reduction in disease severity caused by Xcc. These reductions ranged from 58.3% to 77.8%, corresponding to only 0.16-0.30% of leaf diseased area as opposed to 0.72% on control plants. One transgenic line of Natal sweet orange was significantly more resistant to Xcc, with a reduction of 45.2% comparing to the control plants, with only 0.14% of leaf diseased area. Genetically transformed Pera sweet orange plants expressing attA gene did not show a significant enhanced resistance to Xcc, probably due to its genetic background, which is naturally more resistant to this pathogen. The potential effect of attacin A antimicrobial peptide to control ACC may be related to the genetic background of each sweet orange cultivar regarding their natural resistance to the pathogen.

Holland, R. M., Christiano, R. S. C, Gamliel-Atinsky, E., and Scherni. H. 2014. Distribution of Xylella fastidiosa in blueberry stem and root sections in relation to disease severity in the field. Plant

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.

The combined effect of temperature (15°C,