Prevalence and persistence of oxytetracycline-resistant and copper-tolerant Xanthomonas arboricola pv. pruni in peach orchards of the southeastern USA.

Xanthomonas arboricola pv. pruni (Xap) causes bacterial spot, a major worldwide disease of Prunus species. Very few chemical management options are available for this disease and frequent applications of oxytetracycline (OTC) in the United States peach orchards have raised concerns about resistance development. During 2017–2020, 430 Xap strains were collected from ten peach orchards in South Carolina. Seven OTC-resistant (OTCR) Xap strains were found in 2017 and 2020 from four orchards about 20–270 km apart. Interestingly, the seven strains were also resistant to streptomycin (STR). Six strains grew on media amended with ≤100 μg/mL OTC, while one strain, R1, grew on ≤250 μg/mL OTC. Genome sequence analysis of four representative OTCR strains revealed a 14–20 kb plasmid carrying tetC, tetR, and strAB in each strain. These three genes were transferable to Xanthomonas perforans via conjugation, and they were PCR confirmed in all seven OTCR Xap strains. When tetC and tetR were cloned and expressed together in a sensitive strain, the transconjugants showed resistance to ≤100 μg/mL OTC. When tetC was cloned and expressed alone in a sensitive strain, the transconjugants showed resistance to ≤250 μg/mL OTC. TetC and tetR expression was inducible by OTC in all six wild-type strains resistant to ≤100 μg/mL OTC. However, in the R1 strain resistant to ≤250 μg/mL OTC, tetR was not expressed, possibly due to the presence of Tn3 in the tetR gene, and in this case tetC was constitutively expressed. These data suggest that tetC confers OTC resistance in Xap strains, and tetR regulates the level of OTC resistance conferred by tetC. To our knowledge, this is the first report of OTC resistance in plant pathogenic xanthomonads.

Bacterial spot of peach, caused by Xanthomonas arboricola pv. pruni, causes yield loss every year in southeastern U.S. peach orchards. Management is mainly driven by season-long applications of copper-based products, site location, and choice of cultivar. Although tolerance to copper has not been reported in X. arboricola pv. pruni in the United States, adaptation of populations from frequent use is a concern. We collected X. arboricola pv. pruni from shoot cankers, leaves, and fruit of cultivar O'Henry over 2 years from three conventional farms and one organic farm in South Carolina, one orchard per farm. The four farms had been using copper extensively for years to control bacterial spot. X. arboricola pv. pruni was isolated from four canker types (bud canker, tip canker, nonconcentric canker, and concentric canker) in early spring (bud break), as well as from leaf and fruit tissues later in the season at the phenological stages of pit hardening and final swell. X. arboricola pv. pruni was most frequently isolated from cankers of the organic farm (24% of the cankers) and most isolates (45%) came from bud cankers. X. arboricola pv. pruni isolates were assessed for sensitivity to copper using minimal glucose yeast agar and nutrient agar amended with 38 μg/ml or 51 μg/ml of Cu2+. Two phenotypes of copper tolerance in X. arboricola pv. pruni were discovered: low copper tolerance (LCT; growth up to 38 μg/ml Cu2+) and high copper tolerance (HCT; growth up to 51 μg/ml Cu2+). A total of 26 (23 LCT and 3 HCT) out of 165 isolates in 2018 and 32 (20 LCT and 12 HCT) out of 133 isolates in 2019 were tolerant to copper. Peach leaves on potted trees were sprayed with copper rates typically applied at the stages of delayed dormancy (high rate; 2,397 μg/ml Cu2+), shuck split (medium rate; 599 μg/ml Cu2+), and during summer cover sprays (low rate; 120 μg/ml Cu2+), and subsequently inoculated with sensitive, LCT, and HCT strains. Results indicated that the low and medium rates of copper reduced bacterial spot incidence caused by the sensitive strain but not by the LCT and HCT strains. This study confirms existence of X. arboricola pv. pruni tolerance to copper in commercial peach orchards in the southeastern United States, and suggests its contribution to bacterial spot development under current management practices.

No presente trabalho foram estudadas, em condições de câmara climatizada, a influência da temperatura (15, 20, 25 e 30ºC) e do molhamento foliar (6, 12, 24 e 48 horas) na severidade da mancha bacteriana do tomateiro incitada por Xanthomonas spp. A densidade relativa de lesões foi influenciada pela temperatura e pela duração do molhamento foliar (P<0,05). A doença foi mais severa na temperatura de 25ºC. Os dados foram submetidos à análise de regressão não linear. A função beta generalizada foi usada para ajuste dos dados de severidade e temperatura, enquanto uma função logística foi escolhida para representar o efeito do molhamento foliar na severidade da mancha bacteriana. A superfície de resposta obtida pelo produto das duas funções foi expressa por SE = 0,0001538 * (((x-8)2,4855647 * ((32-x)0,7091962)) * (0,64289/(1+21,26122 * exp(-0,12435*y))), onde SE, representa o valor da severidade estimada (0,1); x, a temperatura (ºC) e y, o molhamento foliar (horas). Este modelo deverá ser validado em condições de campo para aferir o seu emprego como um sistema de previsão da mancha bacteriana do tomateiro.

Bacterial spot, caused by Xanthomonas arboricola pv. pruni, is the most important disease that affects peach production in Okayama Prefecture, Japan. Currently, this disease is managed mainly with copper compounds applied at two stages, before flowering and after harvesting, or with antibiotics applied in May and June. Here we identified the disease risk factors that affect peach at harvest and developed a disease-forecasting model to help growers decide when to apply bactericides. The model was based on parameters for weather data collected for September and October of 2001 through 2012 and for April, May, and June of 2002 through 2013, combined with data on bacterial leaf spot incidence obtained from 28 to 30 fields per year in August from 2001 to 2012 and in May to July from 2002 to 2013. The model, developed using a logistic regression analysis, included the percentage of fields with a bacterial spot incidence (BSI) ≥1 % in mid-August of the previous season and the number of rainy days (≥5 mm/day) during the current June as predictors, and explained 75.0 % of the variability. These results suggest that the previous season’s BSI and weather variables in the present season can be used to predict the risk of bacterial spot.

Passion fruit is infected by many plant pathogens, including Xanthomonas axonopodis pv. passiflorae, causing bacterial spot disease. This disease has been impaired orchards all over Brazil, resulting in low yield and production. In addition, it results in premature death of plants in the field. This study aimed to analyze the reaction of sour passion fruit genotypes to bacterial spot in leaves under protected cultivation (Experiment 1) and the bacterial spot severity in fruits under field conditions (Experiment 2). Experiment 1 was carried out as a randomized block design (RBD) with subdivided plots, composed of 24 treatments (genotypes), three replications, five plants per plot, and five evaluation dates. Bacterial spot incidence and severity were evaluated using a 0 to 5 grading scale. Evaluations were carried out at a 7-day interval after disease symptoms first appeared. Genotypes differed for mean disease severity and incidence. The bacterial spot disease evaluations showed that genotypes MAR20#46 P3 R4 X Rosa Claro R4 and MD 16 P3 X MAR20#39 P1 R4 presented the lowest scores for the disease incidence in the leaves. S2L AP R1, MAR20#19 ROXO R4 X ECRAM P3 R3, and MD 16 P3 X MAR20#39 P1 R4 were the genotypes with the lowest disease severity scores. Experiment 2 consisted of a RBD with 24 treatments, three replications, and seven plants per plot. Severity assessments were monthly performed on five fruits per plot, totaling three evaluation dates. In this experiment, the percentage of total fruit area with necrotic lesions was measured using a 1 to 4 grading scale. Bacterial spot severity assessments in fruits identified interactions between genotypes and evaluation dates (p ≤ 0.05). The lineages S2L MAR 20#15 R4 and S2L MAR20#19 R2 showed the lowest mean severity scores of bacterial spot in fruit. The genotypes with the lowest mean scores were selected to continue the breeding program.

The efficacy of foliar sprays with compost water extracts (compost extracts) in reducing the severity of bacterial spot of tomato caused by Xanthomonas vesicatoria was investigated. Extracts prepared from composted cow manure, composted pine bark, an organic farm compost, or composted yard waste, applied as foliar sprays on tomato transplants, resulted in a moderate but statistically significant reduction in the severity of bacterial spot. The population of X. vesicatoria in infected leaves was reduced significantly by extracts prepared from composted cow manure. Efficacy of the water extracts was not affected by oxygen concentrations in the suspension during extraction, compost maturity, or sterilization by filtration or autoclaving. The degree of control provided by foliar sprays with the most effective compost extracts did not differ from that obtained with the plant activator acibenzolar-S-methyl. In the field in two growing seasons, foliar sprays with compost water extracts did not reduce the severity of foliar diseases, including bacterial spot. During the 1997 season, when the severity of bacterial spot in the field was high, foliar sprays with compost water extracts significantly reduced the incidence of bacterial spot on tomato fruit. Amending plot soil with several rates of composted yard waste did not lead to additional control of fruit disease over those only sprayed with extracts. Foliar sprays with a mixture of chlorothalonil and copper hydroxide or with acibenzolar-S-methyl reduced the severity of bacterial spot as well as incidence of spot on fruit.

Evaluation of a small molecule compound 3-indolylacetonitrile for control of bacterial spot on tomato

FALL ARMYWORM REGIONAL TRAINING AND AWARENESS GENERATION WORKSHOP

Newsletter 116C

12TH ARAB CONGRESS OF PLANT PROTECTION

FIRST INTERNATIONAL CONGRESS OF BIOLOGICAL CONTROL

Peach (Prunus persica) is a fruit crop of significant economic and cultural value, particularly in Japan, where it is cherished for its symbolism of summer and high quality. However, its production is threatened by bacterial spot caused by Xanthomonas arboricola pv. pruni (Xap), a pathogen that also affects other Prunus species such as nectarines, plums, apricots, and almonds. Xap thrives in warm, humid environments and causes symptoms such as water-soaked lesions, necrotic spots, premature defoliation, and fruit blemishes, leading to reduced yield and marketability. Traditional control methods, including copper-based bactericides and antibiotics, are increasingly ineffective due to resistance development and environmental concerns. This review focuses on the biology, epidemiology, and pathogenic mechanisms of Xap, with particular emphasis on its impact on peach production in Japan. We discuss various disease management strategies, such as integrated disease management, biostimulants, cellulose nanofibers, plant defense activators, and biological control agents, alongside novel molecular approaches targeting bacterial virulence factors. By incorporating these innovative and eco-friendly methods with traditional practices, this review offers insights into the potential for sustainable, environmentally friendly solutions to manage bacterial spot and mitigate its impact on peach production.

This study was conducted to assess occurrence of bacterial spot, incited by Xanthomonas cucurbitae , on pumpkin leaves and fruit. Development of the disease was monitored in 9, 14, and 15 commercial pumpkin fields in 2009, 2010, and 2011, respectively, in Illinois. Bacterial spot was observed from 4-leaf growth stage until the green leaf tissue was unavailable. Three types of lesions were observed on leaves, which were described as Type I, Type II, and Type III lesions. Type I lesions were small (1 mm), angular, brown necrotic spots, and numerous on each leaf. Type II lesions were angular, beige in the center with brown halo, numerous on each leaf, and measured 1 to 4 mm. Type III lesions were angular, translucent spots with a narrow chlorotic halo, only a few spots on each leaf, and measured 4 to 8 mm. Incidence and severity of bacterial spot on leaves were the greatest when fruit began to turn to orange. Bacterial lesions were observed on fruit from 1 week after fruit set until harvest. Lesions on fruit were circular, water-soaked, sunken, with beige centers and chlorotic halo, and measured 1 to 3 mm. Occurrence of bacterial spot on fruit was assessed in 17, 50, and 65 commercial pumpkin fields in 2009, 2010, and 2011, respectively. The disease on fruit was observed in 100%, 80%, and 88% of the fields surveyed in 2009, 2010, and 2011, respectively. The incidence of fruit with bacterial spot in all fields surveyed was 46%, 35%, and 24% in 2009, 2010, and 2011, respectively.

Almond production has become a major horticultural industry in Australia over the last ten years. Due to the relatively recent expansion of the industry and limited published information on local diseases and disorders, overseas literature was initially relied on to aid diagnosis of tree health issues. As knowledge of the diseases of almonds in Australia increases, this information will become more relevant as a reference for local growers and researchers. A range of diseases have significant impacts on almond production. Symptoms and key features of major diseases of almonds are described under the categories: foliar or fruit; and trunk, crown and root diseases. The diseases under the respective categories are: hull rot (Rhizopus spp.), bacterial spot (Xanthomonas arboricola pv. pruni), rust (Tranzschelia discolor), shot-hole (Wilsonomyces carpophilus), almond scab (Cladosporium carpophilum); silver leaf (Chondrostereum purpureum), Verticillium wilt (Verticillium dahliae), and trunk and root rot (Phytophthora and Fusarium spp.). Certain foliar disease symptoms are similar to either nutritional or physiological disorders or pesticide toxicity. Correct diagnosis of diseases and disorders is crucial for development and implementation of appropriate disease management strategies.

STRIDER, D. L. 1980. Control of bacterial leaf spot of zinnia with captan. Plant Disease 64:920922. In dense plantings in the greenhouse, bacterial leaf spot of zinnia caused by Xanthomonas nigromaculans f. sp. zinniae (= X. campestris) was reduced from 100 to 2% diseased plants when infected seeds were treated with captan. Control was best using a 30-min seed-soak in 360 g a.i. captan per liter of water containing the wetting agent Tween 80. Dry treatment with captan 50 WP was not as effective, but the percent diseased plants was reduced from 100 to 12.4. In plant beds outside the greenhouse, spread was reduced by weekly foliar applications of captan (4.8 g a.i. / L of water). Streptomycin was effective but moderately phytotoxic. In vitro sensitivity of Xanthomonas nigromaculans (Takimoto) Dowson f. sp. zinniae Hopkins and Dowson (= X. campestris (Pammel) Dowson [2]), the cause of bacterial spot of zinnia, to captan was recently reported (19). That captan has utility in control of bacterial spot of zinnia as a seed treatment and as a foliar spray was hypothesized because 250 Ag/ml a.i. captan in nutrient agar prevented growth of X. nigromaculans f. sp. zinniae (19) and because captan is commonly used at a much higher rate (1,230 Ag/ml a.i.) as a foliar spray for control of many fungal diseases (14). Since its development in the early 1940s, captan has been a useful and versatile fungicide (14), but its activity against X. nigromaculans f. sp. zinniae was surprising. However, a review of the literature revealed several reports on the efficacy of captan as a bactericide (4-9,11,12,15,16,22). In 1960 Diener and Carlton (7) reported that captan appeared to increase the effectiveness of dodine for bacterial spot control of peach caused by Xanthomonas pruni. Daines (4-6) also found that captan reduced the incidence of bacterial spot of peach. Shekharvat and Srivastava (15) reported that a 0.05% captan seed-soak reduced seedling infection of rice by Xanthomonas translucens f. sp. oryzae. A preinoculation spray of seedlings gave reduction of the disease equal to that given by a streptomycin-chlorotetracycline treat