Citrus Canker Incidence Research Articles

Isolation, Antimicrobial Effect and Metabolite Analysis of Bacillus amyloliquefaciens ZJLMBA1908 against Citrus Canker Caused by Xanthomonas citri subsp. citri.

Citrus canker caused by Xanthomonas citri subsp. citri is a devastating bacterial disease with severe implications for the citrus industry. Microorganisms possessing biocontrol capabilities against X. citri subsp. citri offer a highly promising strategy for healthy citrus management. In the present study, a broad-spectrum antagonist strain ZJLMBA1908 with potent antibacterial activity against X. citri subsp. citri was isolated from symptomatic lemon leaves, and identified as Bacillus amyloliquefaciens. Cell-free supernatant (CFS) of strain ZJLMBA1908 also exhibited remarkable antimicrobial activity, especially suppressing the growth of X. citri subsp. citri and Nigrospora oryzae, with inhibition rates of 27.71% and 63.75%, respectively. The antibacterial crude extract (CE) derived from the CFS displayed effective activity against X. citri subsp. citri. A preventive treatment using the CE significantly reduced the severity and incidence of citrus canker in a highly susceptible citrus host. Additionally, the CE maintained activity in the presence of protease and under a wide range of temperature and pH treatments. Applying high-performance liquid chromatography (HPLC) to separate and purify the CE resulted in the discovery of one highly potent anti-X. citri subsp. citri subfraction, namely CE3, which could completely inhibit the growth of X. citri subsp. citri. Liquid chromatography-electrospray ionization-mass spectrometry (LC-ESI-MS) analysis revealed that CE3 mainly consisted of palmitic acid, surfactin C15, phytosphingosine and dihydrosphingosine. Taken together, the results contribute to the possible biocontrol mechanisms of B. amyloliquefaciens ZJLMBA1908, as well as providing a promising new candidate strain as a biological control agent for controlling citrus canker.

Long-term field evaluation of Pera sweet orange selections under the Brazilian humid subtropical climate

Currently, a low number of citrus varieties are commercially planted to supply the Brazilian citrus industry demand. The diversification of citrus scion and rootstock varieties is an important strategy to prevent disease outbreaks and to obtain higher off-season profits, as well as to extend fruit supply for the citrus processing industry. The aim of this study was to evaluate 16 Pera sweet orange selections (Citrus ×sinensis Osbeck) under a humid subtropical climate in the northwest region of the state of Paraná, Brazil regarding tree development, production, and disease incidence. The experimental orchard was planted in December 2012 in a randomized block design with three replications and five trees per plot. All scions were grafted on Rangpur lime rootstock (C. ×limonia Osbeck). The evaluations were performed from 2016 through 2022, and included vegetative growth, yield, fruit quality, estimated tree density and productivity for new plantings, and incidence of diseases as citrus canker, huanglongbing (HLB), and citrus tristeza virus (CTV). All data was submitted to ANOVA and means grouped by the Scott Knott’s test (p ≤ 0.05). Pera Perão was the most vigorous variety, with the heaviest and largest fruit, and was among the selections with the highest content of soluble solids (SS), titratable acidity (TA) and juice. However, this variety showed a light-yellow juice color, and one of the lowest SS·TA−1 ratio. This selection had the lowest percentage of fruits with citrus canker but was among the selections with the highest HLB incidence. The selections Ovale, Clone 9, and Clone 42 also showed higher vegetative growth and fruit length, but lower SS content, juice yield and technological indices. All other Pera selections had lower tree height and canopy volume than Ovale, Clone 9, and Clone 42, and adequate overall fruit quality. Conversely, Pera Comprida, Dibbern, Morretes, and Seleç ão 12 scored lower cumulative fruit yield. Pera Pirangi and Comprida had the highest citrus canker incidence on fruits, while Morretes, Ovo, Pirangi, Seleç ão 12, and Seleç ão 27 had the highest HLB incidence alongside Perão. Low severity level of stem pitting was observed within the evaluated selections. The scores ranged from one (absence of stem pitting) to two (sparse surface stem pitting). IAC, Olímpia, Mel, Bianchi, and IPR 159 revealed to be the most suitable for new plantings under humid subtropical edaphoclimatic conditions, as trees had small vegetative growth, high yield performance and fruit quality, in addition to showing lower incidences of citrus canker, HLB and CTV.

Bioactive copper and Bacillus subtilis for the control and resistance induction against citrus canker in sweet orange [Citrus sinensis (L.) Osbeck] orchard establishment

• The biological product based on Bacillus subtilis lineage QST 731 applied both via drench and foliar sprays reduces the incidence of citrus canker in citrus trees. • The application of Bacillus subtilis lineage QST 731 via drench in combination with foliar sprays of copper hydroxide or bioactive copper increases the efficiency in the control of citrus canker. • Foliar applications of Bacillus subtilis lineage QST 731 reduces the incidence of citrus leafminer injure. • The copper-based product with amino acids, the bioactive copper, increases the expression of the pr2 gene, which encodes the protein β−1,3 glucanase. Resistance is the plant ability to prevent or delay the infection and the development of a pathogen. The objective of this study was to determine the efficiency of treatments with chemical and biological products for the control of citrus canker during the establishment of citrus orchards through resistance induction. The treatments consisted of a combination of foliar and drench applications of products based on Bacillus subtilis (Bs), bioactive copper (BioCu), copper hydroxide (HCu) and acibenzolar-S-methyl (ASM), at the transplanting of the nursery trees to the field and throughout the first year of establishment of the citrus orchard. Evaluations were carried out on mature branches determining the incidence of citrus canker, defoliation, and citrus leafminer (CLM) injures. The vegetative growth of the citrus trees, i.e. tree height and trunk and canopy diameters, were also determined. Under controlled conditions, studies were carried out to determine the activation of the systemic acquired resistance (SAR) by treatments with ASM and Bs applied via drench, and by BioCu and copper sulfate (SCu) applied by spraying on the leaves of the citrus trees. The expression of the PR-2 gene was evaluated by RT-qPCR at 24, 42, and 72 h, and seven days after the application of the treatments. In the field, the ‘IAPAR 73′ sweet orange trees treated via drench with Bs and foliar applications of Bs + HCu, Bs + BioCu and HCu with and without ASM showed reductions in the incidence of citrus canker. HCu treatments reduced defoliation, while foliar treatments with Bs decreased the incidence of CLM injures. The ‘IAPAR 73′ sweet orange trees treated with ASM showed a reduction in the vegetative growth. The BioCu and Bs products were effective for the control of citrus canker in young citrus trees during the orchard establishment. In addition, the BioCu product increased the expression of the PR-2 gene, which encodes the β-1,3 glucanase, a protein associated with the systemic acquired resistance (SAR) in plants.

Late-Season Sweet Orange Selections Under Huanglongbing and Citrus Canker Endemic Conditions in the Brazilian Humid Subtropical Region.

The Brazilian citrus orchards are comprised by few genotypes, which increases the risk of pest and disease outbreaks. The diversification of sweet oranges (Citrus × sinensis) in orchards also generates off-season revenue and extend the fruit processing period. This study aimed to evaluate several horticultural traits of 19 late-season sweet orange selections under citrus canker and huanglongbing (HLB) endemic condition in northwestern Paraná state, Brazil, in a long-term field experiment. Tree size, yield, fruit quality for fresh fruit and industrial markets, estimates of tree density and yield, and citrus canker and huanglongbing (HLB) incidences were assessed. The experimental design was a randomized block with three replicates and five trees per unit. The orchard was drip-irrigated and arranged at tree spacing of 6.5 m × 4.5 m. All scions were graft-compatible with Rangpur lime (C. × limonia). Valencia selections had the tallest trees and largest canopies, particularly Olinda, Frost and #121 with heights and volumes greater than 4.20 m and 43 m3, respectively. Natal África do Sul and Whit’s Late Valencia trees were the most productive with cumulative yields above 640 kg per tree. Most of the selections produced fruits of excellent physicochemical quality attending the fresh fruit and industrial market requirements. All selections showed similar horticultural characteristics for the fresh market, while Natal África do Sul and Charmute de Brotas were more suitable for juice processing. Frost Valencia and Valencia Late Fla. had the highest incidence of citrus canker on fruits (>20%), whereas IPR Folha Murcha, Charmute de Brotas and some Valencia selections (Chafeei Late, Campbell 479, Campbell 294, Olinda, Mutação and Whit’s Late) exihibed low incidence (3.0–17.7%). At 9 years, Valencia Mutação trees had high HLB incidence (93%). In contrast, Natal IAC and Folha Murcha IAC showed the lowest HLB incidence (13%). Our results revealed that Natal IAC, Folha Murcha IAC, IPR Folha Murcha, Natal Murcha, Campbell 479 Valencia and Valencia Late Fla. had the best horticultural performance in addition to low HLB incidence. Together, these late-season sweet oranges are the most advantageous selections for citrus orchard diversification under citrus canker and HLB endemic conditions in humid subtropical regions.

Incidence of Asiatic Citrus Canker on Trifoliate Orange and Its Hybrid Accessions in a Florida Field Planting

Asiatic citrus canker (ACC) foliar lesions were evaluated on progenies of 84 seed-source genotypes (“parent genotypes”) from the Citrus Variety Collection (CVC) of the University of California at Riverside (UCR) of Citrus trifoliata and hybrids between C. trifoliata and other Citrus species and hybrids. Trees were planted Aug. 2013 in a completely randomized design at the Fort Pierce U.S. Department of Agriculture (USDA) grove. Plants were assessed visually Aug. 2017, Sept. 2019, and Sept. 2020 for distinctive ACC lesion incidence and severity. Progeny were compared by parent genotypes using nonparametric analysis. Incidence of ACC [percentage of leaves displaying symptoms, verified by quantitative polymerase chain reaction (qPCR) to be associated with Xanthomonas citri pv. citri] across parent genotypes ranged from 8% to 80% (mean, 49%) of leaves affected in 2017, from 4% to 58% (mean, 29%) in 2019, and 8% to 46% (mean, 25%) in 2020. In 2017, of 49 C. trifoliata parent genotypes, only four separated from the two highest ACC-incidence statistical categories [Citrus Research Center (CRC) 3345, 3484, 3888, and 4017]. whereas 29 of the 35 C. trifoliata hybrids displayed lower ACC incidence, which separated from the two highest statistical categories. In 2019, of the C. trifoliata, only six separated from the highest ACC-incidence statistical category (CRC 3330, 3484, 3547, 3549, 3876, and 3888), whereas all 35 C. trifoliata hybrids displayed lower ACC incidence and separated from the highest statistical category, and 26 hybrids separated from 18 of the C. trifoliata. In 2020, only three C. trifoliata separated from the highest ACC-incidence statistical category (CRC 2861, 3549, and 3888) and 20 hybrids separated from 18 of the C. trifoliata. By parent genotype, ACC incidence correlated substantially between each pair of the 3 years, with r2 values of 0.39, 0.57, and 0.65. Of 34 hybrids validated, similar numbers had C. trifoliata, grapefruit (C. ×aurantium var. racemosa), and sweet orange (C. ×aurantium var. sinensis) chloroplasts. Chloroplast type affected ACC incidence and severity, but not in a consistent manner. Near-isogenic groups within C. trifoliata, as determined by DNA markers, were associated with some statistically different ACC sensitivity. Overall, hybrids of C. trifoliata with other citrus types displayed markedly reduced ACC sensitivity compared with C. trifoliata, indicating that this trait is readily overcome through breeding.

Timing of copper sprays to protect mechanical wounds against infection by Xanthomonas citri subsp. citri, causal agent of citrus canker

Copper sprays are the most important measure for control of citrus canker. Mechanical wounds caused by cultural practices are entry points for the causal bacterium, Xanthomonas citri subsp. citri, and exacerbate the disease. It is not known whether additional applications would protect mechanical wounds from infection by X. citri. This study aimed to assess the predisposition of mechanical wounds to infection and the efficacy of a single copper spray applied at different times for protecting wounds against X. citri under greenhouse conditions. Access of wounds to infection was assessed by spray-inoculating a suspension of the bacterium on leaves from 0 to 7 days after wounding. Timing of a copper application was assessed by treating the wounded leaves at different points before or after bacterial inoculation and/or wounding. The incidence of wounds with citrus canker decreased over time after wounding. No lesions developed in wounds inoculated 7 days after injury. Copper sprayed at all timings reduced the incidence of citrus canker at wound sites compared to the non-treated leaves. Copper sprayed before wounding and/or inoculation showed the greatest reduction (>90%) in the incidence of citrus canker. The reduction in canker associated with wounds ranged from 91.8 to 96.1% for copper sprays applied from 24 h before to 0.5 h after wounding and inoculation, and was 67.8% when copper was applied up to 8 h after wounding and inoculation. As X. citri is splash-dispersed, we conclude that wounds need to be protected with copper sprays if rain is forecast within 7 days of a wounding event. In case no rain is expected, there is no need to protect the trees with copper. However, if trees are wounded when the canopy is wet, a copper spray should be applied before or immediately after the wounding event (but not more than 8 h later). A single appropriately timed copper spray allows the protection of wounds against citrus canker even after the tissue is injured.

Bioagent mediated management of citrus canker

Citrus canker caused by Xanthomonas citri pv. citri (Xcc) is one of the most destructive diseases of citrus. We investigated the citrus canker disease management in Experimental Farm of AAU, Jorhat, Assam (2017-19) using three microbial bioformulations, viz. Biogreen-5 (Combination of Pseudomonas fluorescens, Bacillus thuringiensis, Beauveria bassiana, Metarhizium anisopliae and Trichoderma viride), Bioveer (Trichoderma viride) and Biosona (Beauveria bassiana) in Assam lemon (Citrus limon L. Burm. f ) as a test crop, a premier commercial citrus cultivar of Assam. These bioformulations were observed highly effective in suppressing citrus canker incidence and enhancing fruit yield. Biogreen-5 applied as soil application as well as foliar spray showed maximum reduction in canker incidence (35.55-20.96%) coupled with an enhanced fruit yield (12.43-41.93%). Enhancement of the population count of phylloplane as well as rhizosphere microbes were conspicuously observed in plants treated with microbial bioformulations, offering an environmentally benign alternative of canker management of Assam lemon.

Soil application of acibenzolar-s-methyl and thiamethoxam for the management of citrus canker under subtropical conditions

Drench applications of systemic acquired resistance (SAR) inducing compounds acibenzolar-s-methyl (ASM) and thiamethoxam (THIA) may contribute for the control of citrus canker, caused by Xanthomonas citri subsp. citri. Here, we report the results of a 3-year field experiment on the effect of drench applications of two SAR-inducing compounds, applied alone or in combination with copper oxychloride (CuOX) foliar sprays, on citrus canker control, as well as tree growth and yield, of Natal sweet orange trees under Brazilian subtropical conditions. Leaf disease incidence, tree defoliation, and citrus leaf miner (Phyllocnistis citrella) attack were evaluated every 30 days for 16 months. Soil applications of ASM or THIA combined with foliar sprays of CuOX highly reduced citrus canker incidence on leaves. Effective control of the disease, comparable to regular foliar sprays of cupric bactericides, was obtained with two to four drench applications of SAR-inducing compounds. No differences among the treatments were observed in regard to tree height and canopy volume, and fruit yield of the young citrus trees. Soil applications of SAR-inducing compounds proved to be an important measure for the integrate management of citrus canker under the Brazilian subtropical conditions.

SURVEILLANCE OF CITRUS CANKER IN PESHAWAR DIVISION AND ITS MANAGEMENT THROUGH HOST RESISTANCE

The present study was conducted to record incidence and severity of citrus canker and to screen citrus species against this disease. A detailed survey was conducted in 19 nurseries and 13 commercial orchards of Peshawar division during 2011. It was observed that almost all the nurseries were severely affected by the disease. The highest disease incidence (100%) and severity (31.54%) were observed in New Tarnab nursery in (Mention name of district here). Among the orchards, Shado II showed comparatively more disease severity (2.92%) as compared to other orchards. Bacteria, isolated from symptomatic plant samples on nutrient agar medium produced small, shiny yellow colonies with smooth margins. Biochemical tests further confirmed the bacteria as Xanthomonas axonopodis pv. citri. Detached and attached leaf assays were made to check the resistance of four citrus species i.e. Citrus sinensis (sweet orange), C. aurantifola (kaghazi lemon), C. paradise (Grape fruit) and C. medica (Lime). C. medica was moderately resistant whereas C. sinensis was found to be the most susceptible against the disease. Present study will be useful for developing best possible solution for the management of citrus canker in Peshawar division.

<b>Zero-inflated beta regression model for leaf citrus canker incidence in orange genotypes grafted onto different rootstocks

Data with excess zeros are frequently found in practice, and the recommended analysis is to use models that adequately address the counting of zero observations. In this study, the Zero Inflated Beta Regression Model (BeZI) was used on experimental data to describe the mean incidence of leaf citrus canker in orange groves under the influence of genotype and rootstocks of origin. Based on the model, it was possible to quantify the odds that a null observation to mean incidence comes from a particular plant according to genotype and rootstock, and estimate its expected value according to this combination. Laranja Caipira rootstock proved to be the most resistant to leaf citrus canker as well as Limao Cravo proved to be the most fragile. The Ipigua IAC , Arapongas , EEL and Olimpia genotypes have statistically equivalent chances.

Soluble and insoluble copper formulations and metallic copper rate for control of citrus canker on sweet orange trees

Copper bactericides have been widely studied and used for control of citrus canker caused by Xanthomonas citri subsp. citri. New formulations become available periodically, however, little is known about the relative efficacy of insoluble (ICuF) and soluble (SCuF) copper formulations and the performance of ICuF at the same metallic copper rates. In the present study, we compared three rates of five ICuF (syn. fixed copper) and two SCuF for control of citrus canker during two seasons in a commercial orchard of ‘Valencia’ sweet orange located in Paranavaí, Paraná, Brazil. Control of citrus canker was evaluated as the temporal progress of canker incidence on leaves, cumulative dropped fruit with canker, and incidence of diseased fruit at harvest. All copper formulations and rates reduced the progress of citrus canker incidence on leaves. However, SCuF were not as effective as the ICuF. The incidence of leaves with canker on trees treated with ICuF never exceed 7.3%. In contrast, incidence of leaf canker on trees treated with SCuF ranged from 9.9 to 44.8%. Fruit drop per tree due to citrus canker for trees treated with ICuF did not exceed 6.2 and 17.3 in seasons 1 and 2, respectively. These losses were significantly lower than 20.4 and 38.6, observed for the untreated trees. Fruit drop was 10.4 and 20.9 for trees treated with SCuF in seasons 1 and 2, respectively. The citrus canker incidence on harvested fruit followed the same trend observed for other variables assessed. A non-linear regression analysis showed that the metallic copper rate accounted for a large proportion of the variation in incidence of citrus canker on leaves, fruit and premature fruit drop up to 1 kg of metallic copper/ha/application. Rates exceeding l kg/ha of metallic copper did not consistently improve disease control or reduce the fruit loss.

<b>Modeling the incidence of citrus canker in leaves of the sweet orange variety ‘Pera’

Citrus canker, caused by the bacterium Xanthomonas citri subsp. citri , is one of the most important diseases of citrus. The use of resistant genotypes plays an important role in the management and control of the disease and is the most environmentally sustainable approach to disease control. Citrus canker incidence was recorded in an experiment on nine genotypes of the sweet orange variety ‘Pera’ grafted on four rootstocks. The experiment was started in 2010 and the incidence of citrus canker on the leaves was recorded on a quarterly basis. The incidence data from the experiment were analyzed using a zero-inflated Beta regression model (RBIZ), which is the appropriate method to describe data with large numbers of zeros. Based on the residual analysis, the data fit the model well. The discrete component of the explanatory variable, rootstock, was not significant as a factor affecting the onset of disease, in contrast with the continuous component, genotype, which was significant in explaining the incidence of citrus canker.

Relationships between copper content in orange leaves, bacterial biofilm formation and citrus canker disease control after different copper treatments

Citrus canker is caused by Xanthomonas citri subsp. citri (X. citri), a severe and wide-spread bacterial pathogen. Repeated copper sprays are part of integrated management systems applied in countries where canker is endemic. The objective of this research was to evaluate the effect of a low copper compound, copper sulphate pentahydrate, as soil applications and sprays, on canker intensity, in comparison with traditional application of copper oxychloride. Two independent trials were conducted in 2010 and 2011 in a canker endemic sweet orange orchard. Products were monthly applied during springs. Copper content in young leaves and fruits after each application was assessed through an atomic absorption spectrometer. In addition, the ability of treatments to interfere with bacterial biofilm formation was evaluated under controlled conditions. Soil application of copper sulphate pentahydrate did not reduce citrus canker incidence. In 2011, the pulverization of copper sulphate pentahydrate and copper oxychloride reduced the incidence of the disease compared with untreated trees. Notably, copper content in leaves sprayed with the first compound was significantly lower than in leaves treated with copper oxychloride. However, both treatments showed the same ability to avoid biofilm formation, the first step in establishment of canker disease. Consequently, the pulverization of copper sulphate pentahydrate, integrated with the monitoring of the phenological tree stages and the weather conditions, could constitute a useful tool for the management of citrus canker disease, of high importance to avoid the adverse environmental effects, the risk of phytotoxicity, and the development of copper resistant bacterial populations.

Tree-row-volume-based sprays of copper bactericide for control of citrus canker

Commonly, pre-determined copper spray volumes are indiscriminately applied to citrus orchards of different ages and sizes for control of citrus canker (Xanthomonas citri subsp. citri) leading to a waste of resources, such as water, energy, and chemicals. This study evaluated the effectiveness of copper applied following the tree-row-volume (TRV) methodology for control of citrus canker on sweet orange trees. Treatment design was based on the theoretical runoff volumes previously determined for the exterior and interior of a citrus tree. The volumes 150 (standard), 100 (internal runoff point), 70 (intermediate) and 40 mL (external runoff point) of spray mixture/m3 of tree canopy were tested at the standard copper rate of 0.525 g metallic copper/L. Additionally, 70 and 40 mL/m3 were also tested with copper rate correction by leveling up the theoretical deposition of copper bactericide to that obtained with 100 mL/m3.Untreated control trees (UTC) were not sprayed with copper. Reduction of the spray volume did not affect disease control. While UTC trees presented a peak incidence of citrus canker on leaves of 30% in both years, copper-treated trees showed significantly lower peak incidences of 5–10%. Likewise, the average number of dropped fruits with citrus canker for the UTC was 2.0- to 1.5-fold higher than for copper treatments. At harvest, 30% of fruits from the UTC trees showed citrus canker symptoms. Conversely, only 3.3–9.8% of the fruits from the copper treatments were symptomatic. Moreover, using 40 mL/m3 without adjusting the amount of metallic copper diminished disease control efficiency on leaves. Reduction of spray costs and water needs amounted to 40 and 73%, respectively. In addition to keeping effectiveness of disease control and being more environmentally friendly, the adaptation of the spray volumes and copper rates to the TRV may contribute to reducing production costs.

Quantificação do cancro cítrico em tangerineiras enxertadas sobre diferentes porta-enxertos

Citrus canker caused by the bacteria Xanthomonas citri subsp. citri is an important disease in Santa Catarina State (Brazil) and can reduce citrus plant production. In this work, the objective was to evaluate the incidence of citrus canker in different cultivars of tangerine and rootstocks. Canker occurrence was evaluated along ten years in the scion cultivars ‘Satsuma EEI’, ‘Tankan EEI’, ‘Michal’ and ‘Niva’ budded on ‘Troyer’, ‘C-41’, ‘Cleopatra’ and ‘Sunki’. Evaluated characteristics were citrus canker incidence on leaves and stems, proportion of fallen leaves, the sum of leaves with canker and fallen leaves and the proportion of leaves with citrus leaf miner (Phyllocnistis citrella Stainton). Areas under the progress curve (AUPC) of each variable were calculated and compared. Citrus canker incidence on leaves and stems occurred in all cultivars, but was higher in tangerine ‘Niva’. The rootstocks ‘C-41’ and ‘Sunki’ provided lower incidence of citrus canker on leaves of tangerine cultivars.

Incidence and Severity of Asiatic Citrus Canker on Diverse Citrus and Citrus-related Germplasm in a Florida Field Planting

Xanthomonas citri ssp. citri (Xcc) is the causal agent of Asiatic citrus canker (ACC), a commercially important disease in Florida citrus as well as in many other regions. In this study we evaluated occurrence of foliar lesions from ACC on progenies of 94 seed-source genotypes (hereafter called “parent genotypes”) of Citrus and Citrus relatives in the field in east–central Florida to identify the relative susceptibility to ACC. Eight seedlings per parent genotype were planted in a randomized complete block planting, but the number of plants assessed in some genotype groups was reduced by mortality at some or all sampling dates. Plants experienced ambient exposure to high Xcc inoculum pressure and plants were assessed in Sept. 2010, July 2011, Oct. 2011, May 2012, and Sept. 2012. The incidence and severity of ACC lesions were assessed and evaluated using non-parametric analyses to compare progeny from the 94 parent genotypes. Progeny of 14 parent genotypes did not exhibit ACC symptoms at any date. All were in genera other than Citrus with only Microcitrus and Eremocitrus being cross-compatible with Citrus. The kumquat hybrid C. halimii, two accessions of C. reticulata, C. nobilis, and C. sunki were the only Citrus species in the group that had a low severity (percent total leaf area showing symptoms) on each date of assessment. The aforementioned accessions had an average incidence and severity of ACC lesions of less than 4% in 2011 and 2012, but 26% to 38% in 2010 when no chemical control for ACC was applied in the adjoining citrus groves at our field site. Fourteen of the 16 progeny of C. reticulata and related parent genotypes were in the group with the lowest incidence and severity of ACC on two or more assessment dates. However, for analysis of only the most symptomatic leaves on each plant, 10 C. reticulata parent genotype progenies were in the most resistant category on the Sept. 2012 assessment despite having a low incidence and severity of ACC symptoms overall. Progeny of Poncirus and its hybrids as well as those of C. maxima, C. limon, and related species were the most severely diseased at all assessment dates. There were few instances in which progeny of different accessions of the same species had markedly different responses to ACC: progeny of C. reticulata ‘Fremont’ displayed more severe ACC compared with several other C. reticulata groups and C. aurantium ‘Zhuluan’ displayed much lower incidence and severity of ACC compared with several other accessions designated C. aurantium. Information on ACC susceptibility in diverse Citrus and Citrus relatives may prove useful for breeding programs aimed at reducing ACC susceptibility and will be of value to researchers interested in mechanisms of ACC resistance and susceptibility.

Effect of the duration of inoculum exposure on development of citrus canker symptoms on seedlings of Swingle citrumelo

Citrus canker (Xanthomonas citri subsp. citri, Xcc) is one of the most serious diseases citrus in Florida, and elsewhere in the world. The disease causes yield loss and some fresh fruit trade restrictions may apply. Cultural management techniques such as windbreaks may work by not only reducing wind speed, but also reducing the period of exposure of susceptible foliage or fruit to those wind speeds that support infection from incoming inoculum. To investigate the effect of exposure period to inoculum of Xcc, seedlings of canker-susceptible Swingle citrumelo were exposed to sprayed inoculum for increasing periods at different wind speeds. The incidence and severity of citrus canker was assessed. In three experiments the incidence and severity of citrus canker most often increased with longer periods of exposure to inoculum, especially so at wind speeds of ≥16 m/s compared to wind speeds of ≤5 m/s (wind speed also increased disease incidence and severity). Regression analysis demonstrated relationships between period of exposure to inoculum and the percent infected leaves per plant, the number of lesions per plant, the number of lesions per infected leaf, and for the percent of infected leaves with lesions on the petioles at wind speeds of ≥16 m/s (R2 = 0.16–0.72). Due to the effect of inoculum exposure period and wind speed, attempts should be made to minimize exposure of canker-susceptible citrus when wind speed is highest and inoculum is available. Windbreaks should help minimize periods of exposure to splashed inoculum in high winds.

Prediction of citrus canker epidemics generated through different inoculation methods

Citrus canker epidemics were generated with 108 cfu/ml of Xanthomonas axonopodis pv. citri (ex Hasse) on Citrus limonia cv. China lemon, Citrus reticulate cv. kinnow, Citrus jambhiri, Citrus reticulate cv. Feutral’s early and Citrus limettioides using four inoculation techniques. Natural inoculum was also relied upon for infection. Overall, the injection infiltration method led to maximum disease generation followed by spray, pinprick and smear inoculation methods. Citrus canker incidence along with environmental data were recorded and subjected to stepwise regression analysis. Except relative humidity, the relationship of weekly air temperature (maximum and minimum), rainfall and wind speed with citrus canker disease development in all citrus cultivars was positively correlated and best explained by linear regression. Overall, two environmental variable model containing maximum and minimum air temperature fit the data well explaining 93% variability in disease development. The observed citrus canker incidence values and those predicted by the model were close in most of citrus cultivars. This two environmental variable model can be used to issue advance warning forecasts for the timely management of the citrus canker in Pakistan.

Damage caused by citrus canker on Valencia sweet orange yield

The citrus canker, caused by Xanthomonas axonopodis pv. citri, is one of the main citrus diseases and has threatened the world's citriculture. In this study, the damage caused by citrus canker was quantified in a sweet orange culture, cv. Valencia [Citrus sinensis (L.) Osb.], grafted over 15 year-old Rangpur lime (C. limonia Osb.), in the municipality of Guatambu, SC, Brazil, in the crop of 2004/05. Evaluations of the number of fruits per plant (NF), fruit yield per plant (P), citrus canker incidence on the fruit (IFR), and incidence (IL) and severity (SV) of citrus canker on 80 leaves per orange tree were performed in the harvest period, applying diagrammatical scales. Plants had large variation in SV (0.16%-1.09%), IL (12.50%-56.25 %) and IFR (12.30%-56.09%). Linear regressions were significant between IL x SV, NF x SV, P x IFR, and NF x IFR. There was no significant difference (p>0.05) between the mean estimated values of IL in different quadrants of the crowns of the orange trees (N, S, L and O). Each 1% increase in IFR reduced 2.16 kg and 21.3 fruits per tree

Soil Application of SAR Inducers Imidacloprid, Thiamethoxam, and Acibenzolar-S-Methyl for Citrus Canker Control in Young Grapefruit Trees.

Soil applications of inducers of systemic acquired resistance (SAR) imidacloprid, thiamethoxam, or acibenzolar-S-methyl, at various rates and application frequencies, were evaluated for control of citrus canker caused by Xanthomonas citri subsp. citri in a field trial of 3- and 4-year-old 'Ray Ruby' grapefruit trees in southeastern Florida. Reduction of foliar incidence of canker produced by one, two, or four soil applications of imidacloprid, thiamethoxam, and acibenzolar-S-methyl was compared with 11 foliar sprays of copper hydroxide and streptomycin applied at 21-day intervals. In the 2008 and 2009 crop seasons, canker incidence on each set of vegetative flushes was assessed as the percentage of the total leaves with lesions. By the end of the 2008 season, despite above-average rainfall and a tropical storm event, all treatments significantly reduced foliar incidence of citrus canker on the combined spring-summer-fall flushes. Sprays of copper hydroxide and streptomycin were effective for reducing canker incidence on shoot flushes produced throughout the season compared with the untreated control, whereas soil-applied SAR inducers reduced foliar disease depending on rate, frequency, and timing of application. Except for the treatment of four applications of acibenzolar-S-methyl at 0.2 g a.i. per tree or two applications of imidacloprid, SAR inducers were ineffective for reducing foliar disease on the flushes that were present during the tropical storm. In 2009, all treatments significantly reduced the incidence of foliar canker on the combined spring-summer-fall flushes but not all treatments of spring-summer flushes with SAR inducers were effective compared with the untreated control. Hence, depending on rate, frequency, and timing of application, soil-applied SAR inducers reduced incidence of canker on foliar flushes of young grapefruit trees under epidemic conditions.