Xanthomonas Fragariae Research Articles

Inhibition of Xanthomonas fragariae, Causative Agent of Angular Leaf Spot of Strawberry, through Iron Deprivation.

In commercial production settings, few options exist to prevent or treat angular leaf spot (ALS) of strawberry, a disease of economic importance and caused by the bacterial pathogen Xanthomonas fragariae. In the process of isolating and identifying X. fragariae bacteria from symptomatic plants, we observed growth inhibition of X. fragariae by bacterial isolates from the same leaf macerates. Identified as species of Pseudomonas and Rhizobium, these isolates were confirmed to suppress growth of X. fragariae in agar overlay plates and in microtiter plate cultures, as did our reference strain Pseudomonas putida KT2440. Screening of a transposon mutant library of KT2440 revealed that disruption of the biosynthetic pathway for the siderophore pyoverdine resulted in complete loss of X. fragariae antagonism, suggesting iron competition as a mode of action. Antagonism could be replicated on plate and in culture by addition of purified pyoverdine or by addition of the chelating agents tannic acid and dipyridyl, while supplementing the medium with iron negated the inhibitory effects of pyoverdine, tannic acid and dipyridyl. When co-inoculated with tannic acid onto strawberry plants, X. fragariae’s ability to cause foliar symptoms was greatly reduced, suggesting a possible opportunity for iron-based management of ALS. We discuss our findings in the context of ‘nutritional immunity,’ the idea that plant hosts restrict pathogen access to iron, either directly, or indirectly through their associated microbiota.

Epidemiology and Control of Strawberry Bacterial Angular Leaf Spot Disease Caused by Xanthomonas fragariae.

Strawberry bacterial angular leaf spot (ALS) disease, caused by Xanthomonas fragariae has become increasingly problematic in the strawberry agro-industry. ALS causes small angular water-soaked lesions to develop on the abaxial leaf surface. Studies reported optimum temperature conditions for X. fragariae are 20°C and the pathogen suffers mortality above 32°C. However, at the nursery stage, disease symptoms have been observed under high temperature conditions. In the present study, results showed X. fragariae transmission was via infected maternal plants, precipitation, and sprinkler irrigation systems. Systemic infections were detected using X. fragariae specific primers 245A/B and 295A/B, where 300-bp and 615-bp were respectively amplified. During the nursery stage (from May to August), the pathogen was PCR detected only in maternal plants, but not in soil or irrigation water through the nursery stage. During the cultivation period, from September to March, the pathogen was detected in maternal plants, progeny, and soil, but not in water. Additionally, un-infected plants, when planted with infected plants were positive for X. fragariae via PCR at the late cultivation stage. Chemical control for X. fragariae with oxolinic acid showed 87% control effects against the disease during the nursery period, in contrast to validamycin-A, which exhibited increased efficacy against the disease during the cultivation stage (control effect 95%). To our knowledge, this is the first epidemiological study of X. fragariae in Korean strawberry fields.

Normes OEPP EPPO Standards

Normes OEPP EPPO Standards

Metabolic Response of Strawberry (Fragaria x ananassa) Leaves Exposed to the Angular Leaf Spot Bacterium (Xanthomonas fragariae).

Plants have evolved various defense mechanisms against biotic stress. The most common mechanism involves the production of metabolites that act as defense compounds. Bacterial angular leaf spot disease (Xanthomonas fragariae) of the strawberry (Fragaria x ananassa) has become increasingly destructive to strawberry leaves and plant production. In this study, we examined metabolic changes associated with the establishment of long-term bacterial disease stress using UPLC-QTOF mass spectrometry. Infected leaves showed decreased levels of gallic acid derivatives and ellagitannins, which are related to the plant defense system. The levels of phenylalanine, tryptophan, and salicylic acid as precursors of aromatic secondary metabolites were increased in inoculated leaves, whereas levels of coumaric acid, quinic acid, and flavonoids were decreased in infected plants, which are involved in the phenylpropanoid pathway. In addition, phenylalanine ammonia-lyase (PAL) activity, a key enzyme in the phenylpropanoid pathway, was decreased following infection. These results suggest that long-term bacterial disease stress may lead to down-regulation of select molecules of the phenylpropanoid metabolic pathway in strawberry leaves. This approach could be applied to explore the metabolic pathway associated with plant protection/breeding in strawberry leaves.

FaRXf1: a locus conferring resistance to angular leaf spot caused by Xanthomonas fragariae in octoploid strawberry.

Angular leaf spot is a devastating bacterial disease of strawberry. Resistance from two wild accessions is highly heritable and controlled by a major locus on linkage group 6D. Angular leaf spot caused by Xanthomonas fragariae is the only major bacterial disease of cultivated strawberry (Fragaria×ananassa). While this disease may cause reductions of up to 8% of marketable yield in Florida winter annual production, no resistant cultivars have been commercialized. Wild accessions US4808 and US4809 were previously identified as resistant to the four genetic clades of X. fragariae, and introgression of the trait into commercial quality perennial-type germplasm was initiated. Previous reports indicated high heritability for the trait but proposed both single-locus and multi-locus inheritance models. The objective of this study was to determine the mode of inheritance of resistance, to identify causal loci, and to begin introgression of resistance into Florida-adapted germplasm. Resistance was observed in twoyears of field trials with inoculated plants that assayed four full-sib families descended from US4808 to US4809. Resistance segregated 1:1 in all families indicating control by a dominant allele at a single locus. Using a selective genotyping approach with the IStraw90 Axiom(®) SNP array and pedigree-based QTL detection, a single major-effect QTL was identified in two full-sib families, one descended from each resistant accession. High-resolution melt curve analysis validated the presence of the QTL in separate populations. The QTL was delimited to the 33.1-33.6Mbp (F. vesca vesca v1.1 reference) and 34.8-35.3Mbp (F. vesca bracteata v2.0 reference) regions of linkage group 6D for both resistance sources and was designated FaRXf1. Characterization of this locus will facilitate marker-assisted selection toward the development of resistant cultivars.

A Loop-Mediated Isothermal Amplification Assay and Sample Preparation Procedure for Sensitive Detection of Xanthomonas fragariae in Strawberry.

Xanthomonas fragariae is a bacterium that causes angular leaf spot of strawberry. Asymptomatic infection is common and contributes to the difficulties in disease management. The aim of this study was to develop a loop-mediated isothermal amplification (LAMP) assay as an efficient method for detection of asymptomatic infections of X. fragariae. In addition, a new method of sample preparation was developed that allows sampling of a larger amount of plant tissue, hence increasing the detection rate in real-life samples. The sample preparation procedure includes an overnight incubation of strawberry tissues in phosphate-buffered saline (PBS), followed by a quick sample concentration and a boiling step to extract DNA for amplification. The detection limit of the LAMP assay was approximately 2×103 CFU/mL for pure bacteria culture and 300 CFU/mL for bacteria spiked strawberry leaf and petiole samples. LAMP provided a 2–3 fold lower detection limit than the standard qPCR assay but was faster, and more user-friendly. The LAMP assay should serve as a rapid, sensitive and cost-effective tool for detecting asymptomatic infections of X. fragariae in strawberry nursery stock and contribute to improved disease management.

CATT: a new and non-chemical pest and nematode control method in strawberry planting stock

As an alternative to MeBr fumigation a 48h Controlled Atmosphere Temperature Treatment (CATT) was developed and scaled up by Wageningen UR in cooperation with the Dutch plant propagating association Plantum. This results in an excellent de-infestation and 99.8 % mortality of the strawberry tarsonemid mite (Phytonemus pallidus). This non-chemical and sustainable method provides a healthy production of highly qualified strawberry runners in the field. From 2009 CATT is up scaled to a commercial level and widely applied by Dutch nurseries. In 2011 this CATT method was successfully modified to eradicate also the root knot nematode Meloidogyne hapla (>99.7% mortality), which was not effectively controlled by MeBr fumigation. For an effective killing of the root knot nematodes, temperature must be raised to 40 oC. In several experiments the optimum conditions for a high mortality of both tarsonemids and nematodes was studied. This leads into an adapted CATT of 20 hours at a temperature of 35 oC and 50 % CO2 followed by 20 hours at a temperature of 40 oC. In 2012 this adapted CATT was successfully upgraded and tested under field conditions. Additional research in 2013 leads to the conclusion that cross infection of plants by the bacterial Q-disease Xanthomonas fragariae during CATT treatment is unlikely

딸기 주요 병원균에 대한 친환경제제 NaDCC의 항균활성 및 병 방제효과 평가

딸기에는 여러 가지 병이 발생하여 경제적으로 막대한 손실을 유발하며 많은 양의 농약이 사용되고 있는 실정이다. 최근 유기농산물과 식품안전성에 관한 소비자의 관심이 증가함에 따라 화학농약 대체제에 의한 병해 방제법 개발이 다각적으로 시도되고 있다. 본 연구에서는 광범위 소독제로 사용하고 있는 NaDCC의 딸기 주요 병원균(딸기 시들음병, 딸기 탄저병, 딸기 역병 그리고 딸기 세균성모무늬병)에 대한 항균활성 및 방제효과를 검증하고, 친환경제제로의 가능성을 확인하고자 수행되었다. NaDCC는 150~300 ppm 농도로 처리했을 때 병원진균의 균사생장을 효과적으로 억제하였고, 포자 발아율도 68% 이상 경감시키는 것으로 조사되었다. NaDCC는 딸기 세균성모무늬 병원균에 대하여서도 우수한 방제효과를 나타내었는데, 포장조건에서 딸기 세균성 모무늬병의 발생을 50% 감소시키는 것으로 나타났다. 이상의 결과를 토대로 NADCC는 딸기 병 방제용 방제제 후보물질로 사용될 수 있을 것으로 생각되었다. Various diseases occur in fruits, leave and roots during strawberry cultivation and cause severe economical damage and huge amount of chemical fungicide use. Recently, as consumers' interest in safety of foods and organic agriculture produces have increased, control measures using alternatives for chemical fungicides have been newly developed in various ways. This study was conducted to test antifungal activity and control effect of sodium dichloroisocyanurate (NaDCC), using as disinfectants, against major disease pathogens of strawberry, Fusarium oxysporum (Fusarium wilt), Colletotrichum gloeosporioides (Anthracnose) and Phytophthora sp. (Phytophthora blight), and Xanthomonas fragariae (bacterial angular leaf spot) and evaluate availability as environment-friendly materials. When NaDCC was treated at the concentration range of 150 to 300 ppm, it suppressed significantly hyphal growth and reduced spore germination by more than 28%. In field condition, NaDCC showed excellent control effect (control value: 50%) against the bacterial angular leaf spot disease. Based on above-described results, we suggested that NaDCC can be used as alternative candidates to chemical pesticide alternatives of for controlling strawberry diseases.

Improving survival and storage stability of bacteria recalcitrant to freeze-drying: a coordinated study by European culture collections.

The objective of this study is to improve the viability after freeze-drying and during storage of delicate or recalcitrant strains safeguarded at biological resource centers. To achieve this objective, a joint experimental strategy was established among the different involved partner collections of the EMbaRC project ( www.embarc.eu ). Five bacterial strains considered as recalcitrant to freeze-drying were subjected to a standardized freeze-drying protocol and to seven agreed protocol variants. Viability of these strains was determined before and after freeze-drying (within 1 week, after 6 and 12 months, and after accelerated storage) for each of the protocols. Furthermore, strains were exchanged between partners to perform experiments with different freeze-dryer-dependent parameters. Of all tested variables, choice of the lyoprotectant had the biggest impact on viability after freeze-drying and during storage. For nearly all tested strains, skim milk as lyoprotectant resulted in lowest viability after freeze-drying and storage. On the other hand, best freeze-drying and storage conditions were strain and device dependent. For Aeromonas salmonicida CECT 894(T), best survival was obtained when horse serum supplemented with trehalose was used as lyoprotectant, while Aliivibrio fischeri LMG 4414(T) should be freeze-dried in skim milk supplemented with marine broth in a 1:1 ratio. Freeze-drying Campylobacter fetus CIP 53.96(T) using skim milk supplemented with trehalose as lyoprotectant resulted in best recovery. Xanthomonas fragariae DSM 3587(T) expressed high viability after freeze-drying and storage for all tested lyoprotectants and could not be considered as recalcitrant. In contrary, Flavobacterium columnare LMG 10406(T) did not survive the freeze-drying process under all tested conditions.

PCR-based specific techniques used for detecting the most important pathogens on strawberry: a systematic review.

BackgroundStrawberry diseases are a major limiting factor that severely impact plant agronomic performance. Regarding limitations of traditional techniques for detection of pathogens, researchers have developed specific DNA-based tests as sensitive and specific techniques. The aim of this review is to provide an overview of polymerase chain reaction (PCR)-based methods used for detection or quantification of the most widespread strawberry pathogens, such as Fusarium oxysporum f.sp. fragariae, Phytophthora fragariae, Colletotrichum acutatum, Verticillium dahliae, Botrytis cinerea, Macrophomina phaseolina, and Xanthomonas fragariae. An updated and detailed list of published PCR protocols is presented and discussed, aimed at facilitating access to information that could be particularly useful for diagnostic laboratories in order to develop a rapid, cost-effective, and reliable monitoring technique.MethodsThe study design was a systematic review of PCR-based techniques used for detection and quantification of strawberry pathogens. Using appropriate subject headings, AGRICOLA, AGRIS, BASE, Biological s, CAB s, Google Scholar, Scopus, Web of Knowledge, and SpringerLink databases were searched from their inception up to April 2014. Two assessors independently reviewed the titles, abstracts, and full articles of all identified citations. Selected articles were included if one of the mentioned strawberry pathogens was investigated based on PCR methods, and a summary of pre-analytical requirements for PCR was provided.ResultsA total of 259 titles and abstracts were reviewed, of which 22 full texts met all the inclusion criteria. Our systematic review identified ten different protocols for X. fragariae, eight for P. fragariae, four for B. cinerea, six for C. acutatum, three for V. dahlia, and only one for F. oxysporum. The accuracy and sensitivity of PCR diagnostic methods is the focus of most studies included in this review. However, a large proportion of errors in laboratories occur in the pre-analytical phase of the testing process. Due to heterogeneity, results could not be meta-analyzed.ConclusionsFrom a systematic review of the currently available published literature, effective detection assays to detect the major strawberry pathogens have been developed. These assays can function as a basis for clinical labs, regulatory personnel, and other diagnosticians to adapt or implement for detection of these six important strawberry pathogens.Electronic supplementary materialThe online version of this article (doi:10.1186/2046-4053-4-9) contains supplementary material, which is available to authorized users.

RESISTANCE TO ANGULAR LEAFSPOT DISEASE OF STRAWBERRY: INFLUENCE OF SEEDLING AGE

The bacterium Xanthomonas fragariae (Xf) causes the economically important angular leafspot disease to which a high level of resistance has not been found within the cultivated strawberry Fragaria ×ananassa. The 2002 introduction of resistance donors ‘US 4808’ and ‘US 4809’ by the Fruit Laboratory at the Beltsville Agricultural Research Centre, provided sources of resistance at the octoploid level derived from F. virginiana. Moving this resistance through three generations of back crossing to different recurrent F. ×ananassa parents along with screening progeny (12-week-old seedlings) for resistance with a spray inoculation method, followed by selection based on horticultural traits, has created improved resistant genotypes. Selection from mature plants of seedling populations that had not been screened for Xf resistance gave 50% resistant selections – much higher than expected. Therefore, an experiment was designed to determine if some plants develop resistance after 12 weeks of age. Seven controlled crosses were performed in the greenhouse including three resistant (R) × resistant, three resistant × susceptible (S), and one S×S. The S×S cross was included to provide susceptible seedlings, to ensure that the inoculation procedure was effective. Four replicates of seedling sets of 10 to 12 plants per cross were inoculated at 12 or 17 weeks and rated for disease 3 weeks later. The R×R seedlings inoculated at 12 weeks had a mean disease rating of 1.19 compared to 2.28 for seedlings inoculated at 17 weeks. The R×S seedlings inoculated at 12 weeks had a mean disease rating of 2.75 compared to 3.56 for seedlings inoculated at 17 weeks. When seedlings were classified as resistant or susceptible, data analysis showed no significant differences in percent resistant between the inoculation times. These results argue against our hypothesis that older plants show resistance not identified in young seedlings and validate our procedure of screening young seedlings grown to the four to five trifoliate leaf stage. Resistant to susceptible segregation ratios of approximately 3:1 and 1:1 were found in R×R and R×S progeny, respectively, indicating that resistance shows a measure of dominance over susceptibility.

Resistance and systemic dispersal of Xanthomonas fragariae in strawberry germplasm (Fragaria L.)

The angular leaf spot disease caused by Xanthomonas fragariae is an important plant disease with major impact for the strawberry nursery industry. Currently there is no plant protection product available for controlling the disease effectively. Planting of resistant cultivars seems to be promising, but all commercially used cultivars are susceptible and no donor with a high level of resistance has yet been found. Therefore, a total of 145 genotypes from the Fruit Genebank Dresden (Germany) were evaluated for resistance to X. fragariae by artificial inoculation. Six genotypes were classified as partly resistant, out of which only two (US4808 and US4809) are octoploid. Fragaria vesca f. alba, Fragaria nilgerrensis ‘Yunnan’, F. vesca ‘Illa Martin’ and F. moschata ‘Bauwens’ were also classified as partially resistant, but they are only of limited use for breeding because of their variable ploidy level. Fully resistant genotypes could not be detected. The systemic dispersal of the bacteria in strawberry plants was investigated after inoculation of leaves with X. fragariae strain XF3.9.C and the GFP‐tagged strain XF3.9.C(pKAN). The systemic spread was evaluated after 3, 7, 14 and 28 days post‐inoculation (dpi) by nested PCR and fluorescence microscopy. After 3 dpi, X. fragariae could be found in all tissues tested including the inoculated leaf, its petiole, the rhizome, the heart bud up to the youngest fully expanded leaf and its petiole. The systemic spread was also detectable in partially resistant genotypes.

First Report of Xanthomonas fragariae Causing Angular Leaf Spot on Strawberry Plants in México.

The state of Michoacán is the most important strawberry producer in México. During January 2007, field-grown strawberry plants cv. Aromas showing vein necrosis were observed in 3 ha in Zamora County, in fruit production fields. The average disease incidence in the field was 80%. Infected plants presented water-soaked lesions limited by veins on the lower leaf surfaces, which enlarged to form angular spots (1). Additionally, most affected plants presented severe necrosis in the main veins and reddish to necrotic lesions on the upper leaf surfaces. Gram-negative bacteria were consistently isolated from leaves with water-soaked lesions. Isolated bacteria produced mucoid, yellow colonies on YDC, grew on tween and nutrient agar (NA), but not on SX media. Strains produced non-fluorescent colonies on King's B media, were positive starch hydrolysis, negative esculin hydrolysis; and produced acid from fructose but not from arabinose, galactose, celobiose, and trehalose. Growth was inhibited by 2% NaCl (3). Indirect ELISA analysis (NEOGEN, Lansing, MI) was conducted using antibodies specific for Xanthomonas fragariae. Conventional PCR assay using the primer pairs 241A/241B was performed (2). The ELISA test was positive. The expected 300- and 550-bp bands were observed in the PCR analysis. The bacteria was identified as X. fragariae Kennedy and King. Pathogenicity tests were conducted twice in a greenhouse (24 ± 4°C) on a total of five strawberry cv. Aromas plants. The main vein of each of three leaves per plant were punctured using sterile needles. Pathogen inoculum was obtained from 6- to 8-day-old NA cultures. Bacteria were applied onto the wounds with a sterile cotton swab dipped into the bacterial suspension (105 CFU/ml). Inoculated plants were covered with plastic bags for 48 h. Symptoms resembling those seen in the field developed on all inoculated plants after 9 days. X. fragariae was re-isolated from the necrotic lesions and identified by PCR. Control plants were similarly inoculated with water but did not develop symptoms. To our knowledge, this is the first report of X. fragariae causing angular leaf spot in strawberry in Michoacán, México.

Microbial Assessment of Turkey (<i>Meleagris ocellata</i> L.) and Duck (<i>Anas platyrhynchos</i> L.) Faeces (Droppings) in Akure Metropolis

This study was carried out to determine the occurrence of bacteria and fungi in populations of turkey and duck faeces. The prevalence of bacteria and fungi in the faeces of domesticated turkey and ducks (Meleagris ocellata and Anas platyrhynchos) in the City of Akure (Nigeria) was investigated. Five different locations were sampled in Akure Metropolis in April for each of turkey and duck faeces using standard microbiological methods. The microbial load for bacteria ranged from 16.23 × 105 to 30.04 × 105 cfu/g and 12.60 × 105 to 46.01 × 105 cfu/g for turkey and duck faeces respectively while the fungal count ranged from 12.38 × 105 to 28.05 × 105 s/g and 10.60 × 105 to 34.09 × 105 s/g for turkey and duck faeces respectively. The following bacteria were isolated from turkey faeces: Azomonas agilis, Bacillus cereus, Escherichia coli, Proteus vulgaris, Sarcina maxima, Thiocapsa lumicola, Staphylococcus aureus, Enterococcus spp., Xanthomonas fragariae and Streptococcus spp. while Aeromonas hydrophila, Bacillus cereus, Echerichia coli, Proteus vulgaris, Lactobacillus spp., Sarcina maxima, Streptobacillus moniliformis, Staphylococcus aureus, Enterococcus spp and Streptococcus spp. were isolated from duck faces. Bacteria common to both turkey and duck faeces are Bacillus cereus, Escherichia coli, Proteus vulgaris, Sarcina maxima, Staphylococcus aureus, Enterococcus spp. and Streptococcus spp. The fungal species isolated includes Mucor spp., Cladosporium spp., Aspergillus fumigatus and Aspergillus flavus, Alternaria sp., Candida spp., Fusarium spp., Varicosporium elodea and Penicillium spp. Some of the isolated microorganisms are of major importance in the natural environment as well as food and drug production. It could be concluded that turkey and duck faeces are a potential human health hazard and that accumulation of their droppings may pose a public health risk and can cause illness.

Development of Xanthomonas fragariae populations and disease progression in strawberry plants after spray‐inoculation of leaves

Xanthomonas fragariae is the causative agent of angular leaf spot disease of strawberry. Greenhouse experiments were conducted using a X. fragariae isolate tagged with a green fluorescent protein (GFP) for detailed population dynamic studies in and on leaves after spray‐inoculation. The GFP‐tagged bacteria were monitored with dilution plating of leaf washings and leaf extracts, and analysis of intact leaves using a non‐invasive monitoring system called PathoScreen, based on laser radiation of fluorescent cells in plant tissues and signal recording with a sensitive camera. PathoScreen was also used to monitor bacteria grown on an agar medium after leaf printing. During the first 3 days after inoculation, bacterial populations washed off leaves rapidly decreased by at least a factor of 1000, after which populations remained stable until 14 days post‐inoculation (dpi), when symptoms first started to appear. Thereafter, populations increased to a level of 1012 colony‐forming units (CFU) g−1 of leaf material or higher. Similarly, densities in leaf extracts were low during the first 3 days after inoculation, at a level of 100–1000 CFU g−1 of leaf tissue. Gradually populations increased to a level of 109–1012 CFU g−1 at 28 dpi. Higher densities of epiphytic populations were found on the abaxial side than on the adaxial leaf side during the first 2 weeks after inoculation. After spray‐inoculation of leaves, bacterial populations released from infected plants remained low until symptoms appeared, after which plants became highly infectious, in particular under high humidity.

Xanthomonas arboricola pv. fragariae: what's in a name?

Polyphasic analysis exposed important heterogeneity between bacterial strains catalogued as Xanthomonas arboricola pv. fragariae (Xaf) from different culture collections. Two draft whole‐genome sequences revealed pathogenicity related genes of the type‐three secretion system in strain LMG 19146, whereas none were found in the Xaf pathotype strain LMG 19145. Also, considerable sequence divergence was observed in the phylogenetic marker genes gyrB, rpoD, dnaK and fyuA. Further study of 16 Xaf culture‐collection strains showed that co‐classification is not justified. Partial 16S rRNA gene and gyrB sequencing demonstrated that 12 strains belonged to X. arboricola, but that they did not form one homogeneous group within the species. The four remaining strains were identified as Xanthomonas fragariae and Xanthomonas sp. All sequence‐based identifications were confirmed by MALDI‐TOF MS fingerprinting. Also, the pathogenicity genes hrcQ and avrBs2 were detected in only three of the 12 analysed X. arboricola strains. The X. arboricola and Xanthomonas sp. strains showed pectolytic activity, and upon inoculation in strawberry none of the strains reproduced the leaf blight symptoms reported for Xaf. This study demonstrates that (i) no clear criteria exist for the identification of strains as Xaf, (ii) the name Xaf is currently used for a genetically diverse assortment of strains, and (iii) the species X. arboricola holds many undetermined plant‐associated bacteria besides the described pathovars.

Effect of sugar alcohols, antioxidants and activators of systemically acquired resistance on severity of bacterial angular leaf spot (Xanthomonas fragariae) of strawberry in controlled environment conditions

Bacterial angular leaf spot of strawberry caused by Xanthomonas fragariae is an increasingly important disease for which few management strategies are available. The resistance status of most cultivars is unknown and of those cultivars tested, few have significant levels of resistance. Copper formulations are available and have limited effectiveness but phytotoxicity is a problem with repeated applications. This controlled environment study examined the efficacy of foliar applications of various antioxidants, such as α-tocopherol and mannitol, along with the plant activators acetylsalicylic acid and acibenzolar-S-methyl, and the fungicide fosetyl-Al, for disease control. Mannitol and acibenzolar-S-methyl consistently provided excellent control of disease, while fosetyl-Al and α-tocopherol were significantly less effective. Other antioxidants and sugar alcohols similar to mannitol did not provide the same level of angular leaf spot control. Interestingly, interactions were observed between various sugar alcohols and strawberry cultivars in suppressing disease. The mode of action of sugar alcohols against angular leaf spot is likely not related to their antioxidant activity. Sugar alcohols, especially mannitol, may represent a new and non-toxic approach to bacterial angular leaf spot control.

Breeding Strawberry Plants Resistant to Angular Leafspot Disease

The bacterium Xanthomonas fragariae causes the economically important angular leafspot disease to which a high level of resistance has not been found within the cultivated strawberry Fragaria × ananassa. The 2002 introduction of resistance donors ‘US 4808’ and ‘US 4809’ by the Fruit Laboratory at the Beltsville Agricultural Research Centre provided sources of resistance at the octoploid level derived from F. virginiana. Moving this resistance through three generations of back crossing to different recurrent F. × ananassa parents along with screening progeny for resistance with a spray inoculation method, followed by selection based on horticultural traits, has created improved resistant genotypes. Resistant × susceptible crosses have produced from zero to 21% resistant progeny with a mean of 4.8% for 25 crosses. The presence of undesirable traits, such as small and soft fruit, variegation, pistillate flowers, and susceptibility to powdery mildew, has slowed progress, but this linkage drag can be overcome by increasing screened population size and additional backcross generations. Two resistant × resistant crosses were made in 2009, yielding 32.2 and 32.7% resistant seedlings.

Investigating Alternative Strategies for Managing Bacterial Angular Leaf Spot in Strawberry Nursery Production

This article discusses some of the approaches we have tested for managing the bacterial pathogen Xanthomonas fragariae in infected strawberry nursery stock. X. fragariae causes angular leaf spot in strawberry and is transmitted to production fields almost exclusively through infected nursery stock. Of the methods that we have investigated over the past several years, a modified heat treatment has proven to be very effective at reducing systemic infections in propagation material, but cultivar selection affected the outcome. Surface-sterilizing treatments also had an effect on angular leaf spot. We tried procedures from dipping plants in a solution of 10% chlorine bleach to the use of UV-C radiation to reduce the severity of angular leaf spots in the field. Lastly, a sanitation-type treatment, namely removing or trimming remnant leaf and petiole tissue from nursery-trimmed plants, was found to have a significant impact on angular leaf spot. No one method completely eliminated X. fragariae from the planting stock, but there is good indication that a strategic combination of control practices that includes heat treatment should help to reduce significantly the initial amount of bacteria introduced into a field.

Re‐evaluation of EPPO‐listed pests

In 2011, EPPO conducted a survey among its member countries to re‐evaluate the status of 14 pests currently listed on the EPPO A2 List of pests recommended for regulation as quarantine pests: Cacoecimorpha pronubana, Cacyreus marshalli, Ciborinia camelliae, Dickeya (Erwinia) chrysanthemi pv. dianthicola, Frankliniella occidentalis, Impatiens necrotic spot virus, Leptinotarsa decemlineata, Phialophora cinerescens, Puccina horiana, Quadraspidiotus perniciosus, Raspberry ringspot virus, Tomato spotted wilt virus, Viteus vitifoliae, Xanthomonas fragariae. As a result of the survey, it was concluded that all these pests should be maintained on the A2 List.