Bacterial Leaf Spot Of Tomato Research Articles

Hyperspectral imaging analysis for early detection of tomato bacterial leaf spot disease

Recent advancements in hyperspectral imaging (HSI) for early disease detection have shown promising results, yet there is a lack of validated high-resolution (spatial and spectral) HSI data representing the responses of plants at different stages of leaf disease progression. To address these gaps, we used bacterial leaf spot (Xanthomonas perforans) of tomato as a model system. Hyperspectral images of tomato leaves, validated against in planta pathogen populations for seven consecutive days, were analyzed to reveal differences between infected and healthy leaves. Machine learning models were trained using leaf-level full spectra data, leaf-level Vegetation index (VI) data, and pixel-level full spectra data at four disease progression stages. The results suggest that HSI can detect disease on tomato leaves at pre-symptomatic stages and differentiate bacterial disease spots from abiotic leaf spots. Using VI data as features for machine learning improved overall classification performance by 26–37% compared to the direct use of raw data. Critical wavelength bands and VIs varied across disease progression stages, suggesting that pre-symptomatic disease detection relied more on changes in leaf water content (1400 nm) and plant defense hormone‐mediated responses (750 nm) rather than changes in leaf pigments or internal structure (800–900 nm), which may become more crucial during symptomatic stages. In conclusion, this study provides valuable insights into the dynamics of bacterial spot disease, revealing the potential benefits of leaf structure segmentation and VI group pattern analysis in HSI studies for the early detection of leaf diseases.

The integration of machine learning and IoT for the early detection of tomato leaf disease in real-time

The impact of climate change, pests, and inadequate agricultural practices on crop health is becoming a growing concern. It is estimated that 20-40% of global crop yield is adversely affected by pests and diseases. This has a direct negative impact on food security and nutritional well-being, as staple cereal crops (such as rice, maize, and wheat) and tuber crops (like potato, onion, and tomato) are affected. Advancements in Artificial Intelligence (AI), Computer Vision (CV), and IoT have a significant influence on reducing crop losses by detecting crop diseases at early stages. The primary focus of this research is to develop a real-time system that can detect tomato crop diseases at an early stage by integrating Machine Learning (ML) algorithms and IoT. The most common tomato leaf diseases, such as Tomato Mosaic Virus (TMV), Tomato Bacterial Leaf Spot (TBLS), Tomato Early Blight (TEB), and Tomato Late Blight (TLB), are considered in this work. The hybrid discriminative feature space is derived from the integration of low- and high-level features via Convolution Neural Network (CNN) Layers. The 3-stage Stacked Deep Convolutional Autoencoder is used to optimize the CNN performance by reducing computation complexity. The proposed model is implemented on the Plantvillage benchmark dataset and achieves the highest recognition accuracy of 95.6% for the 5-class problem using 5-fold cross-validation.

Evaluating the Efficacy of Chemicals and Antibiotics in Combating Bacterial Leaf Blight of Tomato

Bacterial leaf spot of tomato caused by Xanthomonas campestris pv. vesicatoria is one of the most destructive biotic stresses that significantly affects the quality and productivity of tomatoes. In the current study, we investigated the effects of ten chemicals, including Score (24.51%), Topsim-M (70%), Cabrio (60%), Fossil (29%), Milvet (80%), Forum Top (53%), Excel (80%), Evcin (80%), Electus Super (30%), and Copper hydroxide (52.4%), as well as ten antibiotics, namely Quinocel (10%), Neflox (30%), Gentam (20%), Velocef (29%), Rithmo (44%), Cefcom (52.4%), Cefstar (52.41%), Novamox-LA (15%), Trisulpha (53%), and Inocef (80%), at different concentrations under laboratory conditions, using the inhibition zone technique. In greenhouse conditions, the most effective chemicals and antibiotics, both individually and in combination, were evaluated at various concentrations against bacterial leaf spot of tomatoes. A Completely Randomized Design was employed for both laboratory and greenhouse experiments. The results from the laboratory conditions indicated that Score exhibited the maximum inhibition zone (34.83 mm), followed by Neflox (29.07 mm) against X. campestris pv. vesicatoria, compared to the control. Disease severity was assessed using a disease rating scale at specific time intervals. The results showed that the combination of Score and Neflox exhibited the least disease severity (7.50%), followed by other treatments.

Chemical control of bacteria Xanthomonas hortorum pv. gardneri and Xanthomonas euvesicatoria pv. perforans in vitro

Tomato bacterial leaf spot is a worldwide disease that causes high losses in processed and fresh tomatoes. This disease is caused by four species of bacteria (Xanthomonas hortorum pv. gardneri, Xanthomonas euvesicatoria pv. perforans, Xanthomonas euvesicatoria pv. euvesicatoria and Xanthomonas euvesicatoria pv. vesicatoria). Xanthomonas hortorum pv. gardneri and Xanthomonas euvesicatoria pv. perforans are the phytopathogenic bacteria most frequently found in tomato crops. The objectives of this work were to identify potential bactericides and to study the mancozeb and copper oxychloride in the management of bacterial spot caused by Xanthomonas hortorum pv. gardneri and Xanthomonas euvesicatoria pv. perforans. Forty-four active ingredients were tested at 1% of the commercial product (1000g c.p./100L H2O) as first screening. Posteriorly, those that inhibited bacterial multiplication were tested at their recommended doses. When used at a dose of 1000g c.p./100L H2O, the products that inhibited the multiplication of X. hortorum pv. gardneri and X. euvesicatoria pv. perforans were benzalkonium chloride, acetic acid, cuprous oxide, mancozeb, copper hydroxide, mancozeb + famoxadone, copper oxychloride, metiram + pyraclostrobin, Bordeaux mixture and Viçosa mixture. At the doses recommended in the package insert, the products that inhibited the multiplication of X. hortorum pv. gardneri and X. euvesicatoria pv. perforans were benzalkonium chloride, acetic acid, mancozeb, mancozeb + famoxadone and metiram + pyraclostrobin. Copper oxychloride at the recommended dose of 0.2% (200g c.p./100L H2O – 168g a.i./100L H2O) did not inhibit the multiplication of X. hortorum pv. gardneri. It has been estimated that 0.004% is resistant to the recommended dose of copper oxychloride. At doses higher than 0.4% (400g c.p./100L H2O – 336g a.i./100L H2O) there was no bacterial growth. However, once the bacteria grow in a culture medium containing 0.2% copper oxychloride of commercial product (200g c.p./100L H2O – 168g a.i./100L H2O), it can be multiplied in a culture media even at a dose of 1000g c.p./100L H2O (840g a.i./100L H2O), which would be equivalent to a dose of 10kg c.p./ha. Mancozeb inhibited bacterial growth from the dose of 0.1% (100g c.p./100L H2O – 80g a.i./100L H2O). However, when X. hortorum pv. gardneri resistant to copper oxychloride 1% (1000g c.p./100L H2O – 840g a.i./100L H2O) is striated in a culture medium containing mancozeb, the bacteria multiply up to a dose of 0.5% mancozeb (500g c.p./100L H2O – 400g a.i./100L H2O). The in vitro results indicate that doses equal to or less than 0.2% copper oxychloride (200g c.p./100L H2O – 168g a.i./100L H2O) select colonies resistant to high doses of copper oxychloride and also decrease the efficiency of mancozeb.

Farklı Konukçu Bitkilerden Toplanan Ökse Otu (Viscum album L.) Ekstraktlarının Bazı Bitki Patojeni Bakteriler Üzerine Etkisi

Bu çalışma ile farklı ağaçlarda yarı parazitik olarak bulunan ökse otu (Viscum album L.)‘ndan elde edilen ekstraktların domates bakteriyel kanser ve solgunluk hastalık etmeni Clavibacter michiganensis subsp. michiganensis, biber ve domates bakteriyel yaprak lekesi hastalık etmeni Xanthomonas axonopodis pv. vesicatoria ve domates bakteriyel benek hastalık etmeni Pseudomonas syringae pv. tomato‘ya olan etkileri belirlenmiştir. Çalışmada çam, armut, akasya ve mahlep ağaçlarından toplanan ökse otunun yaprak ve saplarından elde edilen su ekstraktları son konsantrasyonları %1, %2,5, %5 ve %10 olacak şekilde otoklav edilmeden önce Nutrinet agar besi yerine ilave edilmiştir. Spektrofotometrede 600 nm dalga boyunda 0,2 absorbans değerinde 108 hücre/ml yoğunluğunda hazırlanan bakteri süspansiyonları ekstraktlı besi yerlerine ekilmiştir. Kontrol grubu olarak ekstraktların ilave edilmediği besi yerlerine patojenlerin ekimi yapılmıştır. Çalışma 3 tekerrürlü olarak kurulmuş olup, 2 kez tekrarlanmıştır. İnkübasyon süresi sonunda uygulama yapılan tüm petrilerde gelişen bakteriler toplanarak spektrofotometrede yoğunlukları ölçülmüştür. Yapılan çalışmada, farklı ağaçlardan toplanan ökse otunun yaprak ve saplarından elde edilen ekstraktların %1 ve %2,5‘luk dozları testlenen üç patojen üzerinde etkili olmazken, %5 ve %10‘luk dozları patojen bakterilerin gelişimini tamamen (%100) durdurmuştur. Ayrıca patojenler üzerinde ökse otunun toplandığı konukçu bazında bir farklılığın olmadığı da görülmüştür. In vitro koşullar altında yürütülen bu çalışma sonucuna göre, patojenler üzerinde etkili bulunan ökse otu ekstraktı ile in vivo çalışmaların yürütülmesi gerekmektedir.

Phylogenetic Analyses of Xanthomonads Causing Bacterial Leaf Spot of Tomato and Pepper: Xanthomonas euvesicatoria Revealed Homologous Populations Despite Distant Geographical Distribution.

Bacterial leaf spot of tomato and pepper (BLS), an economically important bacterial disease caused by four species of Xanthomonas (X. euvesicatoria (Xe), X. vesicatoria (Xv), X. gardneri (Xg), and X. perforans (Xp)), is a global problem and can cause over 50% crop loss under unfavorable conditions. Among the four species, Xe and Xv are prevalent worldwide. Characterization of the pathogens is crucial for disease management and regulatory purposes. In this study, we performed a multilocus sequence analysis (MLSA) with six genes (hrcN, dnaA gyrB, gapA, pdg, and hmbs) on BLS strains. Other Xanthomonas species were included to determine phylogenetic relationships within and among the tested strains. Four BLS species comprising 76 strains from different serological groups and diverse geographical locations were resolved into three major clades. BLS xanthomonads formed distinct clusters in the phylogenetic analyses. Three other xanthomonads, including X. albilineans, X. sacchari, and X. translucens pv. undolusa revealed less than 85%, 88%, and 89% average nucleotide identity (ANI), respectively, with the other species of Xanthomonas included in this study. Both antibody and MLSA data showed that Xv was clearly separated from Xe and that the latter strains were remarkably clonal, even though they originated from distant geographical locations. The Xe strains formed two separate phylogenetic groups; Xe group A1 consisted only of tomato strains, whereas Xe group A2 included strains from pepper and tomato. In contrast, the Xv group showed greater heterogeneity. Some Xv strains from South America were closely related to strains from California, while others grouped closer to a strain from Indiana and more distantly to a strain from Hawaii. Using this information molecular tests can now be devised to track distribution of clonal populations that may be introduced into new geographic areas through seeds and other infected plant materials.

Independent Evolution with the Gene Flux Originating from Multiple Xanthomonas Species Explains Genomic Heterogeneity in Xanthomonas perforans.

Xanthomonas perforans is the predominant pathogen responsible for bacterial leaf spot of tomato and X. euvesicatoria for that of pepper in the southeast United States. Previous studies have indicated significant changes in the X. perforans population collected from Florida tomato fields over the span of 2 decades, including a shift in race and diversification into three phylogenetic groups driven by genome-wide homologous-recombination events derived from X. euvesicatoria In our sampling of Xanthomonas strains associated with bacterial spot disease in Alabama, we were readily able to isolate X. perforans from symptomatic pepper plants grown in several Alabama counties, indicating a recent shift in the host range of the pathogen. To investigate the diversity of these pepper-pathogenic strains and their relation to populations associated with tomatoes grown in the southeast United States, we sequenced the genomes of eight X. perforans strains isolated from tomatoes and peppers grown in Alabama and compared them with previously published genome data available from GenBank. Surprisingly, reconstruction of the X. perforans core genome revealed the presence of two novel genetic groups in Alabama that each harbored a different transcription activation-like effector (TALE). While one TALE, AvrHah1, was associated with an emergent lineage pathogenic to both tomato and pepper, the other was identified as a new class within the AvrBs3 family, here designated PthXp1, and was associated with enhanced symptom development on tomato. Examination of patterns of homologous recombination across the larger X. euvesicatoria species complex revealed a dynamic pattern of gene flow, with multiple donors of Xanthomonas spp. associated with diverse hosts of isolation.IMPORTANCE Bacterial leaf spot of tomato and pepper is an endemic plant disease with a global distribution. In this study, we investigated the evolutionary processes leading to the emergence of novel X. perforans lineages identified in Alabama. While one lineage was isolated from symptomatic tomato and pepper plants, confirming the host range expansion of X. perforans, the other lineage was isolated from tomato and acquired a novel transcription activation-like effector, here designated PthXp1. Functional analysis of PthXp1 indicated that it does not induce Bs4-mediated resistance in tomato and contributes to virulence, providing an adaptive advantage to strains on tomato. Our findings also show that different phylogenetic groups of the pathogen have experienced independent recombination events originating from multiple Xanthomonas species. This suggests a continuous gene flux between related xanthomonads associated with diverse plant hosts that results in the emergence of novel pathogen lineages and associated phenotypes, including host range.

Plasma activated water as resistance inducer against bacterial leaf spot of tomato.

Plant bacterial diseases are routinely managed with scheduled treatments based on heavy metal compounds or on antibiotics; to reduce the negative environmental impact due to the use of such chemical compounds, as pollution or selection of antibiotic resistant pathogens, the integrated control management is required. In the frame of a sustainable agriculture the use of bacterial antagonists, biological agents, plant defence response elicitors or resistant host plant genotypes are the most effective approaches. In this work, cold atmospheric pressure plasma (CAP) was applied to sterile distilled water, inducing the production of a hydrogen peroxide, nitrite and nitrate, and a pH reduction. In particular, an atmospheric pressure dielectric barrier discharge (DBD) has been used to produce plasma activated water (PAW), that was firstly assayed in in vitro experiments and then in planta through application at the root apparatus of tomato plants, against Xanthomonas vesicatoria (Xv), the etiological agent of bacterial leaf spot. Moreover, the transcription abundance of five genes related to the plant defense was investigated in response to PAW treatment.PAW did not show direct antimicrobial activity against Xv in in vitro experiments, but it enhanced the tomato plants defenses. It was effective in reducing the disease severity by giving relative protections of ca. 61, 51 and 38% when applied 1 h, 24 h and 6 days before the experimental inoculation, respectively. In addition, the experiments highlighted the pal gene involvement in response to the PAW treatments and against the pathogen; its transcription levels resulted significantly high from 1 to 48 h until their decrease 192 h after PAW application.

Genomic sequence analysis reveals diversity of Australian Xanthomonas species associated with bacterial leaf spot of tomato, capsicum and chilli

BackgroundThe genetic diversity in Australian populations of Xanthomonas species associated with bacterial leaf spot in tomato, capsicum and chilli were compared to worldwide bacterial populations. The aim of this study was to confirm the identities of these Australian Xanthomonas species and classify them in comparison to overseas isolates. Analysis of whole genome sequence allows for the investigation of bacterial population structure, pathogenicity and gene exchange, resulting in better management strategies and biosecurity.ResultsPhylogenetic analysis of the core genome alignments and SNP data grouped strains in distinct clades. Patterns observed in average nucleotide identity, pan genome structure, effector and carbohydrate active enzyme profiles reflected the whole genome phylogeny and highlight taxonomic issues in X. perforans and X. euvesicatoria. Circular sequences with similarity to previously characterised plasmids were identified, and plasmids of similar sizes were isolated. Potential false positive and false negative plasmid assemblies were discussed. Effector patterns that may influence virulence on host plant species were analysed in pathogenic and non-pathogenic xanthomonads.ConclusionsThe phylogeny presented here confirmed X. vesicatoria, X. arboricola, X. euvesicatoria and X. perforans and a clade of an uncharacterised Xanthomonas species shown to be genetically distinct from all other strains of this study. The taxonomic status of X. perforans and X. euvesicatoria as one species is discussed in relation to whole genome phylogeny and phenotypic traits. The patterns evident in enzyme and plasmid profiles indicate worldwide exchange of genetic material with the potential to introduce new virulence elements into local bacterial populations.

Identification of Xanthomonas species associated with bacterial leaf spot of tomato, capsicum and chilli crops in eastern Australia

Several species of Xanthomonas cause bacterial leaf spot, a disease that affects solanaceous crops worldwide. The diversity of 64 Australian isolates of Xanthomonas spp. associated with bacterial leaf spot in tomato, capsicum and chilli crops in eastern Australia was determined using multi-locus sequence analysis of atpD, dnaK, efp and gyrB genes, species-specific PCR assays and biochemical analyses. At least five species of Xanthomonas associated with bacterial leaf spot were identified in Australian tomato, capsicum and chilli crops and their pathogenicity assessed. Phylogenetic and biochemical analyses identified X. euvesicatoria, X. perforans and X. vesicatoria as the most frequently recovered pathogenic species. Non-pathogenic and weakly pathogenic species were also identified. The suitability of the identification methods used and the implications of the detection of these species will be discussed.

New sources of partial resistance to bacterial spot race T2 in processing tomatoes

ABSTRACT Bacterial leaf spot of tomato is caused by four Xanthomonas species, among which Xanthomonas vesicatoria race T2 predominates in Uruguay. Difficulties in integrated disease management and the rapid spread of the pathogen led to investigations of genetic resistance. This study aimed to identify resistance sources to bacterial leaf spot race T2 in tomato for processing. Twelve genotypes were evaluated under field conditions in 2010 and 2011. Plants were spray-inoculated with a suspension of bacteria (108 cfu/mL) 15 days after transplantation. Incubation period, disease severity on leaves, and the percentage of fruits with symptoms at harvest were determined. The incubation period did not differ among the genotypes. The genotype 'Hawaii 7981' had the lowest leaf severity on the leaves, followed by 'Loica'. The lines (derived from the cultivar 'Loica') LB97, LB99, LB60, and LB76, and the cultivar 'Ohio 8245' showed intermediate levels of severity on leaves, whereas 'H9997', 'Cuyano', LB85, and 'NUN6011' presented higher severities. The differences in disease severity of the leaves were similar over the years, while incidence of symptoms in fruit was more variable. Next to 'Hawaii 7981', the cultivars 'Loica' and 'Ohio 8245' were identified as new sources of partial resistance to bacterial spot race T2.

Biofertilizantes no controle da mancha bacteriana (Xanthomonas spp.) do tomateiro

RESUMO A mancha bacteriana do tomateiro, causada por quatro espécies de Xanthomonas pode provocar perdas significativas na produção da cultura e a utilização de biofertilizantes na proteção de plantas tende a reduzir a incidência de doenças. O objetivo do trabalho foi avaliar o efeito dos biofertilizantes no controle preventivo e curativo da mancha bacteriana do tomateiro. Para o controle preventivo da doença, plantas de tomate cultivar Santa Cruz Kada, com 3 a 4 folhas foram pulverizadas com os biofertilizantes (Soil-Set, Agro-Mos e Cop-R-Quick) e água (testemunha); e dois dias após foram inoculadas por aspersão com a suspensão bacteriana nas concentrações 109 UFC mL-1 (OD550=0,5) e 106UFC mL-1, com o isolado UFU A35 de Xanthomonas sp. Para o controle curativo, as plantas foram inoculadas com a suspensão bacteriana, e dois dias após foram pulverizadas com os biofertilizantes e água. A severidade da mancha bacteriana foi avaliada usando uma escala diagramática; aos 3, 5, 8, 11 e 14 dias após a inoculação e calculada a área abaixo da curva de progresso de doença (AACPD). O controle preventivo foi mais eficiente no manejo da mancha bacteriana do tomateiro, e os diferentes biofertilizantes reduziram a severidade da doença.

Progresso da mancha bacteriana do tomateiro em diferentes regimes de pulverização

The control of tomato bacterial spot was carried out based on chemical spraying on a fixed schedule, without considering the progress of the disease. The spraying system based on a mathematical model can predict this progress and also reduce the number of sprays and maximize the disease control. Spraying regimes were established according to estimated severity of 0.05, 0.15, and 0.25 according to a previous published forecast model and compared with standard regimes with chemical applications at intervals of 5 and 7 days. In order to evaluate different spraying regimes on tomato bacterial leaf spot severity, the statistical modeling technique known as mixed models was applied. These models not only include fixed effects but also random effects to each one of the individuals of the population under study. Cumulative bacterial spot severity as a function of weeks after transplanting for five spraying regimes was calibrated to a Logistic model where the fit was obtained for the mixed model where a random effect was adjusted to the upper asymptote. This parameter represents the potential disease severity for each one of the treatments. As a result of model calibration, the treatment with the lowest disease severity was the one sprayed with estimated severity of 0.05 with 36 sprays of fungicide with bactericide action. However, the treatment with disease severity of 0.15 resulted in reduced disease severity of 26.28%.

Green Tea Extract as an Environmentally Friendly Antibacterial Agent Against Pseudomonas syringae pv. tomato on Plants

Green tea contains polyphenols, such as tannins and catechins, that are effective against bacteria. As it is naturally produced by plants, green tea has been explored as an environmentally friendly antibacterial product for use in human health. However, there are very few reports of green tea extract being used as an antibacterial product to protect plants from various bacterial diseases. In this study, we demonstrated that green tea extract could inhibit the growth of Pseudomonas syringae pv. tomato, the causal agent of tomato bacterial leaf spot disease, on agar plates. We also demonstrated that spraying green tea extract on Nicotiana benthamiana plants could inhibit the growth of P. syringae on plant leaves. Therefore, we conclude that green tea extract has the potential to be used as a ‘green’ antibacterial product in agriculture production.

Improved Sample Preparation for PCR-Based Assays in the Detection of Xanthomonads Causing Bacterial Leaf Spot of Tomato

Aims: To develop a sampling procedure for PCR-based screening of bacterial leaf spot (BLS)-causing xanthomonads without DNA extraction from infected tomato plants. Place and Duration of Study: University of Copenhagen, Denmark and Sokoine University of Agriculture, Morogoro, Tanzania between July 2008 and November 2010. Methodology: Flinders Technology Associates (FTA) plant cards and Chromatography paper or Whatman paper strips (WPS) were spotted with bacterial suspensions from 24h-old cultures from reference strains of BLS-causing xanthomonads, or sap obtained by grinding or hand maceration of plant tissue, were used as templates in PCR reactions or isolation of live bacterial cells on Nutrient agar (NA) media. Samples were tested by PCR with Xan 7 genus/-specific Xanthomonas primers or in multiplex with 26S rRNA primers. Isolation of bacteria was done by streaking aliquots of 75 μl of a suspension from a disc Research Article British Biotechnology Journal, 3(4): 556-574, 2013 557 (2-mm-punch by Harris Micro Punch®) in triplicate, removed from each of the FTA plant card and WPS onto NA media. Results: The FTA plant card spotted with pure cultures of reference strains of xanthomonads and sap from grinding or direct maceration of plant tissue resulted in more clear PCR bands (402 bp) and (594 bp of rRNA gene in multiplex) than the WPS samples. Sensitivity of detection by the FTA paper-based PCR was ≈ 5.0 x 10, while that of the WPS was > 1.0 x 10 CFU/ml. The WPS (but not the FTA) was proved to be useful for saving living bacteria cells for up to one week of storage at ambient temperatures. Conclusion: Both FTA plant card and WPS can be used for PCR detection of BLScausing xanthomonads in tomato. However, the FTA plant card is recommended as it produced clearer PCR products than WPS. WPS is recommended for experiments requiring isolation of live bacterial cells on NA media.

Evaluation of essential oils as seed treatment for the control of Xanthomonas spp. associated with the bacterial leaf spot of tomato in Tanzania

Bacterial leaf spot (BLS) caused by Xanthomonas spp. is a serious and a major constraint to tomato production worldwide. The seed-borne nature of BLS, the complex nature of the disease, which is caused by different bacterial species and the current ineffective control measures have made the evaluation of alternative control compounds for seed treatment necessary. Therefore, the efficacy of 11 essential oils applied as seed treatments to control Xanthomonas perforans in tomato and the effect on seed germination and seedling growth was evaluated. Seed treatment with oils of eucalyptus (Eucalyptus globules Labill.), rosemary (Rosmarinus officinalis L.) and niaouli (Melaleuca viridiflora Sol. ex Gaertn.) at 2% concentration inhibited the growth of X. perforans in the in vitro assays and consistently reduced the incidence and severity of BLS in planta tests. Negative effects on seed germination and seedling growth were not observed when tomato seeds were treated with the essential oils of eucalyptus and rosemary. Therefore, eucalyptus and rosemary oils can be used for seed treatment to control BLS in tomato. Further investigation is required on the shelf life of these oils, mode of action and their effects on other seedborne pathogens of tomato and in other crop systems in Tanzania.

The effect of plant extracts as seed treatments to control bacterial leaf spot of tomato in Tanzania

Bacterial leaf spot (BLS) caused by seed-borne xanthomonads is a serious disease of tomato (Solanum lycopersicum L.), causing significant losses in both yield and quality. To identify more effective control measures, we evaluated crude extracts from 84 plant species in in vitro and in planta assays for antibacterial activity against BLS of tomato. In the in vitro assays, 20.2 % of the tested plant extracts totally inhibited growth of bacteria when seed washings from treated seeds were plated on nutrient agar medium. In the in planta assays, 17.8 % of the tested plant extracts reduced BLS incidence by 100 % in tomato seedlings. The most effective seed treatments were obtained with extracts from Aloe vera, Betula pendula, Coffea arabica, Glycyrrhiza uralensis, Juniperus communis, Ocimum basilicum, Quercus robur, Rheum palmatum, Rosmarinus officinalis, Ruta graveolens, Sinapis alba, Yucca schidigera and Salvia officinalis. Seed treatment of tomato with these extracts completely inhibited Xanthomonas perforans in both in vitro and in planta assays. Extracts from A. vera, C. arabica and Y. schidigera were tested three times using tomato seeds of cultivars Tanya, Cal-J and Moneymaker in Tanzania. Treatment of tomato seeds with these extracts had a positive effect on the number of normal seeds and had no effect on seedling vigor, height and weight. These results indicate that plant extracts from A. vera, C. arabica and Y. schidigera are potential candidates for seed treatment against seed-borne xanthomonads of tomato in Tanzania.

Pathogenic variability among five bacterial isolates of Xanthomonas campestris pv. vesicatoria, causing spot disease on tomato and their response to salicylic acid

Symptoms of a bacterial disease caused by Xanthomonas campestris pv. vesicatoria were observed on tomato plants growing in Al-Kharj area of Saudi Arabia. Symptoms included irregular yellow-necrotic areas on tomato leaves and spotting on the stem. Five bacterial isolates were isolated from the infected tomato seedlings. The pathogen identified was confirmed biochemically and physiologically, hypersensitive reaction occurred in tobacco plants and pathogenicity tests were conducted. Significant variability was shown among the bacterial isolates. Isolates T2, T4 and T5 were the most pathogenic isolates as expressed by the disease severity index. The highest severity index (57.8%) was observed by isolate T5, followed by the two isolates T4 (55.3%) and T2 (43.1%) two weeks post inoculation. In an attempt to control this disease, Salicylic acid was used to study its ability to inhibit the growth of X. campestris pv. vesicatoria in vitro, and its potentiality in suppressing bacterial spot of tomato under artificial inoculation conditions. All concentrations tested had inhibitory effects against X. campestris pv. vesicatoria. The inhibition zone increased with increasing concentrations. SA at 1.5 ppm caused the largest inhibition zone (40 mm) followed by 1 ppm (33 mm). Application of SA at 1.5 ppm reduced the severity of bacterial leaf spot of tomato. Application as foliar treatment was more effective than seedling treatment. SA was the most effective at 1.5 ppm as foliar treatment where the number of lesions/leaf and disease severity were 1.8 and 14.1, respectively.

Diagnosis of bacterial spot of tomato using spectral signatures

Ultraviolet, visible, and near-infrared reflectance spectroscopy was used to determine the disease severity of tomato ( Lycopersicon esculentum) leaves infected with Xanthomonas perforans, the causal agent of bacterial leaf spot of tomato. Chemometric methods were used to identify significant wavelengths and create spectral-based prediction models. Significant wavelengths were identified through analysis of the B-matrix from partial least squares (PLS) regression, analysis of a correlation coefficient spectrum, and through the use of a stepwise multiple linear regression (SMLR) procedure. These analysis methods revealed several significant regions wavelengths and produced predictive models of disease severity based on absorbance spectra. The best model predicted the disease severity of the validation data set with a root mean square difference (RMSD) of 4.9% and a coefficient of determination ( R 2) of 0.82. The results of this initial study indicate the potential for the use of spectral technology to detect bacterial leaf spot of tomato in the field.

Evaluation of spray programmes for the management of leaf spot incited by Pseudomonas syringae pv. syringae on tomato cv. Cuore di bue

Four experimental trials were carried out in northern Italy under greenhouse conditions against bacterial leaf spot of tomato, incited by Pseudomonas syringae pv. syringae, a disease recently observed in the country. The efficacy of spray programmes, based on the use of several copper-based compounds, acibenzolar-S-methyl, fosetyl Al and two biological control agents, Bacillus subtilis strain QST 713 and Pseudomonas chlororaphis MA 342, was tested. The best results were obtained with the alternation of copper oxychloride, used alone or in mixture with acibenzolar-S-methyl, with acibenzolar-S-methyl, applied for a total of two sprays. Acibenzolar-S-methyl applied alone was also effective. However, due to its possible slight phytotoxic effect, observed in our case in one trial, strategies relying on a limited number of its application are preferred. The efficacy of acibenzolar-S-methyl is higher under lower inoculum conditions. Among the tested copper-based fungicides, copper oxychloride provided the best and most consistent results, with no or limited phytotoxicity. Other copper compounds were much less effective and sometimes caused a reduction in plant height. The disease suppression achieved with B. subtilis strain QST 713 was only partial, while P. chlororaphis was not effective. The advantages of programmes based on rotation of copper compounds and acibenzolar-S-methyl are discussed.