Bacterial Diseases in Horticulture: Advances in the Study of Pathogens, Interaction with the Host, Diagnosis and Sustainable Control

Wild apples and their hybrids are valued as a source of genetic resistance to biotic and abiotic stress. Malus niedzwetzkyana is an endangered ornamental apple species endemic to Southeast Kazakhstan, the center of Malus domestication. To test the fire blight resistance of M. niedzwetzkyana, eight plant genotypes were inoculated with a local strain of Erwinia amylovora. The genotypes possess different genetic backgrounds, which was confirmed via SSR profiling. Four out of eight displayed moderate to severe symptoms of fire blight infection, while the three wild genotypes proved resistant. To search for the source of the resistance, the samples were tested for the presence of FBF7 QTL using SCAR markers, where seven genotypes tested positive for one of the markers (AE10-375) and one for the other (GE80-19). No correlation between resistance phenotype and FBF7 QTL was confirmed, indicating the source lies elsewhere. Developing detailed genetic and phenotypic profiles of wild apple species helps advance both the preservation efforts and marker-assisted selection in apple breeding. Full article

In 2022, a test performance study (TPS) assessing the influence of different master mixes on the performance of the tetraplex real-time PCR (TqPCR) assay was organized. TqPCR allows for the specific detection and identification of Xylella fastidiosa (Xf) subspecies in a single reaction. Eighteen official laboratories of the Italian National Plant Protection Organization received a panel of 12 blind samples, controls, primers, probes, and different master mixes to participate in the TPS. Furthermore, the Research Centre for Plant Protection and Certification of the Council for Agricultural Research and Economics performed an intra-laboratory study (ITS) on spiked plant matrices to evaluate the analytical sensitivity of TqPCR employing the selected master mixes with the best performance. Naturally infected samples were analyzed for subspecies identification via TqPCR compared with the official multilocus-sequence-typing (MLST) method. The best results in this comparative study were obtained using Fast Universal PCR Master Mix (Applied Biosystems) and Brilliant multiplex QPCR Master Mix (Agilent), and they confirmed that the TqPCR test is reliable, offering the advantage of identifying this subspecies at the same time, thus saving time and resources. The TqPCR assay is suggested among the tests to be used by laboratories performing the official diagnosis of Xf to support the activities of official monitoring. Full article

Fire blight of fruit crops is one of the most dangerous diseases for apple trees and other plants of the Rosaceae family, and in Kazakhstan, it is subject to quarantine. To study the spread of fire blight, a phytopathological evaluation of 59 apple varieties of domestic and foreign breeds was carried out in various regions of the south and southeast of Kazakhstan while also considering climatic conditions. The susceptibility of an apple tree to fire blight is influenced by the climatic conditions prevailing in a particular fruit region of Kazakhstan. Samples were collected from various varieties of apple trees with fire blight symptoms for molecular genetic analysis. The phytopathological evaluation and results of the PCR analysis made it possible to identify the causative agent of the disease and its spread to apple varieties in the main fruit regions of Kazakhstan. A molecular study of the resistance to the fire blight pathogen was carried out using the most effective molecular markers. A set of 10 (FBE-1_Y320; FBE-2_Y192; FBE-2_Y495; FBE-2_Y551; FB-MR5-K35; FB-MRS-R240; FB-MR5-R249; FB-MR5-rp16k15_M106; RLP1a; and RLP1b) SNPs was selected, including SNPs reported to be associated with three trait loci, as well as the two markers AE10-375 and GE-8019. Interestingly, the SNP analysis revealed that for all ten markers linked to fire blight resistance, the genotypes of all 59 apple cultivars were identical. No differences in the presence or absence of these markers were observed among the studied varieties. The 26 apple varieties of domestic and foreign breeds most resistant to fire blight were identified in the molecular analysis using the markers AE-375 and GE-8019. Among the studied 59 apple varieties, 23 varieties were identified using the AE-375 marker and 7 varieties with the GE-8019 marker. Samuret, Honeycrisp, Pinova, and Red Topaz were found to be resistant using markers AE-375 and GE-8019. The most promising apple varieties for further breeding for resistance to fire blight programs were selected. Full article

Nanotechnology could play a huge role in ensuring safer and greener agriculture in the years ahead by providing sustainable tools to control plant diseases. In this study, the possibility of using cellulose nanocrystals (CNCs) obtained from tomato waste to control the bacterial speck disease’s causal agent was evaluated for the effects on plant development. Biocompatibility was assessed by studying seeds’ germination, leaf area, biomass and nitrogen balance index of tomato seedlings treated with CNC. Since epiphytic survival represents a relevant phase in early and later infections provoked by Pseudomonas syringae pv. tomato (Pst), the CNC’s ability to lower the level of bacterial cells in the plant canopy was evaluated in treated seedlings at 1, 7 and 14 days after being artificially inoculated. Leaflets were collected and washed to quantify the epiphytic bacterial population and observed through electron microscopy. Obtained results indicate that CNCs are non-toxic, compatible nanomaterials, highlighting at the same time their potential in counteracting bacterial speck disease by decreasing the level of epiphytic population after two weeks from inoculation by up to one log unit (3.08 CFU cm⁻²) compared to the control (3.94 CFU cm⁻²). Moreover, we were able to demonstrate that it is possible to cut in half the amount of copper without losing effectiveness in controlling the bacteria by mixing it with CNCs, concluding that CNCs could be used to design innovative sustainable plant protection strategies. Full article

In recent decades, the use of nanocomposites (NCs) in crop production has been actively studied. We have previously shown that chemically synthesized selenium nanocomposites (Se NCs) based on natural polymeric matrices of arabinogalactan (Se/AG NCs), carrageenan (Se/CAR NCs), and starch (Se/ST NCs) stimulated the growth and development of potatoes in vitro and had an antibacterial effect against the phytopathogen Clavibacter sepedonicus. It is important to confirm that the results obtained in the laboratory are also reproducible in the field conditions of crop cultivation. In addition, the applications of NCs can be expanded if a wider range of their biological activity is revealed, and the effect on other types of cultivated plants and phytopathogens is shown. In this regard, the purpose of this research was to study the effect of nanopriming with Se NCs on the productivity of potatoes in a field experiment, the germination of soybeans, and the viability of the phytopathogen Pectobacterium carotovorum under in vitro conditions. The study included the use of traditional methods of conducting a field experiment in natural growing conditions, also using microbiological cultures and studying the bacterial profile from the endosphere of potato tubers by the imprint method, germinating Glycine max L. soybean seeds after nanopriming in Petri dishes, followed by the measurement of morphometric and biochemical parameters such as the activity of antioxidant enzymes and content of diene conjugates. Based on the results of field experiments, it was found that the preplant treatment of tubers with Se/AG and Se/CAR NCs stimulated a significant increase in the number of stems in potato plants and tubers per plant, but for Se/AG NC, the increase was observed only in 2020 and 2022, and the stimulating effect of Se/AG and Se/CAR NCs on the weight of tubers was observed only in 2020. In the yield structure, the proportion of marketable seed tubers was increased under the influence of all Se NCs. Endosphere cultures from tubers showed that bacteria were present in only 56% of the tubers in the control, while 78% of the tubers obtained from plants grown from seed tubers treated with Se/AG NC and 100% of the tubers from plants grown from seed tubers treated with Se/ST and Se/CAR NCs contained bacteria. The bacteria were represented by both spore-forming Gram-positive and Gram-negative bacteria of various sizes. The results presented in this article and previously published data showed that the maximum diversity of bacteria was observed after treatment with Se/ST NC. Experiments on potatoes showed that Se/AG NC had the greatest biological effect. In addition, the growth-stimulating effect of these NCs on the biomass of the roots of soybean seedlings was shown. It activated the antioxidant enzymes and reduced the level of diene conjugates (DCs), the amount of which increased significantly after seed biopriming with the phytopathogen Pectobacterium carotovorum. The antibacterial effect of Se/AG NC in relation to the P. carotovorum was observed as a reduction in the growth of the bacterial culture, biofilm formation, and dehydrogenase activity of cells. Thus, it has been shown that Se/AG NC has a stimulating effect on such agriculturally important crops as potatoes and soybeans, as well as an antibacterial effect against Gram-negative and Gram-positive phytopathogenic bacteria. Full article

The yew tree (Taxus baccata L.) is considered in folklore a symbol of immortality due to its qualities of longevity and regeneration. Despite its poisonous reputation, the yew tree has a long history of medicinal use, particularly in the form of extracts from its leaves and bark. In the work presented herein, gas chromatography–mass spectrometry (GC–MS) chemical profiling was applied to the aqueous ammonia/hydromethanolic extracts of several plant organs of T. baccata, leading to the identification of different bioactive compounds than those previously characterized by high-performance liquid chromatography with tandem mass spectrometry (HPLC–MS/MS) in other extraction media. The leaf aqueous ammonia extract was rich in 2-hexylthiophene and 3-O-methyl-d-fructose; 9-octadecenoic and hexadecanoic acid were the main constituents of the bark aqueous ammonia extract; and the fruit hydromethanolic extract contained methyl 2-O-methyl-α-d-xylofuranoside, 1,3-dioxolane derivatives, and erysimoside. The antimicrobial activity of the extracts was assayed against four bacterial pathogens responsible for the soft rot and blackleg diseases of potatoes, viz. Pectobacterium carotovorum subsp. carotovorum, Pectobacterium atrosepticum, Pectobacterium parmentieri, and Dickeya chrysanthemi, resulting in minimum inhibitory concentration (MIC) values as low as 187 μg·mL⁻¹. Bioassays on potato slices confirmed the efficacy of the leaf extract at this dose when applied as a preventive treatment before artificial inoculation with P. carotovorum subsp. carotovorum. In view of this high activity, these extracts may find application in the integrated pest management of soft rot Pectobacteriaceae (SRP) diseases. Full article

Acidovorax citrulli, the causal agent of bacterial fruit blotch, can be divided into two groups. Group I is mainly isolated from melon, and group II is mainly isolated from watermelon. In this study, group I and II strains were used to assess competition in vivo and in vitro by evaluating inhibition activity assays and by measuring population growth dynamics. Our results indicated that there is no mutual inhibition of growth between the two groups of strains in King’s B medium. The bacterial populations of A. citrulli strains were counted within 7 days after inoculation in melon and watermelon cotyledons and plotted against time to generate population growth curves. Area under the population growth curves was calculated. When the two groups of strains were inoculated separately into melon and watermelon cotyledons, the results of analysis of variance show that there was no significant difference. In this study, for the first time in an experimental setting, we inoculated two A. citrulli groups into melon and watermelon cotyledons at the same time and determined the population dynamics of each strain. The results showed that there was no significant difference between two group strains in melon cotyledons. However, in watermelon cotyledons, the area under population growth curves of group II strain were significantly higher than those of group I strain. Three-way analysis of variance results showed that there was interaction between host and grouping on the influence of strain population level (p < 0.0001). These data provide additional information on the host preference of different groups of A. citrulli. Full article

Pistachio crops have a great economic potential, as their global production has increased dramatically over the past few decades. Therefore, it is important to maintain the healthy phytosanitary status of pistachio crops. In a Chilean pistachio orchard, a dieback of the trees was observed, with blighting of twigs and severe necrosis in the trunk and twigs. Bacterial isolation, pathogenicity tests and molecular characterization were conducted to determine the causal agent of the observed disease. The bacterial isolation and analysis of 16S rRNA gene led to the identification of Pantoea genus bacteria. Pathogenicity tests carried out on fruits inoculated with Pantoea isolates induced large necrosis seven days post-inoculation. Further inoculations were carried out on pruning cuttings and on the trunk of 18-month-old pistachio plants. Thirty-one weeks after inoculation, necrotic lesions were observed in the wood of pistachio plants. Sequence analysis of housekeeping genes enabled the isolated bacterium to be identified as Pantoa agglomerans, and to verify its role as the causal agent of the pistachio dieback with necrotic lesions. This is the first report of an occurrence of P. agglomerans inducing dieback in pistachio. Full article

Tomatoes are susceptible to bacterial diseases, mainly related to some Pseudomonas syringae pathovars. Many Pseudomonas species are considered innocuous, but some have shown the ability to opportunistically infect tomato plants. Antimicrobial compounds have been used to control pathogenic organisms, and this can lead to environmental selection of phenotypically resistant bacteria. We assessed the diversity of Pseudomonas species associated with tomato plants from Chilean orchards and analyzed antimicrobial resistance among the isolated strains. A total of 64 Pseudomonas isolates (P. syringae, P. viridiflava, P. fluorescens, P. koreensis, P. gessardii, and P. azotoformans) were evaluated for their phenotypic resistance to seven antimicrobial compounds, including copper, streptomycin, and five other antibiotics typically not used in agriculture. The results showed that 95%, 86%, 70%, 53%, 45%, and 1.6% of the isolates were resistant to rifampin, ampicillin, copper, chloramphenicol, streptomycin, and tetracycline, respectively, with no isolates being resistant to gentamicin. A total of 96.9% of Pseudomonas isolates exhibited a multiresistant phenotype to at least two of the antimicrobials tested. The most frequent multiresistance phenotype was Cu-Str-Amp-Cm-Rif (23.4%). The presence of Pseudomonas strains tolerant to conventional bactericides, metals, and other antimicrobials makes these bacteria an emerging threat to the agriculture industry and to human health. Full article

Bananas are one of the most crucial fruit crops worldwide and significantly contribute to food security in developing countries. However, blood disease of bananas caused by Ralstonia syzygii subspecies celebensensis has become a threat to banana production. Rapid and accurate diagnosis of BDB for on-site detection is pivotal at an early stage for an effective disease control strategy. This study developed LAMP with specific primers targeting BDB, followed by a flocculation assay for visualising positive amplification in the LAMP assay. The assay was sensitive to picogram amounts of gDNA (0.5 pg). LAMP assay on BDB gDNA showed flocculation, but negative results on Fusarium oxysporus cubense and Ralstonia solanacearum confirming the specificity of the assays. Field testing conducted at MARDI headquarters and Taman Pertanian Universiti discovered that the LAMP-flocculation assays were successful in detecting BDB on symptomatic samples as well as on samples from a healthy plot with no symptom observed at the sampling stage, revealing that this assay can detect BDB at an early infection stage. The validation results showed that the LAMP-flocculation assay was comparable with the PCR technique. This newly developed technique is highly specific and sensitive for the early detection of BDB for the adoption of precautionary control measures. Full article

Pseudomonas syringae is a Gram-negative bacterium that infects a wide range of plants, causing significant economic losses in agricultural production. The pathogen exhibits a high degree of genetic and phenotypic diversity, which has led to the classification of P. syringae strains into different pathovars based on their host range and disease symptoms. Copper-based products have traditionally been used to manage infections in agriculture, but the emergence of copper-resistant strains has become a significant concern. Biological control is a promising strategy to manage P. syringae, as it offers an environmentally friendly and sustainable approach to disease management. The review includes an overview of the biology and epidemiology of P. syringae, and of the mechanisms of action of various biological control agents, mainly microorganisms (antagonistic bacteria, and fungi) and bacteriophages. Specifically, this review highlights the renewed interest in bacteriophages (bacteria-infecting viruses) due to their advantages over other eco-friendly management methods, thanks to their bactericidal properties and potential to target specific pathogenic bacteria. The potential benefits and limitations of biological control are also examined, along with research directions to optimize the use of this approach for the management of P. syringae. Full article

Jackfruit-bronzing disease, caused by Pantoea stewartii subsp. stewartii, has recently become more common in the jackfruit crop. Jackfruit-bronzing disease was first discovered in the Philippines in 2014 and spread to Malaysia and Mexico in 2017. Outbreaks of the disease reduced the quality of fresh jackfruit, lowered the market value of local jackfruit, and caused yield losses to the production and financial setbacks to the processors. This disease is more aggressive toward jackfruits with a sweeter flavor and high Brix composition. Symptoms are observable when the fruit is cut open, indicated by the appearance of rusty specks and yellowish-orange to reddish discoloration of the infected pulps and rags. Extensive research is needed to better understand the pathogen’s nature and pathogenicity, supporting future disease prevention and recognition of the pathogen-host interaction. This review explores the significance of the jackfruit-bronzing bacterium, its biology, diagnostics, and pathogenomics, emphasizing the pathogen’s virulence and the management strategies to mitigate this disease. Understanding this destructive bacterium will guide growers and agricultural practitioners to develop the most efficient and sustainable jackfruit-bronzing control methods. Full article