Hyperspectral imaging for the detection of plant pathogens in seeds: recent developments and challenges.

Food security, a critical concern amid global population growth, faces challenges in sustainable agricultural production due to significant yield losses caused by plant diseases, with a multitude of them caused by seedborne plant pathogen. With the expansion of the international seed market with global movement of this propagative plant material, and considering that about 90% of economically important crops grown from seeds, seed pathology emerged as an important discipline. Seed health testing is presently part of quality analysis and carried out by seed enterprises and governmental institutions looking forward to exclude a new pathogen in a country or site. The development of seedborne pathogens detection methods has been following the plant pathogen detection and diagnosis advances, from the use of cultivation on semi-selective media, to antibodies and DNA-based techniques. Hyperspectral imaging (HSI) associated with artificial intelligence can be considered the new frontier for seedborne pathogen detection with high accuracy in discriminating infected from healthy seeds. The development of the process consists of standardization of methods and protocols with the validation of spectral signatures for presence and incidence of contamined seeds. Concurrently, epidemiological studies correlating this information with disease outbreaks would help in determining the acceptable thresholds of seed contamination. Despite the high costs of equipment and the necessity for interdisciplinary collaboration, it is anticipated that health seed certifying programs and seed suppliers will benefit from the adoption of HSI techniques in the near future.

Colorimetric LAMP assay for detection of Xanthomonas phaseoli pv. manihotis in cassava through genomics: a new approach to an old problem.

Bacterial spot caused by Xanthomonas phaseoli pv. manihotis (Xpm) is considered the main bacterial disease that affects cassava, causing significant losses when not properly managed. In the present study, a fast, sensitive, and easy-to-apply method to detect Xpm via colorimetric loop-mediated isothermal amplification (LAMP) was developed. In order to ensure the use of a unique to the target pathovar core region for primer design, 74 complete genomic sequences of Xpm together with different bacterial species and pathovars were used for comparative genomics. A total of 42 unique genes were used to design 27 LAMP primer sets, from which nine primers were synthesized and only one (Xpm_Lp1 primer set) showed sufficient efficiency in preliminary tests. The sensitivity, assessed by a serial dilution of the type strain (IBSBF 278) DNA, yielded high sensitivity, detecting up to 100 fg. The LAMP primers showed high specificity, not cross-reacting with other bacterial species or other pathovars tested, and amplifying only the Xpm isolates. Tests confirmed the high efficiency of the protocol using infected or inoculated macerated cassava leaves, without the need for additional sample treatment. The LAMP test developed in this study was able to detect Xpm in a fast, simple, and sensitive way, and it can be used to monitor the disease under laboratory and field conditions.

Morphologic, molecular, and pathogenic characterization of Phytophthora palmivora isolates causing flower rot on azalea.

The objective of this work was to characterize two Phytophthora palmivora isolates causing floral blight and rot in azalea plants and to evaluate the pathogenicity of this oomycete pathogen on several plant species. Azalea plants with symptoms of flower blight and rot were obtained in the municipality of Holambra-SP. After an attempt of isolation, colonies with Phytophthora characteristics grown only on selective V8 medium. Molecular identification of the isolates was done by amplification and sequencing of ITS and COX2 regions. In the phylogenetic analysis, the azalea isolates clustered with reference isolates of P. palmivora. Morphological characteristics were similar to those described for P. palmivora. Isolates were inoculated in healthy azalea plants and caused leaf blight and floral rot. The pathogen was re-isolated from symptomatic plants completing Koch's postulates. In a host range test, the azalea isolates were able to cause lesions on leaves of vinca, snapdragon, basil, and tomato, and affected both leaves and flowers of geranium. Fruit rot was observed on tomato, potato, sweet pepper, scarlet eggplant, zucchini, cucumber, maroon cucumber, onion, apple, papaya, guava, and carrot. This is the first report of the species P. palmivora causing flower blight and rot in azalea plants in Brazil and probably in the world.

An isolate of sweet potato chlorotic stunt virus from Brazil with a distinct genome organization.

Sweet potato chlorotic stunt virus (SPCSV; genus Crinivirus, family Closteroviridae), is an economically important pathogen of sweet potato. In the present work, the nucleotide sequences of two RNA segments of SPCSV (isolate SPCSV-UNB-01) were determined by MiSeq Illumina sequencing of samples of sweet potato plants grafted onto Ipomoea setosa. A comparative analysis of the genome organization of SPCSV-UNB-01 and other SPCSV sequences showed that RNA1 was lacking p22, and p5.1 and that p5.2. was absent in RNA2, indicating a unique genomic pattern. SPCSV-UNB-01 contained longer p6 and p5 regions, with little similarity to orthologous sequences. Sequence comparison did not reveal any previously identified functional domains within these open reading frames (ORFs). No recombination or rearrangement events were detected. Phylogenetic analysis suggested the possibility of separate entries of SPCSV into South America based on the genetic distance between SPCSV-UNB-01 and the Peruvian isolate m2-47. Samples from northeastern Brazil (State of Pernambuco) were positive for SPCSV when tested using specific primers for the major coat protein (CP) gene. This is the first full-length genome sequence of SPCSV-UNB-01 from Brazil.

Green synthesis of silver nanoparticles using tomato leaf extract and their entrapment in chitosan nanoparticles to control bacterial wilt.

BACKGROUND: Silver nanoparticles (AgNPs), particularly those entrapped in polymeric nanosystems, have arisen as options for managing plant bacterial diseases. Among the biopolymers useful for the entrapment of AgNPs, chitosan is promising because of its low cost, good biocompatibility, antimicrobial properties and biodegradability. The present study aimed: (i) to greenly-synthesize AgNPs using different concentrations of aqueous extract of tomato leaves followed by entrapment of AgNPs with chitosan (CH-AgNPs); (ii) to characterize the optical, structural and biological properties of the nanosystems produced; (iii) to evaluate the antimicrobial activities of AgNPs and nanomaterials; and (iv) to assess the effectiveness of AgNPs and nanomaterials for controlling tomato bacterial wilt caused by Ralstonia solanacearum. RESULTS: Spherical and oval AgNPs had incipient colloidal instability, although the concentration of the tomato leaf extract influenced both size (< 87 nm) and the polydispersity index. Nanomaterials (< 271 nm in size) were characterized by a highly stable matrix of chitosan containing polydisperse AgNPs. Free AgNPs and CH-AgNPs were stable for up to 30 days, with no significant alteration in physicochemical parameters. The AgNPs and nanomaterials had antibacterial activity and decreased bacterial growth at micromolar concentrations after 48 h. Morphological changes in R. solanacearum cells were observed after treatment with CH-AgNPs. The application of CH-AgNPs at 256 µmol L-1 reduced the incidence of bacterial wilt in a partially resistant tomato genotype but not in the susceptible line. CONCLUSION: Greenly-synthesized chitosan-derived nanomaterials containing AgNPs produced with leaf extracts from their own species appear to comprise a promising and sustainable alternative in an integrated management approach aiming to reduce the yield losses caused by bacterial wilt. © 2019 Society of Chemical Industry.

History and Status of Selected Hosts of the Ralstonia solanacearum Species Complex Causing Bacterial Wilt in Brazil.

Bacterial wilt induced by the Ralstonia solanacearum species complex is endemic to Brazil, where it can cause variable losses in many hosts. Its economic importance, however, cannot be precisely measured due to Brazil's continental size, subject to variable weather conditions which directly affect disease expression. The objectives of this paper were (i) to gather scattered information on historical facts; (ii) to show the current distribution of the pathogen in the country, and (iii) to comment on future trends on the importance of the disease in economically important current and potential hosts, based on the pathogen's variability and the global climate change under way.

Complete Genome Sequences of Sweet potato feathery mottle virus and Sweet potato virus G from Brazil.

In Brazil, Potyvirus species in sweet potatoes have been detected mostly by serology. Here, we report the complete genome sequences of two Potyvirus species, Sweet potato feathery mottle virus strain (SPFMV-UNB-01) and Sweet potato virus G strain (SPVG-UNB-01).