Using Plant-Based Preparations to Protect Common Bean against Halo Blight Disease: The Potential of Nettle to Trigger the Immune System

Halo blight disease of beans (Phaseolus vulgaris L.), caused by the bacterium Pseudomonas syringae pv. phaseolicola (Pph), is responsible for severe losses in crop production worldwide. As the current agronomic techniques used are not effective, it is necessary to search for new ones which may prevent disease in common bean. In this study, we challenged four plant-based preparations (PBPs), with no other agronomic uses, as they come from industrial waste (grapevine pomace (RG) and hop residue (RH)) or wild plants (Urtica dioica (U) and Equisetum sp. (E)), to be used as immune defense elicitors against Pph in common bean. After studying their inhibitory effect against Pph growth by bioassays, the two most effective PBPs (RG and U) were applied in common bean plants. By measuring the total H2O2, lipid peroxidation, and antioxidant enzymatic activities, as well as the expression of six defense-related genes—PR1, WRKY33, MAPKK, RIN4, and PAL1, it was observed that U-PBP application involved a signaling redox process and the overexpression of all genes, mostly PR1. First infection trials in vitro suggested that the application of U-PBP involved protection against Pph. The elicitation of bean defense with U-PBP involved a decrease in some yield parameters, but without affecting the final production. All these findings suggest a future use of U-PBP to diminish halo blight disease.

First report of halo blight of hop (Humulus lupulus) caused by Diaporthe humulicola in Quebec, Canada

Halo blight of hop caused by Diaporthe humulicola has recently been reported in Michigan and Connecticut (Higgins et al. 2021, Allan-Perkins et al 2020). In August 2020 growers in Quebec, Canada reported necrotic foliar lesions and desiccation of the hop strobile (cone) on Chinook and Nugget cultivars. The foliar lesions were dry concentric circles with a chlorotic halo surrounding the lesions; no pycnidia were observed on leaves or cones. Up to 100% of the infected bract tissue was dry and easily shattered, the grower estimated that more than 90% of the plants in the hopyard exhibited symptoms. Twenty-six isolates were obtained from surface-sterilized leaf and cone tissue by plating the leading edge of lesions on potato dextrose agar. Fungal isolates were hyphal tipped and were incubated at 22°C with a 12 h photoperiod. After 21-days, all cultures were white to beige with pycnidia. DNA was extracted from cultures using the MagMAX Plant DNA Isolation Kit (Applied Biosystems, Foster City, CA). DNA amplification of a representative isolate (CD6C) was performed with primers ITS1/ITS4 (White et al. 1990) for the internal transcribed spacer (ITS), CYLH3F/H3-1b (Glass and Donaldson 1995) for histone 3 (HIS), and Ef1728f/EF1-986R (Carbone and Kohn 1999) for translation elongation factor 1-α (TEF). Amplification primers were used for bidirectional Sanger sequencing, reads were assembled using Geneious Prime (Biomatters, New Zealand), and identified using NCBI BLAST. BLAST results showed that the sequences for TEF, ITS, and HIS all had 100% pairwise identity to Diaporthe sp. 1-MI (MT909101, MT909099, MT909093, OK001342, MZ934713, OK001341). Futhermore, BLAST results showed that ITS and HIS have 100% pairwise identity D. humulicola (MN152929, MN180214). The TEF sequence also had 99.7% pairwise identity to D. humulicola (MN180209). Koch’s postulates were conducted by inoculating six 3-mo-old ‘Chinook’ plants with conidia harvested from 28-day-old cultures and spraying 50 ml of inoculum (6 x 105 conidia/ml) or water to each plant. Plants were then stored in a greenhouse at 100% relative humidity at 22°C with a 14-h photo period. Lesions appeared on the adaxial side of the leaf after 21 days. D. humulicola was re-isolated from all infected leaf tissue, but not from any water inoculated plants and identified by conidial morphology using descriptions from Higgins et al. (2021). So far, Diaporthe sp. 1-MI appears to be synonymous with Diaporthe humulicola, but currently two names are being utilized (i.e. Diaporthe leaf spot and halo blight). In Higgins et al., (2021) it was proposed that the name halo blight might be more appropriate because disease symptoms are not confined to the leaves and cause significant blighting of cones. Halo blight caused by D. humulicola appears widespread in Michigan and Canada and may become an issue in other eastern North American growing regions with humid conditions.

Molecular Characterization and Taxonomic Assignment of three Phage Isolates from a Collection Infecting Pseudomonas syringae pv. actinidiae and P. syringae pv. phaseolicola from Northern Italy

Bacterial kiwifruit vine disease (Pseudomonas syringae pv. actinidiae, Psa) and halo blight of bean (P. syringae pv. phaseolicola, Pph) are routinely treated with copper, leading to environmental pollution and bacterial copper resistance. An alternative sustainable control method could be based on bacteriophages, as phage biocontrol offers high specificity and does not result in the spread of toxic residues into the environment or the food chain. In this research, specific phages suitable for phage-based biocontrol strategies effective against Psa and Pph were isolated and characterized. In total, sixteen lytic Pph phage isolates and seven lytic Psa phage isolates were isolated from soil in Piedmont and Veneto in northern Italy. Genome characterization of fifteen selected phages revealed that the isolated Pph phages were highly similar and could be considered as isolates of a novel species, whereas the isolated Psa phages grouped into four distinct clades, two of which represent putative novel species. No lysogeny-, virulence- or toxin-related genes were found in four phages, making them suitable for potential biocontrol purposes. A partial biological characterization including a host range analysis was performed on a representative subset of these isolates. This analysis was a prerequisite to assess their efficacy in greenhouse and in field trials, using different delivery strategies.

Chemical Composition, Production of Secondary Metabolites and Antioxidant Activity in Coffee Cultivars Susceptible and Partially Resistant to Bacterial Halo Blight

Coffee production is one of the main agricultural activities in Brazil, and several coffee cultivars with disease resistance have already been developed. The secondary metabolites produced by plants are closely associated with defense strategies, and the resistance of coffee cultivars to bacterial halo blight (BHB) can be related to these compounds. Therefore, this study aims to compare a partially resistant coffee cultivar (Iapar-59) and a susceptible cultivar (Mundo Novo 376/4) to BHB (Pseudomonas syringae pv. garcae) in relation to the chemical composition and antioxidant activity of the leaf extracts. In addition, this study determined the total phenolic and flavonoid contents and phenolic profiles of the Iapar-59 leaf extracts of plants inoculated with P. syringae pv. garcae. The Iapar-59 extract showed a higher content of phenolic compounds and flavonoids than the Mundo Novo 376/4 extract. Both cultivars contained gallic, chlorogenic and caffeic acids; however, the highest contents were quantified in the Iapar-59 cultivar. The leaf extracts from the Iapar-59 cultivar exhibited higher antioxidant activity. Higher concentrations of gallic, caffeic and chlorogenic acids and the presence of vanillin were detected in the extract of cultivar Iapar-59 inoculated with P. syringae pv. garcae.

Immune Priming Triggers Cell Wall Remodeling and Increased Resistance to Halo Blight Disease in Common Bean

The cell wall (CW) is a dynamic structure extensively remodeled during plant growth and under stress conditions, however little is known about its roles during the immune system priming, especially in crops. In order to shed light on such a process, we used the Phaseolus vulgaris-Pseudomonas syringae (Pph) pathosystem and the immune priming capacity of 2,6-dichloroisonicotinic acid (INA). In the first instance we confirmed that INA-pretreated plants were more resistant to Pph, which was in line with the enhanced production of H2O2 of the primed plants after elicitation with the peptide flg22. Thereafter, CWs from plants subjected to the different treatments (non- or Pph-inoculated on non- or INA-pretreated plants) were isolated to study their composition and properties. As a result, the Pph inoculation modified the bean CW to some extent, mostly the pectic component, but the CW was as vulnerable to enzymatic hydrolysis as in the case of non-inoculated plants. By contrast, the INA priming triggered a pronounced CW remodeling, both on the cellulosic and non-cellulosic polysaccharides, and CW proteins, which resulted in a CW that was more resistant to enzymatic hydrolysis. In conclusion, the increased bean resistance against Pph produced by INA priming can be explained, at least partially, by a drastic CW remodeling.

Lines derived from coffee BA-10 with resistance to Pseudomonas syringae under field conditions with simultaneous natural infections of the pathovars garcae and tabaci

Bacterial-halo-blight and bacterial-leaf-spot are important coffee diseases caused by Pseudomonas syringae pv. garcae and pv. tabaci, respectively. The most suitable method to control these diseases is the use of resistant cultivars. There are no studies on resistance to Pseudomonas syringae (PS) in coffee derived from BA-10 genotypes. Therefore, the aim of this study was to evaluate the resistance to PS in lines derived from BA-10 under field conditions with simultaneous natural infections of the pathovars garcae and tabaci. 38 F4 and two F5 lines derived from BA-10 were evaluated in a field trial in Londrina, PR, Brazil. The Catuaí Vermelho IAC 81 and IAPAR 59 were the susceptible and intermediate resistant controls, respectively. Resistance to PS was evaluated in January 2017 after 45 months of planting. The grading scale varied from 1 to 5, where grade 1 was plants with more resistance and 5 plants more susceptible. Two F5 lines showed 100% of resistant plants (grades 1 and 2) and the F4 line IAPAR 12201 showed 60% of plants with a high level of resistance to PS (grade 1), while the control Catuaí showed no resistant plant. IAPAR 59 and several F4 lines showed high frequency of plants with intermediate resistance to PS

Halo Blight of Mungbean in Australia

Halo blight, one of the major diseases of mungbean, is caused by the bacterium Pseudomonas savastanoi pv. phaseolicola. The pathogen infects the foliar parts of the plant, causing water-soaked spots that eventually develop surrounding yellow margins. The disease is particularly destructive under moderate temperature and high humidity, especially when it occurs during late vegetative through to early reproductive stage. In such conditions, severely infected crops could experience a yield loss up to 70%. Halo blight can be widespread on mungbeans grown in Southern Queensland and Northern New South Wales. However, due to its seedborne and cryptic nature of transmission, the disease is likely to be under-reported. This report addresses major aspects of halo blight symptomology, pathology and epidemiology.

Management of bacterial halo blight and other coffee crop diseases

Bacterial halo blight (BHB), caused by Pseudomonas syringae pv. garcae, has reemerged as an important disease in Brazil, especially in coffee cultivated at high altitude in the states of Minas Gerais and São Paulo. In this study we evaluated copper-based antimicrobial compounds (CBACs), the antibiotic kasugamycin and the resistant inducers acibenzolar-S-methyl (ASM) and phosphite for BHB management in four experiments carried out in coffee crops in the municipalities of Caconde and Altinópolis, São Paulo State, Brazil. Fungicides to control brown leaf spot (BLS), a disease caused by Boeremia exigua pv. coffea, were also included in two experiments, because both diseases frequently occur simultaneously. Copper oxychloride, copper hydroxide, antibiotic, and ASM mixture with copper hydroxide, and phosphite reduced BHB incidence and had no phytotoxic effects on flowers or pin-head berries. Mixtures of boscalid or pyraclostrobin with copper hydroxide were compatible and effective for the simultaneous control of BHB and BLS. In this study, we showed that August-September is the most important period to control BHB in Brazil and lasts until December, when disease incidence increases, and flowers and pin-head berries are being formed.

Ethiopian coffee germplasm is a valuable resistance gene pool to brazilian Pseudomonas syringae PVS garcae and tabaci

ABSTRACT Seven wild accessions of Coffea arabica from Ethiopia prospected by FAO Coffee Mission 1964-1965 were investigated concerning the resistance to 18 Brazilian strains and two Kenyan strains of Pseudomonas syringae pv. garcae and four P. syringae pv. tabaci strains, causal agents of bacterial halo blight and bacterial leaf spot, respectively. The cultivars of C. arabica IPR 102, resistant to the diseases, and Mundo Novo IAC 376-4, susceptible, were used as experimental controls. Our results indicated that the Ethiopian accessions presented high levels of resistance to all Brazilian strains of P. syringae pv. garcae but were susceptible to infection caused by Kenyan strains, which causes different levels of severity in wild accessions and experimental controls. Ethiopian accessions were also considered resistant to the four P. syringae pv. tabaci strains, with low susceptibility observed, one point on the severity scale, in access E-268 in response to a strain of the bacterium.