Baseline Sensitivity and Control Efficacy of Pyraclostrobin Against Botryosphaeria dothidea Isolates in China

Bioactivities evaluation of an endophytic bacterial strain Bacillus tequilensis QNF2 inhibiting apple ring rot caused by Botryosphaeria dothidea on postharvest apple fruits

Apple ring rot, one of the most common apple postharvest diseases during storage, is caused by Botryosphaeria dothidea. Presently, the disease management is primarily dependent on chemical fungicide application. Here we demonstrated an endophyte bacterium Bacillus tequilensis QNF2, isolated from Chinese leek (Allium tuberosum) roots considerably suppressed B. dothidea mycelial growth, with the highest suppression of 73.56 % and 99.5 % in the PDA and PDB medium, respectively in vitro confront experiments. In in vivo experiments, B. tequilensis QNF2 exhibited a control efficacy of 88.52 % and 100 % on ring rot disease on postharvest apple fruits inoculated with B. dothidea disc and dipped into B. dothidea culture, respectively. In addition, B. tequilensis QNF2 volatile organic compounds (VOCs) also manifested markedly inhibition against B. dothidea mycelial growth and the ring rot on postharvest apple fruits. Moreover, B. tequilensis QNF2 severely damaged the mycelial morphology of B. dothidea. Finally, B. tequilensis QNF2 significantly repressed the expression of six pathogenicity-related genes, such as adh, aldh, aldh3, galm, pdc1, pdc2, involved in glycolysis/gluconeogenesis of B. dothidea. The findings of the study proved that B. tequilensis QNF2 was a promising alternative for controlling apple ring rot of postharvest apple fruit.

Exploring the Phylogenetic Diversity of Botryosphaeriaceae and Diaporthe Species Causing Dieback and Shoot Blight of Blueberry in Serbia

Identifying the precise pathogens responsible for specific plant diseases is imperative for implementing targeted and efficient interventions and mitigating their spread. Dieback and shoot blight significantly diminish the lifespan and productivity of blueberries, yet the causative agents remain largely unidentified. To determine the identity and prevalence of the causal agents of branch dieback and shoot blight, we conducted multiyear and multisite sampling of diseased highbush blueberries (Vaccinium corymbosum cultivar Duke) in Serbia. Sixty-nine monosporic isolates were collected and characterized based on morphological, physiological features and multilocus phylogenetic analysis of internal transcribed spacer, β-tubulin, and translation elongation factor 1-α sequence data. Five species were identified as causal agents: Diaporthe eres (36 isolates), D. foeniculina (3 isolates), Neopestalotiopsis vaccinii (9 isolates), Neopestalotiopsis rosae (6 isolates), and Neofusicoccum parvum (15 isolates). The results of the pathogenicity tests performed with the 23 representative isolates confirmed the role of these species as primary pathogens in causing dieback and shoot blight of blueberry, with Neofusicoccum parvum being the most aggressive and D. eres the least. Our study underscores the diversity of genera and species of ascomycetes capable of causing blueberry dieback and shoot blight. Furthermore, our findings indicate that the agents responsible for the disease in Serbia differ from those identified in other regions worldwide.

The Mitogen-Activated Protein Kinase Hog1 Regulates Fungal Development, Pathogenicity, and Stress Response in Botryosphaeria dothidea

The high-osmolarity glycerol mitogen-activated protein kinase (HOG-MAPK) pathway plays a central role in environmental stress adaptation in eukaryotes. However, the biological function of the HOG-MAPK pathway varies in different fungi. In this study, we investigated the HOG-MAPK pathway by inactivation of the core element Hog1 in Botryosphaeria dothidea, the causal agent of Botryosphaeria canker and apple ring rot. Targeted deletion of BdHOG1 resulted in the loss of conidiation ability and significant reduction of virulence. In addition, the ΔBdHog1 mutant exhibited hypersensitivity to osmotic stress but resistance to phenylpyrrole and dicarboximide fungicides. Comparative transcriptome analysis revealed that inactivation of BdHog1 influenced multiple metabolic pathways in B. dothidea. Taken together, our results suggest that BdHog1 plays a crucial role in development, virulence, and stress tolerance in B. dothidea, which provides a theoretical basis for the development of target-based fungicides.

Molecular characterization of a novel ourmia‑like virus from the phytopathogenic fungus Botryosphaeria dothidea

Here, we describe a novel ourmia-like virus, Botryosphaeria dothidea ourmia-like virus 2 (BdOLV2), derived from the phytopathogenic fungus Botryosphaeria dothidea strain ZM180192-1 infecting maize in Henan province of China. The complete genome sequence of BdOLV2 consists of a positive-sense single-stranded RNA (+ ssRNA) segment with a length of 2,532 nucleotides (nt). The sequence contains a large open reading frame (ORF) encoding a putative RNA-dependent RNA polymerase (RdRp) consisting of 605 amino acids (aa) with a molecular mass of 68.59 kDa. This RdRp protein contains eight typical conserved motifs associated with ourmia-like viruses. BLASTp analysis revealed that the RdRp protein of BdOLV2 had the highest similarity (62.10%, 58.15%, and 55.75% identity, respectively) to a virus previously identified as "Botourmiaviridae sp.", Macrophomina phaseolina ourmia-like virus 2, and Macrophomina phaseolina ourmia-like virus 2-A. Phylogenetic analysis based on the RdRp aa sequence indicated that BdOLV2 is a new member of the genus Magoulivirus in the family Botourmiaviridae.

Endophytic bacterium Pseudomonas protegens suppresses mycelial growth of Botryosphaeria dothidea and decreases its pathogenicity to postharvest fruits

Apple (Malus domestica Borkh.), one of the most economically important fruits widely consumed worldwide, has been suffering from apple ring rot caused by Botryosphaeria dothidea, which dramatically affects its quality and yield. In the present study, we demonstrated that Pseudomonas protegens, isolated from Chinese leek (Allium tuberosum), significantly suppressed the mycelial growth and propagation of B. dothidea, respectively, further displayed a considerably inhibitory effect on the apple ring rot of postharvest fruits. In addition, P. protegens significantly improved the total soluble solid/titrable acidity (TSS/TA) ratio and soluble sugar/titrable acidity (SS/TA) ratio and drastically maintained the fruit firmness. Further analysis manifested that P. protegens substantially induced the defense-related genes such as MdGLU, MdPAL, MdPOD, MdCAL, and transcription factors related to the resistance to B. dothidea, including MdWRKY15, MdPUB29, MdMyb73, and MdERF11 in apple fruits. Meanwhile, P. protegens considerably restrained the expressions of the pathogenicity-related genes in B. dothidea, including the BdCYP450, BdADH, BdGHY, BdATS, Bdα/β-HY, and BdSTR. By inference, P. protegens inhibited the apple ring rot on postharvest fruits by activating the defense system of apple fruit and repressing the pathogenic factor of B. dothidea. The study provided a theoretical basis and a potential alternative to manage the apple ring rot on postharvest fruits.

Antifungal Activity and Possible Mechanism of Bacillus amyloliquefaciens FX2 Against the Postharvest Apple Ring Rot Pathogen

Botryosphaeria dothidea-induced apple ring rot is one of the most serious postharvest diseases in apple production. In our preliminary work, we isolated a bacterial strain (FX2) from an infested apple orchard. Here, we confirmed the strong antifungal activity of FX2 on B. dothidea. Through phylogenetic analysis and morphological observations, we identified FX2 as a Bacillus amyloliquefaciens strain. We also found that 10% cell-free supernatant (CFS) of FX2 significantly affected mycelial growth and morphology and almost completely inhibited spore germination and germ tube elongation in B. dothidea. Furthermore, 10% CFS damaged the cell ultrastructure, resulting in a remarkable increase in cellular leakage in B. dothidea mycelia. Thus, CFS has the potential to effectively reduce in vivo B. dothidea infection, reduced lesion diameters to 64.7% compared with the control group, and reduced disease incidence by 15%. Finally, ultrafiltration, desalting chromatography, and anion exchange chromatography showed that the antifungal constituents in CFS are composed mainly of antifungal proteins. We further characterized these potential antifungal proteins via liquid chromatography-tandem mass spectrometry (LC-MS/MS) analysis. Herein, we provide novel insights into the antifungal mechanisms of B. amyloliquefaciens FX2, and we highlight its potential as a novel biocontrol agent for controlling postharvest apple ring rot.

Reduced Sensitivity to Tebuconazole in Botryosphaeria dothidea Isolates Collected from Major Apple Production Areas of China

China has the largest acreage and is the greatest producer of apples in the world. Apple ring rot (ARR) caused by Botryosphaeria dothidea is one of the major diseases affecting apple fruit production. Tebuconazole, a sterol demethylation-inhibiting (DMI) fungicide, has been increasingly applied to manage ARR in China. Resistance to tebuconazole in field populations of B. dothidea may be selected and accumulate to higher degrees over time. Establishment of tebuconazole sensitivity monitoring programs is critically important for resistance management and effective ARR control. However, the resistance level of B. dothidea to tebuconazole in China remains largely unknown. In this study, in vitro mycelial growth assays of B. dothidea in media amended with tebuconazole were conducted, and the sensitivity of B. dothidea to tebuconazole was determined with a set of 390 isolates collected from the major apple production provinces in China between 2006 and 2014. Results showed that the 50% effective concentration (EC₅₀) value ranged from 0.011 to 0.918, 0.040 to 1.621, and 0.052 to 1.925 μg ml^-1 with a median value of 0.194, 0.386, and 0.782 μg ml^-1 in the isolates collected in 2006, 2010, and 2014, respectively. The frequency distribution of EC₅₀ for tebuconazole was a nonnormal distribution (P < 0.05), suggesting that subpopulations with reduced sensitivity to tebuconazole had emerged in these B. dothidea isolates. The frequency distribution of the B. dothidea isolates collected in 2006 fit a unimodal curve and could be regarded as the baseline sensitivity to tebuconazole. The resistance levels increased over time with the average occurrence frequency of 43.3% and resistance index of 0.38. Positive cross-resistance was observed between tebuconazole and metconazole, which is another DMI fungicide, but multiple resistance was not detected between tebuconazole and non-DMI fungicides. Our results demonstrated that regular long-term resistance monitoring combining with prudent fungicide use should be implemented to prolong the lifespan of tebuconazole in management of ARR in apples.

The heat shock protein 20 gene editing suppresses mycelial growth of Botryosphaeria dothidea and decreases its pathogenicity to postharvest apple fruits

Apple ring rot caused by Botryosphaeria dothidea is an essential and prevalent disease in the apple orchard in China. Our previous study demonstrated that dimethyl trisulfide (DT) from Chinese leek (Allium tuberosum) significantly suppressed the mycelial growth of B. dothidea and inhibited the incidence of apple ring rot postharvest. However, the mechanism underlying the inhibitory role of DT against B. dothidea is not fully understood. Comparing the control and the DT-treated B. dothidea mycelial transcriptomes revealed that heat shock protein 20 (Hsp20) strongly responded to DT treatment. This study identified four Hsp20 genes throughout the B. dothidea genome (BdHsp20_1-4). Each BdHsp20 gene had a conserved ACD with a variable N-terminal region and a short C-terminal extension. The segmental duplication event has contributed to the expansion of the BdHsp20 gene family. Compared to the wild-type strain, the CRISPR/Cas9 gene-edited BdHsp20 mutant (ΔBdHsp20) decreased the mycelial growth by 55.95% and reduced the disease symptom in postharvest apple fruit by 96.34%. However, the BdHsp20 complemented strain (ΔBdHsp20_C) significantly restored the growth and pathogenicity, which suggested that the BdHsp20 gene was closely involved in the growth and pathogenicity of B. dothidea. This study would accelerate the exploration of the molecular mechanism of the inhibitory effect of DT against B. dothidea and also provide new insights for the management of apple ring rot disease.

Efficacy of Dimethyl Trisulfide on the Suppression of Ring Rot Disease Caused by Botryosphaeria dothidea and Induction of Defense-Related Genes on Apple Fruits

Apple ring rot caused by Botryosphaeria dothidea is prevalent in main apple-producing areas in China, bringing substantial economic losses to the growers. In the present study, we demonstrated the inhibitory effect of dimethyl trisulfide (DT), one of the main activity components identified in Chinese leek (Allium tuberosum) volatile, on the apple ring rot on postharvest fruits. In in vitro experiment, 250 μL/L DT completely suppressed the mycelia growth of B. dothidea. In in vivo experiment, 15.63 μL/L DT showed 97% inhibition against the apple ring rot on postharvest fruit. In addition, the soluble sugar content, vitamin C content, and the soluble sugar/titratable acidity ratio of the DT-treated fruit were significantly higher than those of the control fruit. On this basis, we further explored the preliminary underlying mechanism. Microscopic observation revealed that DT seriously disrupted the normal morphology of B. dothidea. qRT-PCR determination showed the defense-related genes in DT-treated fruit were higher than those in the control fruit by 4.13–296.50 times, which showed that DT inhibited apple ring rot on postharvest fruit by suppressing the growth of B. dothidea, and inducing the defense-related genes in apple fruit. The findings of this study provided an efficient, safe, and environment-friendly alternative to control the apple ring rot on apple fruit.

Occurrence and Detection of Carbendazim Resistance in Botryosphaeria dothidea from Apple Orchards in China

Botryosphaeria dothidea causes white rot, which is among the most devastating diseases affecting apple crops globally. In this study, we assessed B. dothidea resistance to carbendazim by collecting samples from warts on the infected branches of apple trees or from fruits exhibiting evidence of white rot. All samples were collected from different orchards in nine provinces of China in 2018 and 2019. In total, 440 B. dothidea isolates were evaluated, of which 19 isolates from three provinces were found to exhibit carbendazim resistance. We additionally explored the fitness and resistance stability of these isolates, revealing that they were no less fit than carbendazim-sensitive isolates in terms of pathogenicity, sporulation, and mycelial growth and that the observed carbendazim resistance was stable. Sequencing of the β-tubulin gene in carbendazim-resistant isolates showed the presence of a substitution at codon 198 (GAG to GCG) that results in an alanine substitution in place of glutamic acid (E198A) in all 19 resistant isolates. A loop-mediated isothermal amplification (LAMP) method was then developed to rapidly and specifically identify this E198A mutation. This LAMP method offers value as a tool for rapidly detecting carbendazim-resistant isolates bearing this E198A mutation and can thus be used for the widespread monitoring of apple crops to detect and control the development of such resistance.

Butylated Hydroxytoluene Induced Resistance Against Botryosphaeria dothidea in Apple Fruit

Apple ring rot caused by Botryosphaeria dothidea is an important disease in China, which leads to serious economic losses during storage. Plant activators are compounds that induce resistance against pathogen infection and are considered as a promising alternative strategy to traditional chemical treatment. In the present study, butylated hydroxytoluene (BHT), a potential plant activator, was evaluated for its induced resistance against B. dothidea in postharvest apple fruits. The physiological and molecular mechanisms involved in induced resistance were also explored. The results showed that BHT treatment could trigger strong resistance in apple fruits against B. dothidea, and the optimum concentration was 200 μmol L^–1 by immersion of fruits. BHT treatment significantly increased the activities of four defensive enzymes and alleviated lipid peroxidation by increasing antioxidant enzyme activities. In addition, salicylic acid (SA) content was enhanced by BHT treatment as well as the expression of three SA biosynthesis-related genes (MdSID2, MdPAD4, and MdEDS1) and two defense genes (MdPR1 and MdPR5). Our results suggest that BHT-conferred resistance against B. dothidea might be mainly through increasing the activities of defense-related enzymes and activating SA signaling pathway, which may provide an alternative strategy to control apple ring rot in postharvest fruits.

Residue analysis and kinetics modeling of thiophanate-methyl, carbendazim, tebuconazole and pyraclostrobin in apple tree bark using QuEChERS/HPLC-VWD

Winter is the key period for the control of apple diseases, and fungicides are needed to protect the trunk or main branches. Fungicide residue in apple tree bark is an important basis for the action of the pesticide, but there are no reports on analytical methods or dissipation patterns. In this work, thiophanate-methyl, carbendazim, tebuconazole and pyraclostrobin were selected as typical fungicides and a new QuEChERS-HPLC-VWD(QuEChERS extraction followed by high-performance liquid chromatography detection with a variable wavelength detector) analytical method was developed to estimate their residue kinetics in apple tree bark during the winter months. In the pretreatment step, the sorbent for the clean-up of extracts was optimized as 60 mg/ml primary secondary amine and a gradient-elution model followed by a variable wavelength detection was developed for instrumental analysis. Then this method was validated and applied to the analysis of apple tree bark samples with the linearity range of 0.010-50.00 mg/L, quantification limit range of 0.028-0.080 mg/kg and recovery range of 86.1-101.4%. The dissipation kinetics of thiophanate-methyl and pyraclostrobin could be described by the first-order and two-phase kinetics models, respectively. For carbendazim and tebuconazole, two new models were developed to describe their residue kinetics.

Evaluating the Sensitivities and Efficacies of Fungicides with Different Modes of Action Against Phomopsis asparagi

Asparagus stem blight caused by Phomopsis asparagi is a major hindrance to asparagus production worldwide. Currently, fungicides are used to manage the disease in commercial production, but resistance to common fungicides has emerged in the wild population. In the present study, 132 isolates of P. asparagi collected from different provinces in China were tested for sensitivities to pyraclostrobin, tebuconazole, and fluazinam. We also determined the efficacies of six fungicides against P. asparagi. The frequency distributions of EC₅₀ values of the isolates tested were unimodal, but the curves for pyraclostrobin and tebuconazole had long right-hand tails. The mean EC₅₀ values for pyraclostrobin, tebuconazole, and fluazinam were 0.0426 ± 0.0029, 0.6041 ± 0.0416, and 0.0314 ± 0.0013 μg/ml, respectively. In addition, the EC₅₀ values for pyraclostrobin were very similar with or without salicylhydroxamic acid (SHAM), 20 μg/ml, indicating that SHAM is not needed to determine the sensitivity of P. asparagi to pyraclostrobin when using the mycelial growth inhibition assay. In greenhouse assays, Merivon (42.4% fluxapyroxad plus pyraclostrobin SC), Frown-cide (500 g/liter fluazinam SC), Cabrio (250 g/liter pyraclostrobin EC), and Nativo (75% trifloxystrobin plus tebuconazole WG) showed excellent preventive efficacy against P. asparagi. And these fungicides were more effective before inoculation than when they were applied after inoculation (P < 0.05). Therefore, these fungicides should be applied prior to infection to control stem blight. In field trials, Frown-cide, Merivon, Nativo, and Cabrio also performed good control effects, ranging from 75.2 to 86.0% in 2017 and 75.4 to 87.1% in 2018. We demonstrated that Frown-cide, Merivon, Nativo, and Cabrio had considerable potential to manage asparagus stem blight. In addition, rotations of these fungicides are essential for precluding or delaying the development of resistance and for controlling the disease.