Category Archives: Journal of Phytopathology

Effect of environmental factors, fungicide sensitivity, and pathogenicity of Fusarium spp. associated with fruit rot of melon

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Abstract

Fusarium rot is one of the main postharvest diseases of melons, directly interfering with the quality and commercial use of the fruit. The purpose of this study was to assess the effect of environmental factors (temperature, pH and salinity) and the pathogenicity of four Fusarium species (F. falciforme, F. kalimantanense, F. pernambucanum and F. sulawesiense) belonging to three different species complexes on disease development on melon, as well as the in vitro and in vivo sensitivity of these species to the fungicides azoxystrobin + fludioxonil, imazalil and thiabendazole. The results showed optimum fungal growth temperatures from 24.5 to 27.2°C, and optimum pH from 6.10 to 8.37 for all Fusarium species. NaCl concentrations (250–1000 mM) significantly reduced in vitro mycelial growth for all isolates. All species were pathogenic to melon plants and fruit, with an isolate of F. falciforme being the most aggressive, causing the highest disease severity in plants and fruit (43.3% and 62.5%, respectively). The isolate of F. sulawesiense tested showed high in vitro sensitivity to two fungicides (azoxystrobin + fludioxonil and imazalil), with EC50 values below 1 mg/L of a.i. Fruit inoculated with the selected isolates of F. falciforme and F. pernambucanum showed a reduction in the disease severity on the peduncle when treated with the fungicide thiabendazole (9.0% and 4.0%, respectively). Azoxystrobin + fludioxonil was responsible for the lowest disease severity in the epicarp caused by the same isolates (0.0% and 2.5%, respectively). These results are relevant to improve management strategies for diseases of melon caused by Fusarium spp.

Powdery mildew of chayote (Sechium edule) caused by Podosphaera xanthii race 2 in Brazil

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The causal agent of chayote (Sechium edule) powdery mildew (PM) was identified in Brazil. Microscopic examinations revealed hyaline and ovoid conidia, borne in chains and displaying distinctive extra-vacuolar fibrosin bodies. Morphology agreed to the standard description of the anamorph of Podosphaera xanthii. To confirm this identification, genomic DNA was extracted from field isolates and used in PCR assays with ITS primers. Amplicons were Sanger-sequenced and the nucleotide alignments of all isolates displayed identity levels above 97% with P. xanthii isolates in GenBank database. Phylogenetic analyses indicated only P. xanthii isolates as the PM-inducing pathogens in chayote. Inoculation assays were performed using chayote, zucchini and a set of muskmelon differential cultigens for determination of the physiological races of P. xanthii. All chayote isolates were pathogenic to chayote and zucchini seedlings. Five isolates were also pathogenic to the muskmelon cultigens ‘Hale's Best Jumbo’, ‘Védrantais’, ‘PMR-45’ and ‘Edisto 47’, whereas ‘PMR-5’ was asymptomatic during the experimental period. The virulence profile of chayote-infecting isolates allowed their classification as P. xanthii race 2. To the best of our knowledge, this is the first worldwide report of P. xanthii race 2 in chayote. This report has relevant epidemiological and disease management implications, allowing more sustainable chayote cultivation under tropical and subtropical conditions.

Identification of Clonostachys rogersoniana as a causal agent of sisal leaf blight disease in South China

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A novel strain of leaf blight disease affecting sisal leaves was discovered at the Hongshan farm of Yulin City, Guangxi Province, in 2022. Ten fungal isolates were identified in the infected sisal leaves and classified into six categories (22GX3-1 to 6). Pathogenicity testing revealed that only 22GX3-2 and two morphologically similar isolates induced symptoms similar to those detected in the field. Based on morphological analysis and nucleotide sequencing of the internal transcribed spacer (ITS) of the rRNA gene, translation elongation factor 1 alpha (TEF1) and tubulin beta chain 2 (TUB2) genes, the fungal isolate 22GX3-2 was identified as Clonostachys rogersoniana. To the best of our knowledge, this is the first study reporting sisal leaf blight caused by Clonostachys rogersoniana.

Characterization and pathogenicity of Fusarium species causing sugar beet root rot in Morocco

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The sugar beet crop ranks second globally among the plant species grown mainly for sugar production. In Morocco, the area under sugar beet farming occupies approximately 57,000 ha yielding more than 3 million tons of roots. However, sugar beet root-tip rot (RTR) caused by Fusarium spp. dramatically reduces the anticipated yields, the purity of the resulting juice, and the sugar concentration. The current study aimed at identifying and characterizing the Fusarium species responsible for the root rot in sugar beet grown in the Khenifra-Beni Mellal region of Morocco. In this survey, 69 isolates of Fusarium were sampled from sugar beet roots showing typical symptoms of root rot from 2019 to 2021. After screening based on the pathogenicity test, 28 isolates were selected and identified based on morphological features and sequence analyses of the ribosomal internal transcribed spacer (ITS) region and translation elongation factor 1 α 34 (TEF-1 α). Fusarium oxysporum was the most frequently identified species, followed closely by F. solani, F. equiseti, F. nygmai, F. brachygibbosum, F. proliferatum, F. culmorum, and F. falciforme. Six weeks after inoculations under greenhouse conditions, the studied isolates caused internal vascular discoloration and tip rot of sugar beet roots, with disease incidences ranging from 37.5% to 100.0% and a disease index between 30.3% and 70.5%. Isolates belonging to F. solani were the most aggressive. Moreover, the majority of isolates significantly reduced plant growth. To our knowledge, this research article is the first report of Fusarium species inducing RTR in sugar beet in Morocco.

Improper crop rotation may enrich soil‐borne pathogens of Panax notoginseng

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Soil-borne diseases are the main cause of yield reduction of Panax notoginseng (Sanqi), and are mainly caused by the enrichment of pathogenic fungi during continuous cropping. In the Wenshan district of Yunnan province, China, where Sanqi is widely cultivated, the rotation of Foeniculum vulgare (fennel) crops with Sanqi crops is assumed to help reduce occurrences of rot in Sanqi roots. However, in a field investigation, we found that this practice actually increased incidences of root rot in Sanqi crops. Using fennel plants obtained from the cropping system, we tested the hypothesis that fennel crops enriched communities of pathogenic fungi of Sanqi. We isolated six endophytic fungi from the roots of fennel plants and identified these based on their morphological characteristics and a sequencing analysis. The isolates were identified as Fusarium oxysporum (FV-1-R-1, FV-2-R-1), Alternaria alternata (FV-3-R-1, FV-14-R-1), Pyricularia grisea (FV-8-R-1) and Colletotrichum truncatum (FV-11-R-4). In a series of inoculation experiments, we verified the pathogenicity of these fungi to Sanqi based on Koch's postulates, and demonstrated that all isolates caused root rot in Sanqi. Our results suggest that fennel is an inappropriate crop choice for rotation with Sanqi because it may serve as an intermediate host for the pathogenic fungi that cause root rot in Sanqi, and thereby exacerbate crop diseases. Our empirical findings provide useful information for enhancing the cultivation of Sanqi and the practice of crop rotation.

The presence of the plant pathogen Fusarium graminearum as a soil inoculum enhances the rhizosphere survival of bacterial biocontrol strains aimed at the pathogen

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The serious wheat pathogen Fusarium graminearum causes both root rot and head blight. Some classical biocontrol tests were first used to explore the biocontrol ability of 39 Pseudomonas fluorescens strains. The five most antifungal strains B4, P13, UTPf127, UTPf125 and UTPf105 were selected to screen known antifungal antibiotic genes and greenhouse experiments. The ability of bacteria to colonize wheat rhizosphere and their effect on plant growth in the presence and absence of soil F. graminearum inoculum was studied under greenhouse conditions. Overall, biocontrol bacteria populations were significantly higher in both wheat endo-and ectorhizosphere of pathogen-inoculated soil than in healthy soil. The population of all strains differently decreased with time. On day 28, endorhizosphere populations of strain B4 could be detected in inoculated but not healthy soil, while UTPf127 populations remained high in endorhizospheres at all tested times. Isolate B4 and UTPf105 showed the most substantial plant growth in pathogen-inoculated soil compared to pathogen-inoculated soil without added bacteria. UTPf127-treated plants grew better in control soil than when the pathogen was present. In contrast, UTPf125 and P13 showed little effect on plant growth. These results point to complex interactions between pathogen and biocontrol bacteria and suggest that a fungal pathogen in the soil can affect the survival of potential bacterial biological control agents. Additionally, they highlight the importance of screening and evaluating potential biocontrol bacteria against soilborne fungal pathogens by in vivo tests rather than relying on plate screenings.

Characterization of causal agents of bacterial canker on apricot plantations and risk mapping using GIS in Aras Basin (Türkiye)

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Bacterial canker of stone fruits caused by Pseudomonas syringae pv. syringae (Pss) and Pseudomonas syringae pv. morsprunorum (Psm race-1/Psm race-2) may lead to significant yield and crop losses in apricot (Prunus armeniaca L.) cultivation areas in Türkiye. Strains pathogenic to apricot were isolated from trees with symptoms (mainly necrotized buds and dieback) of bacterial canker in orchards in Aras Basin. Pathogens were characterized using pathogenicity tests, phenotypic assays, end-point PCR and multilocus sequence analysis (MLSA). Fifteen Pseudomonas syringae strains were isolated from 205 plant samples collected from apricot orchards showing symptoms of bacterial canker. As a consequence of the diagnostic tests, all isolates were identified as P. syringae pv. syringae. In this study, Pss, Psm R1 and Psm R2 strains in stone fruits were separated into different phylogroups (Pg-2, Pg-3, sPg-1b) based on MLSA. Turkish strains obtained from stone fruits, particularly apricot, showed genetic heterogeneity, and clustered in different sub-phylogroups (sPg-2b, sPg-2c, sPg-2d). All these strains except strain K258 are also clustered in the same sub-phylogroups (sPg-2b and sPg-2d) with other strains from different countries especially Iran, Lebanon, etc. Strain K258 isolated from apricot was clustered in sPg-2c with Pss strain 642 (USA). The risk of bacterial canker disease in apricot growing areas is considered using GIS in this study. It was determined that a significant part of the Iğdır Plain, the biggest agricultural area in the Aras Basin, is at very high risk.

Occurrence of copper‐resistant Pseudomonas syringae pv. actinidiae strains in kiwifruit orchards of Central Italy

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Bacterial isolates belonging to Pseudomonas syringae pv. actinidiae, the causal agent of kiwifruit bacterial canker, have been isolated, during 2021 and 2022, from Actinidia chinensis orchards of Central Italy planted with cultivars Hayward and Zespri G3®. A total of 23 isolates were obtained from twigs and leaves that showed the typical symptoms of bacterial canker, including leaf spotting and twig canker. All of them showed resistance to 1.2 mM copper sulphate added to mannitol–glutamate–yeast extract (MGY) medium. No other Pseudomonas syringae pv. actinidiae strains were isolated from the orchards. The P. s. pv. actinidiae strains that showed resistance to copper have the copA and copD genes that regulate the efflux of copper through the inner membrane and the copper sensor copS gene. They do not possess the cusABC complex that confers resistance to high concentration of copper. Representative copper-resistant strains were proven pathogenic to kiwifruit plants upon artificial inoculation. This is the first record of copper-resistant P. s. pv. actinidiae strains in Italy and, as far as we know, in Europe.

Lentil (Lens culinaris), a new host of Alternaria chlamydosporigena

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During a survey conducted in June 2019, chlorosis symptoms were observed on lentil leaves in a field located at Relizane, Algeria. Based on morphological and molecular identification, the pathogen was identified as Alternaria chlamydosporigena. A pathogenicity test was carried out and Koch's postulates were verified and fulfilled. To our best knowledge, this is the first report of A. chlamydosporigena as new pathogen on lentil worldwide.