First report of canker caused by Cytospora sorbicola on Sweet Cherry (Prunus avium) in Chile

Chile leads cherry exports in the southern hemisphere with a total of 415.315 t exported in the 2022 to 2023 season (IQonsulting, 2023). Cytospora canker, produced by Cytospora spp., causes destructive infections and limit the productivity of sweet cherry orchards (Luo et al. 2019). This study was focused on isolating Cytospora strains to identify and characterize the species present in sweet cherry. During the period 2019-2022, ten samples of stem or branch presenting canker, dieback, gummosis or dead buds, were collected from sweet cherry cultivars 'Skeena', 'Lapins', 'Santina', 'Sweetheart', and 'Regina', in the regions Ñuble and O'Higgins, Chile. Five mm pieces from the necrotic wood margins of the samples were rinsed with sterile deionized water, placed on potato dextrose agar (PDA, Difco) and incubated at 20±2 ºC for 5 days. One isolate was recovered from each sample, resulting in ten Cytospora-like strains. Single hyphal tips were transferred onto PDA plates and all isolates were deposited in the Chilean Collection of Microbial Genetic Resources (CChRGM). Colonies grown on PDA reached 89 mm in diameter in 10 d at 25 °C, showing irregular margin, lacking aerial mycelium, initially off-white to cream that turned greenish gray in the center, which darkens with age. After 20 days of culturing on pine needle agar (Chen et al. 2015), isolates produced conidiomata pycnidial, semi-immersed, black, and subglobose (362)445-555(681)×(357)528-700(1053) µm (n=10), generating amber slimy conidia masses; Conidiophores were phialidic, cylindrical, aseptate, hyaline (6.77)9-10.04(12.88)×(0.82)1.1-1.28(1.99) µm (n = 30); conidia were abundant, allantoid, hyaline to light brown, aseptate (3.39)4.28-4.57(5.36)×(0.69)0.96-1.09(1.47) µm (n = 30) (Supplementary Figure 1). No sexual morph was observed. With the exception of the strain RGM 3390, all the isolates shared morphological characters to the descriptions of Cytospora sorbicola Norphanph., Bulgakov, T. C. Wen & K. D. Hyde (Norphanphoun et al. 2017). Isolates were identified at species level, by sequencing DNA regions described by Pan et al. (2020): ITS1-5.8S-ITS2, LSU; act, tef-1α, and tub2 with the exception of the RBP2, because this region could not be amplified in seven out of ten isolates. The consensus tree included the concatenated sequences of the ten isolates and those of reference Cytospora species reported by Ilyukhin et al. (2023) using a maximum likelihood analysis with the tool IQ-TREE webserver. MLSA confirmed the taxonomic affiliation of nine of the isolates with C. sorbicola and one isolate with Cytospora sp. (RGM 3390), that might represent a novel species (Supplementary Figure 2). The isolates RGM 3399 and RGM 3400, were selected randomly for pathogenicity tests. Inoculations were performed on 2-year-old sweet cherry cv. 'Lapins' grow in pots in a greenhouse at 26±6°C. Seven plants per isolate were cut to about 6-cm length from the main stem, and inoculated onto fresh cuts with 5-mm mycelium PDA plugs of 5-d-old culture and wrapped in moist sterile cotton and parafilm to keep moisture. Six plants were inoculated with non-colonized PDA agar plugs as control. The average canker length 3 months after inoculation was 3.1 and 0.8 cm, for RGM 3389 and RGM 3400, respectively (Supplementary Figure 1). Symptomatic twigs were incubated in moist chambers at 20±2 ºC for 10 d, resulting in the re-isolation of Cytospora strains that produced pycnidia and conidia structures in agreement with C. sorbicola. Both strains were reidentified to fulfill Koch's postulates, control twigs remained asymptomatic and no fungus was isolated from these twigs. This is the first report of C. sorbicola causing canker on sweet cherry in Chile. Our findings suggest that this species could be the most recurrent in cherry in central Chile, coinciding with it found in California where C. sorbicola has been described as the main causal agent of Cytospora canker of stone fruits in California (Lawrence et al. 2018).

Silicon inhibits gummosis by promoting polyamine synthesis and repressing ethylene biosynthesis in peach

Silicon is a beneficial element for plant growth, as well as for improving plant resistance to multiple biotic and abiotic stresses. Gummosis is a common harmful disease in peach and is induced by many factors. However, the effect of silicon on gummosis of peach has not been determined yet. In this study, we reported that application of silicon significantly reduced gummosis by regulating biosynthesis of ethylene and polyamines in peach. Ethylene promoted the development of gummosis by inducing the expression of genes encoding cell wall degrading enzymes. While application of different types of polyamines, including spermidine and spermine, dramatically inhibited the occurrence of gummosis. Moreover, polyamines inhibited the ethylene biosynthesis by down-regulating expression of ethylene biosynthetic gene PpACS1 (1-aminocyclopropane -1-carboxylic acid synthase), as well as the enzymatic activity of ACS. We further found that application of silicon significantly restricted the development of gummosis in peach. Exogenous silicon dramatically inhibited expression of PpACS1 and the enzymatic activity of its product to reduce ethylene biosynthesis. Simultaneously, the activity of S-adenosylmethionine decarboxylase, a key enzyme in ployamines biosynthesis, was increased by 9.85% under silicon treatment, resulting in elevated accumulation of polyamines. Thus, our data proved that application of silicon restricted gummosis development by activating ployamines biosynthesis and inhibiting ethylene synthesis in peach.

Management of Phytophthora parasitica causing gummosis in citrus using biogenic copper oxide nanoparticles

AIM

The main aim of the present study is to develop nanotechnology-based solutions for the management of a fungus, Phytophthora parasitica causing gummosis in citrus.

METHODS AND RESULTS

Biogenic copper nanoparticles (CuONPs) were synthesized using two different biocontrol agents, Pseudomonas fluorescens and Trichoderma viride and characterized using different analytical techniques. Further, in vitro (at the concentrations of 10 mg/L, 15 mg/L, 30 mg/L, 50 mg/L, 70 mg/L, 100 mg/L and 150 mg/L) and in vivo (at the concentration of 100 mg/L) activities of these nanoparticles were evaluated for their antifungal efficacy against P. parasitica. The results obtained confirmed the synthesis of irregular shaped CuONPs having a size in the range of 40-100 nm in case of P. fluorescens, whereas, spherical CuONPs in the size range of 20-80 were recorded in case of T. viride. As far as the in vitro antifungal efficacies of both these CuONPs is concerned, the maximum percent growth inhibition was observed in case of CuONPs synthesized from T. viride compared to CuONPs from P. fluorescens. However, in case of in vivo antifungal efficacies, CuONPs synthesized from T. viride showed the activity significantly higher than the conventionally used Bordeaux mixture.

CONCLUSIONS

It can be concluded that biosynthesized CuONPs can be effectively used as a potential fungicide against P. parasitica.

SIGNIFICANCE AND IMPACT OF THE STUDY

The application of nanoparticles having antifungal activities can be used as alternative fungicides to the conventional chemical fungicides. It has the potential to revolutionize the existing management strategies available for plant pathogenic fungi.

Rathayibacter agropyri (non O'Gara 1916) comb. nov., nom. rev., isolated from western wheatgrass (Pascopyrum smithii)

Aplanobacter agropyri was first described in 1915 by O'Gara and later transferred to the genus Corynebacterium by Burkholder in 1948 but it was not included in the Approved Lists of Bacterial Names in 1980 and, consequently, is not recognized as a validly published species. In the 1980s, bacteria resembling Corynebacterium agropyri were isolated from plant samples stored at the Washington State Mycological Herbarium and from a diseased wheatgrass plant collected in Cardwell, Montana, USA. In the framework of this study, eight additional isolates were recovered from the same herbarium plant samples in 2011. The isolates are slow-growing, yellow-pigmented, Gram-stain-positive and coryneform. The peptidoglycan is type B2γ containing diaminobutyric acid, alanine, glycine and glutamic acid, the cell-wall sugars are rhamnose and mannose, the major respiratory quinone is MK-10, and the major fatty acids are anteiso-15 : 0, anteiso 17 : 0 and iso-16 : 0, all of which are typical of the genus Rathayibacter. Phylogenetic analysis of 16S rRNA gene sequences placed the strains in the genus Rathayibacter and distinguished them from the six other described species of Rathayibacter. DNA-DNA hybridization confirmed that the strains were members of a novel species. Based on phenotypic, chemotaxonomic and phylogenetic characterization, it appears that strains CA-1 to CA-12 represent a novel species, and the name Rathayibacter agropyri (non O'Gara 1916) comb. nov., nom. rev. is proposed; the type strain is CA-4^T (=DSM 104101^T;=ATCC TSD-78^T).

PAMPs, PRRs, effectors and R-genes associated with citrus-pathogen interactions

BACKGROUND

Recent application of molecular-based technologies has considerably advanced our understanding of complex processes in plant-pathogen interactions and their key components such as PAMPs, PRRs, effectors and R-genes. To develop novel control strategies for disease prevention in citrus, it is essential to expand and consolidate our knowledge of the molecular interaction of citrus plants with their pathogens.

SCOPE

This review provides an overview of our understanding of citrus plant immunity, focusing on the molecular mechanisms involved in the interactions with viruses, bacteria, fungi, oomycetes and vectors related to the following diseases: tristeza, psorosis, citrus variegated chlorosis, citrus canker, huanglongbing, brown spot, post-bloom, anthracnose, gummosis and citrus root rot.

Gene Expression Changes during the Gummosis Development of Peach Shoots in Response to Lasiodiplodia theobromae Infection Using RNA-Seq

Lasiodiplodia theobromae is a causal agent of peach (Prunus persica L.) tree gummosis, a serious disease affecting peach cultivation and production. However, the molecular mechanism underlying the pathogenesis remains unclear. RNA-Seq was performed to investigate gene expression in peach shoots inoculated or mock-inoculated with L. theobromae. A total of 20772 genes were detected in eight samples; 4231, 3750, 3453, and 3612 differentially expressed genes were identified at 12, 24, 48, and 60 h after inoculation, respectively. Furthermore, 920 differentially co-expressed genes (515 upregulated and 405 downregulated) were found, respectively. Gene ontology annotation revealed that phenylpropanoid biosynthesis and metabolism, uridine diphosphate-glucosyltransferase activity, and photosynthesis were the most differentially regulated processes during gummosis development. Significant differences were also found in the expression of genes involved in glycometabolism and in ethylene and jasmonic acid biosynthesis and signaling. These data illustrate the dynamic changes in gene expression in the inoculated peach shoots at the transcriptome level. Overall, gene expression in defense response and glycometabolism might result in the gummosis of peach trees induced by L. theobromae.

Outbreaks of Yuzu Dieback in Goheung Area: Possible Causes Deduced from Weather Extremes

Starting in 2012, severe diebacks usually accompanied by abundant gum exudation have occurred on yuzu trees in Goheung-gun, Jeonnam Province, where severely affected trees were occasionally killed. On-farm surveys were conducted at 30 randomly-selected orchards located at Pungyang-myeon, Goheung-gun, and the resulting disease incidences were 18.5% and 39.6% for dieback and gumming symptoms, respectively. Black spots on branches and leaves also appeared on infected trees showing a typical dieback symptom. Morphological and molecular identifications of the isolated fungal organisms from lesions on the symptomatic leaves and branches revealed that they are identical to Phomopsis citri, known to cause gummosis. In order to find the reason for this sudden epidemic, we investigated the weather conditions that are exclusively distinct from previous years, hypothesizing that certain weather extremes might have caused the severe induction of pre-existing disease for yuzu. There were two extreme temperature drops beyond the yuzu's cold hardiness limit right after an abnormally-warm-temperature-rise during the winter of 2011-12, which could cause severe frost damage resulting in mechanical injuries and physiological weakness to the affected trees. Furthermore, there was an increased frequency of strong wind events, seven times in 2012 compared to only a few times in the previous years, that could also lead to extensive injuries on branches. In conclusion, we estimated that the possible damages by severe frost and frequent strong wind events during 2012 could cause the yuzu trees to be vulnerable to subsequent fungal infection by providing physical entries and increasing plant susceptibility to infections.