isolation-behavioral-identification-pathogenicity-assessment

Isolation, Behavioral Identification, and Pathogenicity Assessment of Entomopathogenic Fungi from a Forest Wood Borer

Forest wood borers (FWB) cause severe tree damage and economic losses worldwide. The release of entomopathogenic fungi (EPF) during the FWB emergence ...

Zhejiang A&F University

Huzhou University

Pine wilt disease, caused by the pinewood nematode Bursaphelenchus xylophilus (PWN), is an important destructive disease of pine forests worldwide. In addition to behaving as a plant-parasitic nematode that feeds on epithelial cells of pines, this pest relies on fungal associates for completing its life cycle inside pine trees. Manipulating microbial symbionts to block pest transmission has exhibited an exciting prospect in recent years; however, transforming the fungal mutualists to toxin delivery agents for suppressing PWN growth has received little attention.

Suppressing a plant-parasitic nematode with fungivorous behavior by fungal transformation of a Bt cry gene.

 is a latent fungal pathogen that causes cankers or diebacks on a variety of host woody plants worldwide. The symptomatic necrosis on the host plants can be triggered by abiotic stress, such as drought and soil acidification. Here we report a high-quality genome assembly and announcement of the  strain CK16 (CGMCC 19654), which causes trunk canker disease on  in China. The genome sequence of strain CK16 will be useful for studying the evolution, host adaption, and pathogenicity of , which will be beneficial for a better understanding of the mechanisms of host-pathogen interaction during the endophytic period.

Genome Sequence Resource of  CK16, a Fungal Pathogen Causing Chinese Hickory Trunk Canker Disease.

Cherry (Prunus avium) has become an important economical fruit in China. In October 2020, a leaf spot disease was found on cherry in the orchard of Taizhou Academy of Agriculture Sciences, Zhejiang, China. The symptoms appeared as small, water-soaked spots on the leaves, which later became larger, dark brown, and necrotic lesions of 1 cm to 3 cm in width, 4 cm to 8 cm in length. Disease incidences of approximately 60% of the leaves were observed by sampling five locations. To isolate the causing agent, small fragments from five target symptomatic leaves were surface-sterilized with 1.0% sodium hypochlorite solution for 1 min and then rinsed three times with sterilized water. Afterwards the leaf fragments were air-dried, plated onto potato dextrose agar (PDA) medium, and incubated at 25 ℃ in the dark for 2 days. The pure cultures were obtained by transferring hyphal plug of 2 mm in diameter onto PDA, which followed single spore isolation. The colony morphology showed light to dark gray, cottony mycelium, with the underside of the culture became brownish after 7 days. Conidia (n = 28) were hyaline, smooth-walled, cylindrical, aseptate, broadly rounded ends, and average size around 3.84 × 12.82 μm (2.99 to 4.87 × 10.27 to 15.68 μm). Appressoria (n = 27) were mostly brown, ovoid and slightly irregular in shape, and average size around 8.04 × 9.68 μm (6.29 to 9.67 × 9.32 to 12.06 μm). Perithecia average size is 106.25 μm, textura angularis, thick-walled. Asci 26.35-49.18 × 5.00-12.03 μm (average size 37.44 × 7.80 μm, n = 17), unitunicate, thin-walled, clavate or cymbiform. Ascospores 13.69-20.93 × 3.86-6.69 μm (average size 16.00 × 5.42 μm, n = 30), one-celled, hyaline, one or two large guttulate at the centre, slightly rounded ends. The morphological characteristics matched well with previous descriptions of Colletotrichum species of C. gloeosporioides species complex, including C. fructicola (Prihastuti et al. 2009; Fu et al. 2019). The identity of two representative isolates (cf2-3 and cf4-4) from different leaves was confirmed by means of multi-locus gene sequencing. To this end, genomic DNA was extracted by the Plant Direct PCR kit (Vazyme Biotech Co., Ltd, China). Molecular identification was conducted by sequencing the internal transcribed spacer (ITS) rDNA region, partial glyceraldehyde-3-phosphate dehydrogenase (GAPDH) gene, partial actin (ACT) gene, partial beta-tubulin 2 gene (TUB2), and partial chitin synthase gene (CHS). The obtained sequences have been deposited in GenBank under accession numbers MW581851 and MW581852 (ITS), MW590586 and MW590587 (GAPDH), MW616561 and MW616562 (ACT), MW729380 and MW729381 (TUB2), MW729378 and MW729379 (CHS). The results of Basic Local Alignment Search Tool (BLAST) analysis revealed that the ITS, GAPDH, ACT, TUB2 and CHS sequences of both isolates matched with 100% identity to Colletotrichum fructicola culture collection sequences in GenBank database (JX010165, JX009998, JX009491, JX010405, and JX009866 respectively). These morphological characteristics and molecular analyses allowed the identification of the pathogen as C. fructicola. Koch's postulates were performed with healthy detached cherry leaves of cultivar namely 'HongMi' from Taizhou Academy of Agriculture Sciences. Surface-sterilized leaves were inoculated with five-day-old cultures of C. fructicola mycelial discs of 2 mm in diameter after being wounded with a needle or non-wounded. Control leaves were inoculated with discs of same size PDA agar. Treated leaves were incubated at 25 ℃ in the dark at high relative humidity. Anthracnose symptoms appeared within 3 days both on non-wounded and wounded inoculation approaches. Mock-inoculated controls remained asymptomatic. Biological repetitions were carried out three times. The fungus was reisolated from infected leaves and confirmed as C. fructicola following the methods described above. Until recently, it has been found that C. fructicola can infect tea, apple, pear, Pouteria campechiana in China (Fu et al. 2014; Li et al. 2013; Shi et al. 2018; Yang et al. 2020). To the best of our knowledge, this is the first report of C. fructicola on cherry in China.

First report of Colletotrichum fructicola causing anthracnose on cherry (Prunus avium) in China.

Carya cathayensis Sarg. (Chinese hickory) is one of the important economic forest plants, mainly distributed in Zhejiang and Anhui provinces in China. In September 2020, leaf spot disease occurred on 90% C. cathayensis in a 2.6 km2 plantation with 500 hickorys in Shangshu Village (30°26'N, 119°32'E), Huzhou, Zhejiang, China. Symptoms initially appeared as small brown spots. Later, the spots became dark brown, and joined into irregular shapes. Twenty diseased leaves with typical symptoms were collected and used to isolate the pathogen. The leaf tissues (5 × 5 mm) at junction of diseased and healthy portion were cut and surface-sterilized with 75% ethanol for 15 s, 0.1% NaClO for 2 min, and rinsed 3 times in sterile water, then placed on potato dextrose agar (PDA) plates and incubated at 25°C in the darkness for 3 days. Eight isolates with similar morphological characterizations were obtained after pure cultures by transferring hyphal tips. The colony growing on PDA for 7 days was circular, dense, white cotton-like hyphae, and light gray-black hyphae can be seen inside. The conidia were cylindrical, aseptate, hyaline, with rounded ends, and 12.5 to 20.0 × 5.0 to 7.5 µm (n = 50). The appressoria were brown to dark brown, ovoid to clavate, slightly irregular to irregular, and were in the range of 6.4 to 10.2 × 5.0 to 6.7 µm (n = 50). The morphologies of the isolates were consistent with the genus description of Colletotrichum (Fuentes-Aragón et al. 2018; Liu et al. 2015). The internal transcribed spacer (ITS) regions, glyceraldehyde-3-phosphate dehydrogenase (GAPDH), calmodulin (CAL), chitin synthase (CHS-1), beta-tubulin 2 (TUB2) and actin (ACT) genes were amplified from genomic DNA for the isolates using the primers described by Weir et al. (2012). The sequences of eight isolates were consistent and the representative isolate CFZJ-64 were deposited in GenBank under the following accession numbers: ITS, OK145563; ACT, OK216738; CAL, OK216739; CHS-1, OK216740; GAPDH, OK216741; and TUB2, OK216742. A phylogenetic tree was generated by combining ITS, ACT, CAL, CHS-1, TUB2, and GAPDH sequences in MEGA11. Three representative isolates CFZJ-42, CFZJ-53 and CFZJ-64 clustered in the C. fructicola clade with 90% bootstrap support. Based on morphological characteristics and phylogenetic analysis, the isolates were identified as C. fructicola. To confirm pathogenicity, 9 detached healthy leaves and 9 healthy leaves on 3-year-old C. cathayensis seedlings were inoculated with conidial suspension of each isolate (20 µL, 1 × 106 conidia/mL). The control leaves were treated with distilled water (20 µL). Each tested leaf was covered with a clean ziplock bag and incubated for 48h at about 27°C, and 14h photoperiod. After five days, 7 of 8 isolates caused on all detached leaves or part of the leaves on the seedlings developed lesions similar to those observed in the field, whereas controls were asymptomatic. The same fungus was re-isolated from all the diseased leaves and identified by sequencing, confirming Koch's postulates. As far as we know, this is the first report of C. fructicola causing anthracnose on C. cathayensis. This study not only expands the knowledge on this important pathogen of C. cathayensis in China, but also provides the foundation to further investigate the biology, epidemiology, and control of the disease.

First report of Colletotrichum fructicola causing anthracnose on Carya cathayensis Sarg. in China.

Entomopathogen-based biocontrol is crucial for blocking the transmission of vector-borne diseases; however, few cross-latitudinal investigations of entomopathogens have been reported for vectors transmitting woody plant diseases in forest ecosystems. The pine sawyer beetle  is an important wood borer and a major vector transmitting pine wilt disease, facilitating invasion of the pinewood nematode  (PWN) in China. Due to the limited geographical breadth of sampling regions, species diversity of fungal associates (especially entomopathogenic fungi) on .  adults and their potential ecological functions have been markedly underestimated. In this study, through traditional fungal isolation with morphological and molecular identification, 640 fungal strains (affiliated with 15 genera and 39 species) were isolated from 81 beetle cadavers covered by mycelia or those symptomatically alive across five regional populations of this pest in southern China. Multivariate analyses revealed significant differences in the fungal community composition among geographical populations of . , presenting regionalized characteristics, whereas no significant differences were found in fungal composition between beetle genders or among body positions. Four region-representative fungi, namely,  (Zhejiang),  (Sichuan),  (Fujian), and .  (Guangxi), as well as the three fungal species , , and , showed significantly stronger entomopathogenic activities than other fungi. Additionally, insect-parasitic entomopathogenic fungi (. , . , . , and . ) exhibited less to no obvious phytopathogenic activities on the host pine , whereas . , , and certain species of  spp.-isolated from .  body surfaces-exhibited remarkably higher phytopathogenicity. Our results provide a broader view of the entomopathogenic fungal community on the vector beetle . , some of which are reported for the first time on  spp. in China. Moreover, this beetle might be more highly-risk in pine forests than previously considered, as a potential multi-pathogen vector of both PWN and phytopathogenic fungi.

Discovery of entomopathogenic fungi across geographical regions in southern China on pine sawyer beetle  and implication for multi-pathogen vectoring potential of this beetle.

During April 2022, leaf spot was observed on strawberry (Fragaria × ananassa Duch.) with a disease incidence of approximately 45% among 100 plants. Strawberry was cultivated in a nursery at Huzhou University (30.87゜N, 120.13゜E), Zhejiang Province, China. In the strawberry greenhouse, the average temperature was 15-18 degrees, 40%-60% humidity. Early symptoms appeared as dark brown or black spotted necrotic lesions, which expanded from 2 to 6 mm in diameter. Dark brown spots with yellow halos occupied half of the leaf area and eventually developed leaf blight with large yellow halos. To isolate the causal agent, 0.5 cm x 0.5 cm fragments were cut from three symptomatic leaves, and were surface sterilized with 75% ethanol for 30 s and then rinsed three times with sterilized water. The airdried leaf fragments were placed on PDA with 50 μg/ml ampicillin and incubated in the dark at 25℃ for two days. Isolates were obtained by transferring hyphal plugs of 1 mm in diameter onto PDA. The colony morphology was circular and dark brown on the upperside and black on the underside, with cottony mycelium and an large amount of gray aerial mycelium. Conidia were large, light olive-brown to dark olive-brown and light olive-black and septate. The typical conidia were oval or rod-shaped, rarely curved, and dark septa defined the basal and apical cells. In the two typical forms of conidia, the average size of oval conidia was approximately 18.77 × 54.92 μm (11.99 to 26.97 × 35.13 to 74.59 μm, n = 20), and the average size of the rod-shaped conidia was approximately 14.80 × 103.24 μm (11.24 to 24.64 × 73.11 to 131.51 μm, n = 20). The morphological characteristics matched well with previous descriptions of Exserohilum rostratum (Sharma et al. 2014; Liu et al. 2021). The identity of C1-L and C1-S from symptomatic tissues was confirmed by means of multi-locus gene sequencing. Genomic DNA was extracted from the mycelium using the CTAB (cetyltrimethylammonium bromide) method (Griffith & Shaw 1998). Molecular identification was conducted by sequencing the internal transcribed spacer (ITS) rDNA region, partial glyceraldehyde-3-phosphate dehydrogenase (GAPDH) gene, partial actin (ACT) gene, and partial beta-tubulin 2 (TUB2), using the primers ITS1/ITS4 (White et al. 1990), GDF/GDR (Templeton et al. 1992), ACT512F/ACT783R (Carbone and Kohn 1999), T1 (O'Donnell and Cigelnik 1997) and Bt2b (Glass and Donaldson, 1995). The obtained sequences of C1-L and C1-S were the same. Moreover, the sequences have been deposited in GenBank under accession numbers ON982516 (ITS), ON996915 (GAPDH), ON996916 (ACT), and ON996917 (TUB2). The results of Basic Local Alignment Search Tool (BLAST) analysis revealed that the ITS, GAPDH, and ACT had 100% identity with the sequences of E. rostratum (GenBank Accession No. LT837834, LT883550, and LT837672, respectively), the TUB2 had 99.61% similarity with BLAST sequences of E. rostratum (LT899391). These morphological characteristics and molecular analyses allowed the identification of the pathogen as E. rostratum. Koch's postulates were performed with five healthy detached strawberry leaves with three inoculations per leaf of the 'Akihime' strawberry variety. Surface-sterilized leaves were wounded with an aseptic needle, and inoculated with 2 mm diameter mycelial plugs from 5-day-old cultures of E. rostratum. Control leaves were also wounded with the aseptic needle, and inoculated with a sterile PDA agar plug. The leaves were incubated at 25℃ in Petri plates with petioles wrapped in moist sterile cotton. The diseased symptoms included black spots on the epidermis of the wounded leaves within 5, 10, and 20 days after inoculation. Mock-inoculated controls remained asymptomatic, and three biological repetitions were conducted. The fungus reisolated from the diseased leaves was confirmed as E. rostratum by sequencing. Abundant reports have shown that E. rostratum can infect many economically important crops such as maize, rice, and pineapple (Sun et al. 2021; Kabore et al. 2022; Luo et al. 2012). To the best of our knowledge, this is the first report of E. rostratum on strawberry in China and worldwide.

First worldwide report of Exserohilum rostratum causing leaf spot on strawberry (Fragaria × ananassa Duch.).

Abiotic stresses can affect the outcome of plant-pathogen interactions, mostly by predisposing the host plant to infection; however, the crosstalk between pathogens and plants related to such predisposition remains unclear. Here, we investigated the predisposition of Carya cathayensis to infection by the fungal pathogen Botryosphaeria dothidea (Bd) caused by drought in the host plant. High levels of drought stress resulted in a significant increase in plant susceptibility to Bd. Drought significantly induced the accumulation of H2O2 and the free amino acids Pro, Leu, and Ile, and in the phloem tissues of plants, and decreased the content of non-structural carbohydrates. In vitro assays showed that Bd was sensitive to H2O2; however, Pro played a protective role against exogenous H2O2. Leu, Ile, and Pro induced asexual reproduction of Bd. Our results provide the first analysis of how drought predisposes C. cathayensis to Botrysphaeria canker via amino acid accumulation in the host plant, and we propose a model that integrates the plant-pathogen interactions involved.

Liqin Zhang

School of Forestry and Biotechnology

Zhejiang A&F University

Isolation, Behavioral Identification, and Pathogenicity Assessment of Entomopathogenic Fungi from a Forest Wood Borer