First Report of Southern Blight Caused by Athelia rolfsii on Pepper in Yiyang, China

Pepper(Capsicum annuum L.) is the vegetable with the largest production area China (Zou and Zou 2021). In the summer of 2020 and 2021, disease symptoms were observed in C. annuum L. cv. bola in a 10-ha field in Yiyang(28.35°N, 112.56°E), Hunan province of China. The disease incidence ranged from 10% to 30%. The symptoms initially appeared as tan lesions, which were colonized by fast-growing white mycelia, at the soil line. Affected plants eventually became wilted. Wilting was accompanied by girdling of the stem at the base, and signs of the pathogen, mycelia and golden-brown colored sclerotia. The spatial distribution of the disease was either single plants or small foci of affected plants. Diseased stem sections (1.0~1.5 cm) of 20 plants from the field in 2021 with typical symptoms were surface sterilized with 75% ethanol for 30 s, followed by 60 s in 2.5% NaClO, rinsed thrice with sterile water, air dried and plated on potato dextrose agar (PDA), and incubated at 28℃ in the dark for 5 days to isolate the causative pathogen. Twenty fungal isolates with similar colony morphology were collected and purified. These isolates formed radial colonies, and abundant sclerotia were observed after 5 to 10 days of incubation at 28℃. The color of the sclerotia with a diameter of 1.39 ± 0.15 mm (1.15 to 1.60, n=50) gradually changed from white to light yellow, and finally to dark brown. The representative isolate YYBJ20 was selected for further molecular identification. The internal transcribed spacer region and elongation factor-1alpha gene were amplified using the primers, ITS1/ITS4 (White et al. 1990) and EF1-983F/EF1-2218R (Rehner and Buckley 2005), respectively. The ITS and EF1α amplicons were sequenced and deposited in GenBank with the accession numbers OQ186649 and OQ221158, respectively. Sequence analysis revealed that the ITS and EF1α sequences of the YYBJ20 isolate exhibited ≥99% of identity with the ITS (MH260413 and AB075300) and EF1α (OL416131 and MW322687) sequences of Athelia rolfsii, respectively. Phylogenetic analysis classified YYBJ20 into a common clade with different A. rolfsii strains, but different from other Athelia or Sclerotium species. For pathogenicity tests, PDA plugs (6 mm diam.) colonized by 3-day-old mycelia were inoculated into the stem bases of 30-day-old pepper seedlings (n=10). Another 10 seedlings were inoculated with noncolonized PDA plugs were used as noninoculated controls. The pepper seedlings were incubated at 28 ± 2℃ and 60 to 80 % relative humidity under a 14h-10h of light-dark cycle. After 10 days of incubation, ten YYBJ20-inoculated plants were wilted with similar symptoms to those observed in the field, while control plants remained healthy. The pathogenicity tests were repeated three times. The fungal strain re-isolated from the infected seedlings (100% re-isolation frequency) showed the same morphological and molecular traits as the original isolates from the diseased plants. No fungi were isolated from the control plants, which is consistent with the Koch's postulates. Based on the morphological and sequencing results, the causative fungus was identified as A. rolfsii (anamorph Sclerotium rolfsii). To our knowledge, this is the first report of A. rolfsii causing southern blight on pepper in China. Due to the broad host range of and serious consequences caused by A. rolfsii (Lei et al. 2021; Zhang et al. 2022; Zhu et al. 2022), this research will be beneficial to develop strategies to mitigate future losses of pepper in China.

First Report of Athelia rolfsii Causing Damping-Off and Leaf Blight on Basella alba in Malaysia

Basella alba is an evergreen perennial vine that grows as an edible leafy vegetable in Malaysia (Nordin et al. 2007). During January 2021, a cottony white hypha associated with aggregates of white to brown sclerotia and symptoms of damping-off were visualized on the stem base of B. alba at the soil surface in an isolated field (~0.03 ha) located in the district of Penampang, Sabah province, Malaysia (5°56'51.0"N 116°04'31.8"E). Yellowing and wilting of leaves, and defoliation were observed after four days of the development of damping-off. Survey was conducted on 100 plants which 30 were found infected. The disease severity (90%) on stems was estimated using IMAGEJ. Symptomatic stem tissues were surface sterilized with 75% of ethanol for 1 min, washed with 2% of sodium hypochlorite solution for 1 min, rinsed thrice with sterile distilled water, air dried and plated on potato dextrose agar (PDA). Plates were incubated for 7 days at 25°C in the dark. After 7 days, fungi were isolated; colony color was white and had a cottony appearance. On day 14, white to brown sclerotia 1.0 to 2.2 mm in diameter were produced. Hyaline septate hyphae with clamp connections and multiple nuclei were seen. Conidia and conidiophores were absent from the colony on PDA. Genomic DNA of fungi was extracted based on Khoo et al. (2022a and 2022b). PCR amplification (Khoo et al. 2022b) was performed using primer set ITS1/ITS4, EF983/EF2218 and LR0R/LR05 to amplify the internal transcribed spacer (ITS) region of rDNA, partial translation elongation factor 1 alpha (TEF-1α) gene and partial large subunit ribosomal RNA (LSU rRNA) gene, respectively (Vilgalys and Hester 1990; White et al. 1990; Carbone and Kohn, 1999; Rehner 2001). Phylogenetic analysis indicated that the isolates formed a supported clade to the related Athelia rolfsii sequences. The sequencing result (GenBank Accession Nos. OK465460, OP809607, OP857217) had a 99% identity over 625 bp, 941 bp, and 1,101 bp with the corresponding gene sequence of A. rolfsii (GenBank Accession Nos. MN622806, AY635773, MW322687) after analysis in BLASTn program. Pathogenicity test was performed based on Le (2011). Three 8-week-old B. alba plants cultivated on sterilized soil were inoculated with 5-mm mycelia plugs from 7-day-old culture. A plug was put on the upper soil surface layer 2 cm away from the base of the stem of B. alba plant before fully covered with a layer of sterilized soil. Plants that were inoculated with sterile PDA plugs served as the control treatment. Plastic bags were used to cover the plants after inoculation for 24 h before keeping them in a glasshouse under ambient conditions. Water-soaked and brown lesions were visualized on the stem base of all inoculated plants after four days of inoculation. Symptom of damping-off and leaf blight was observed after another 3 days. No symptoms developed on the mock controls. The pathogenicity test was repeated twice. Re-isolation was performed from the symptomatic tissues of inoculated plants and mock controls. The isolates reisolated from the symptomatic tissues were verified as A. rolfsii based on morphology and molecular characterization, thus fulfilling Koch's postulates. No pathogens were isolated from the mock controls. To our knowledge, this is the first report of A. rolfsii causing damping-off and leaf blight on B. alba in Malaysia, as well as worldwide. Our findings documented the wider geographical and host range of A. rolfsii and indicate its potential threat to B. alba production in Malaysia.

First report of southern blight caused by Athelia rolfsii on Spotted laurel (Aucuba japonica) in China

Aucuba japonica, also known as spotted laurel, is a woody, broadleaf, evergreen shrub with variegated leaves in the Garryaceae family, widely used in urban parks, green spaces and landscaping. In October 2019, an outbreak of a disease with southern blight symptoms was observed on A. japonica planted as a green barrier in Kunshan city, Jiangsu province of China (N31°32'37", E120°00'41"). The disease incidence was estimated up to 30%. The infected plants showed symptoms including brown to black necrotic stems, white mycelium and white to dark reddish brown sclerotia at the base of the stem and decayed tissues. Fifteen samples (10 sclerotia and 5 mycelial fragments) were collected from symptomatic plants for causal agent isolation. The sclerotia were disinfected with 70% ethanol for 2 to 3 s and 5% NaClO for 2 min, rinsed three times with sterile water, then cultivated on potato dextrose agar (PDA) plate at 25°C. Mycelial fragments were transferred to PDA plates by an inoculation needle directly. In total 15 fungal strains were obtained and purified by transferring single hyphal tips to fresh media. All the strains showed consistent phenotype, white mycelia on PDA, with an average growth rate of 13.6 to 16.9 mm/day (n=30), and mycelia with clamp connections were observed under the microscope. Globose sclerotia formed at 4 days post inoculation (dpi), initially whitish, turning to beige and eventually dark reddish brown. The number of sclerotia produced per plate ranged from 280 to 486 (mean = 378; n = 30), and the diameter of mature sclerotia ranged from 0.8- to 1.6-mm (mean = 1.24; n = 150). Three strains YKY2020.02, YKY2020.03, and YKY2020.07 were selected for further molecular identification. Genomic DNA was extracted from these strains using a CTAB method (Mahadevakumar et al. 2018). ITS primer pair ITS1/ITS4 was used to amplify the internal transcribed spacer region (White et al. 1990). PCR products were then sequenced by Sangon Biotech (Shanghai, China), and subsequently, the ITS sequences (686 bp) were deposited in GenBank under accession number OM647806, OP279917 and OP279918, respectively. All sequences showed 99-100% similarity with Athelia rolfsii sequences from GenBank by BLAST analysis in NCBI. The phylogenetic tree of ITS sequences generated by the neighbor-joining analysis in MEGA-X also shows that all selected strains clustered with different strains of A. rolfsii into one big branch, indicating that these strains are the same. Based on morphological and molecular characteristics, these strains were identified as A. rolfsii (Curzi) C.C. Tu & Kimbr. (syn. Sclerotium rolfsii) (Stevens 1931; Paul et al. 2017). Pathogenicity tests were conducted on healthy plants of A. japonica (n = 15). Five-day-old mycelial discs (5 mm) were inoculated at the basal part of the plants with mycelial side inward and secured with wet absorbent cotton, while plants inoculated with sterile water were used as a control (n = 5). All plants were kept in a greenhouse with a temperature of 26 to 33°C and an average relative humidity higher than 65%. At 5 dpi, all inoculated plants showed symptoms similar to those observed in fields. Control plants remained asymptomatic. To fulfill Koch's postulates, identities of all the causal pathogens were confirmed by reisolation in PDA and identification by morphology. To our knowledge, this is the first report of A. rolfsii causing southern blight on A. japonica worldwide. Our findings are important for future disease control strategy development.

First Report of Sclerotium rolfsii Causing Collar Rot of Guar (Cyamopsis tetragonoloba) in Mexico

Cyamopsis tetragonoloba (Fabaceae), also known as guar or cluster bean, is an annual legume grown mainly for industrial purposes and also as an ingredient for animal feed. In October 2021, collar rot symptoms were observed in five guar fields located in Guasave, Sinaloa, Mexico. Abundant white mycelium, and later brown and small sclerotia were observed at the base of the stems. Diseased plants showed reduced growth, wilting, and drying of the entire plant. Disease incidence ranged from 15 to 40%. Samples were collected from each field at two phenological stages (vegetative and reproductive). For fungal isolation, symptomatic stems pieces were surface sterilized with 2% sodium hypochlorite for 2 min, rinsed in sterilized distilled water two times, placed on PDA medium and incubated at 28°C in darkness for 3 days. Sclerotium-like colonies were consistently obtained and five isolates from five different fields were purified by the hyphal-tip method. Fungal colonies were white, cottony, and often forming fans. Sclerotia (1 to 2 mm diameter) were white at first and then gradually turned dark brown. Microscopic examination showed septate hyphae with some cells having clamp connections. A representative isolate was deposited in the Culture Collection of Phytopathogenic Fungi of the Faculty of Agriculture of Fuerte Valley at the Sinaloa Autonomous University under Accession no. FAVF647. For molecular identification, genomic DNA was extracted, and the internal transcribed spacer (ITS) region was amplified by PCR and sequenced using the primer pair ITS5/ITS4 (White et al. 1990). The sequence was deposited in GenBank (accession no. OM510466). BLASTn searches in GenBank showed 99.21 to 100% identity with the available sequences of Sclerotium rolfsii (accession nos. MK926446, MH854711, and KY175225). A phylogenetic analysis using the maximum Likelihood method placed isolate FAVF647 in the same clade as S. rolfsii. Pathogenicity tests were performed by inoculating 10 healthy guar seedlings (15-day-old) grown in pots. Four sclerotia were directly placed on the stem base of each plant. Five uninoculated guar seedlings were used as control. All plants were placed in a moist chamber at 25°C with a 12-h photoperiod for 2 days. Collar rot symptoms appeared on inoculated plants after 3 days, whereas control plants remained symptomless. Pathogenicity test was performed twice with similar results. The fungus was reisolated from the artificially inoculated plants, thus fulfilling Koch's postulates. Sclerotium rolfsii has been reported on guar plants in Australia, Brazil, Fiji, India, and the United States (Farr and Rossman 2022). To our knowledge, this is the first report of Sclerotium rolfsii causing collar rot of guar in Mexico. The disease is very common in guar fields in Sinaloa, Mexico, therefore additional studies are needed to develop effective disease-management strategies.

Peanut disease epidemiology under dynamic microclimate conditions and management practices in North Florida

Diverse field characteristics, weather patterns, and management practices can result in variable microclimates. The objective was to relate in-field microclimate conditions with peanut diseases and yield and determine the effect of irrigation and fungicides within these environments. Irrigation did not have a major impact on disease and yield. Stem rot (Athelia rolfsii) and early (Passalora arachidicola) and late (Nothopassalora personata) leaf spot were most affected by changes in environmental patterns across seasons. Average non-treated stem rot was 12.9% in 2017 which dropped considerably in 2018 to 0.2% but emerged again in 2019 to 3.2%. Stem rot incidence varied across the field, and the response to fungicides depended on management zone. Leaf spot defoliation in non-treated plots was severe in 2019 reaching an average of 73% at 126 days after planting but only reached 15% in 2017 and 35% in 2019 at the same stage. A low-input fungicide schedule was able to reduce foliar disease in all zones and seasons, but the microclimatic conditions in the low-lying area favored leaf spot in 2017 and 2018 although not in the dryer 2019 season. Seasonal differences in disease and plant growth affected the level of protection against average yield loss using a standard low-input program which in 2017 (527 kg/ha) was not as great as 2018 (2,235 kg/ha) or 2019 (1,763 kg/ha). Disease prediction models built on dynamic environmental factors in the context of multiple pathogens and natural field conditions could be developed to improve within-season management decisions for more efficient fungicide inputs.