Genetic Analyses of Resistance to Fusarium Basal Rot in Onion

Fusarium basal rot (FBR) is a serious disease of onion (Allium cepa). We identified sources of FBR resistance, assessed efficacy of selection for increased resistance, and investigated its genetic control. Onion accessions were evaluated for FBR resistance, and percentage survival ranged from 0% to 78%. Survivors were intercrossed, and progenies from one cycle of selection showed increased survival by 18% to 52%. Selections were crossed to male-sterile lines, and hybrids showed specific combining ability for FBR resistance. Segregating families were produced, and quantitative trait loci (QTLs) were identified on chromosomes 2 and 4 conditioning FBR resistance. A second QTL on chromosome 4 was identified that decreased FBR resistance. Plants from families with different genotypes across the 1.5 logarithm of odds (LOD) regions on chromosomes 2 and 4 were self-pollinated, and resulting families were evaluated for FBR survival. Genomic regions on chromosomes 2 and 4 associated with resistance were validated at p = 0.05 and 0.10, respectively. The region on chromosome 4 associated with increased susceptibility was validated at p = 0.05. These results are in agreement with previous studies reporting high heritability and specific combining ability for FBR resistance and should be useful for selection of FBR-resistant onion.

Karakterisasi Jamur Penyebab Penyakit Busuk Pangkal Batang (Basal Rot) pada Bawang Wakegi (Allium x wakegi Araki)

Bawang wakegi merupakan hasil persilangan alami antara bawang merah (Allium cepa L. aggregatum group) dan bawang daun (Allium fistulosum L.). Salah satu faktor yang mengganggu pertumbuhan dan mempengaruhi hasil produksi bawang wakegi di lapang adalah keberadaan penyakit busuk pangkal batang (basal rot) atau layu Fusarium. Penyakit ini disebabkan oleh banyak spesies dari jamur patogen Fusarium spp yang berbeda. Penelitian ini bertujuan untuk menentukan karakteristik jamur patogen busuk pangkal batang yang menyerang bawang wakegi sehingga dapat dipastikan patogen penyebabnya. Tahapan penelitian meliputi pengambilan sampel, isolasi jamur, uji patogenisitas, dan karakterisasi secara makroskopis dan mikroskopis. Hasil karakterisasi jamur patogen yang menginfeksi bawang wakegi mengarah pada karakteristik jamur F. oxysporum f.sp. cepae.

Improving Fusarium Basal Rot Resistance of Onion Cultivars through Artificial Inoculation and Selection of Mature Bulbs

Fusarium basal rot (FBR) of onion, caused by the soil-borne fungus Fusarium oxysporum f.sp. cepae (FOC), results in a substantial storage loss of marketable bulbs. Seedling and field screening methods, which were used to generate FBR resistant long-day and intermediate-day onion cultivars, were found to be ineffective at increasing FBR resistance in short-day onion cultivars. This study attempted to improve the FBR resistance of seven commercial short-day onion cultivars and two check cultivars when evaluated at their mature bulb stage. Mature bulbs were artificially inoculated with 1 cm diameter potato dextrose agar plugs containing conidia of a virulent FOC isolate, CSC-515, at a concentration of 3 × 104 spore mL−1, after transversely cutting the basal plate tissue. Incubated bulbs, which had few or no FBR symptoms, were selected after 20 d using visual scoring, from 1 (no disease) to 9 (>70% basal plate is infected), and combined in seed production cages to produce the selected generation of a cultivar. Multiple cycles of phenotypic recurrent selection were conducted, and the resultant populations were compared with their respective original populations for FBR severity and incidence, from 2016 to 2019, using the same conidial inoculation method. A variable amount of progress was achieved in reducing FBR severity and incidence in the seven cultivars, with maximum improvement in the most advanced selected populations. FBR development in the advanced selected populations differed between mature bulbs of each entry and was influenced by yearly environmental variation. The progress of FOC infections was slower in resistant bulbs when compared to susceptible bulbs. These results indicated a partial or quantitative resistance against FBR. The partial FBR resistant cultivar populations could be used to develop synthetic short-day FBR resistant cultivars after multi-locational and multi-seasonal field trials. These populations could also be used to study the mechanism(s) of FBR resistance in onion, which has yet to be determined.

Comparing Visual and Image Analysis Techniques to Quantify Fusarium Basal Rot Severity in Mature Onion Bulbs

Development of resistant cultivars for Fusarium basal rot (FBR), a fungal disease caused by the soil-borne fungus Fusarium oxysporum f.sp. cepae (FOC), is a major breeding goal for onion breeding programs around the world. Currently, the selection of FBR-resistant bulbs is based on a visual interval or category scale, which is an entirely subjective method. This study aimed to develop an objective approach using digital image analysis to quantify symptom development in the basal plate of dormant bulbs. Digital image analyses were performed after artificially inoculating dormant bulbs of eighty-five United States Department of Agriculture (USDA) Allium cepa accessions with a virulent FOC isolate, ‘CSC-515’. An analysis with confocal microscopy identified bright blue–green autofluorescence from FOC-infected tissue, effectively differentiating diseased from healthy tissue. Visual scoring of the FBR symptom was aided by stereo fluorescence microscopic images captured using a green fluorescence protein dual filter to quantify accurately FBR severity in the basal plate tissue. An automatic stepwise image segmentation method was developed that was relatively more accurate than a visual estimation. This method exhibited comparable reliability and precision to visual scoring, but it tended to underestimate FOC infection. To our best knowledge, this is the first comprehensive study to investigate the potential use of image analysis as a viable alternative to conventional visual scoring for FBR symptom development. This method could be used for developing resistant cultivars for onion breeding programs in the near future.

Duohua huangjing (Polygonatum cyrtonema Hua) seedling basal rot caused by Fusarium redolens in China

Duohua huangjing (Polygonatum cyrtonema Hua) seedling basal stem rot caused by Fusarium redolens in China Tao Tang1, Fanfan Wang1, Jie Guo1, Xiaoliang Guo1, Yuanyuan Duan1,Jingmao You1* 1 Institute of Chinese Herbal Medicines, Hubei Academy of Agricultural Sciences, Enshi, 445000, China. Duohua huangjing (Polygonatum cyrtonema Hua), a herbal medicine, that is mostly planted in several provinces in China. In April 2020, severe diseases with about 40% seedling losse was found in the Huangjing seedling base in Shiyan city, Hubei province. The symptoms included softening and decay of the roots and stem bases, a progressive yellowing and wilting of leaves, and finally being completely rotted. Small pieces of symptomatic stems (0.5 cm in length) and leaves (0.5 × 0.5 cm in size) were surface sterilized with 75% ethanol for 30 s, followed by 0.1% HgCl2 for 1 min, rinsed three times with sterile water, and then dried with sterilized absorbent paper. The sections were placed on potato dextrose agar (PDA) medium containing 10 µg/ml of ampicillin and incubated at 25°C in the dark. After 3 days incubation, eight isolates with the same colony morphology were sub-cultured and purified by hyphal tip isolation. Macroconidia were sickle-shaped, 15.8 - 32.3 × 3.1 - 5.6 μm (n = 25), and three to five septate. Microconidia were oval or kidney-shaped, 5.2 - 11.4 × 2.0 - 3.2 μm (n = 25), and zero to one septate. To confirm the identity of the pathogen, molecular identification was performed with strain HJCD1. Following DNA extraction, PCR was performed using the TSINGKE 2×T5 Direct PCR Mix kit. Target areas of amplification were the internal transcribed spacer (ITS) and translation elongation factor 1α (TEF-1α) using ITS1/4 (White et al. 1990) , EF1/EF2 (Taylor et al. 2016), respectively. Following BLAST searches and phylogenetic reconstruction, the ITS region (GenBank MW485770.1) showed 99% identity with those of Fusarium redolens in GenBank (KU350713.1) and the TEF-1α (GenBank MW503930.1) showed 100% identity with F. redolens GenBank (MK922537.1). Pathogenicity tests were performed to fulfill Koch’s postulates. Huangjing seedlings were rinsed with sterile water, wiped clean with sterile absorbent paper, and transferred to a tray covered with wet filter paper to maintain high humidity. The mycelial piugs of F. redolens HJCD1 were inoculated onto the surface of leaves and basal stems. Controls were inoculated with sterile PDA plugs. The inoculated seedlings were sealed with plastic wrap, and then cultivated in a 25 ℃ growth chamber with 16 h of light per day. The pathogen-inoculated plants exhibited etiolation and typical wilt symptoms after 4 days, whereas no symptoms were observed in the control plants. F. redolens was reisolated from the infected tissues, and colony morphology and ITS sequence of re-isolates were same as that of HJCD1. The pathogen has been reported previously in american ginseng in China (Fan et al. 2021), lentil in Pakistan (Rafique et al. 2020), and wild rocket in United Kingdom (Taylor et al. 2019). However, to the best of our knowledge, this is the first report of F. redolens causing seelding basal rot on Duohua huangjing in China. References: White, T. J., et al. 1990. Page 315 in: PCR Protocols: A Guide to Methods and Applications. Academic Press, San Diego, CA. Taylor, A., et al. 2016. Mol. Plant Pathol. 17:1032. https://doi.org/10.1111/mpp.12346 Fan, S. H., et al. 2021. Plant Dis. https://doi.org/10.1094/PDIS-11-19-2519-PDN Rafique, K., et al. 2020. Plant Dis. 9:104. https://doi.org/10.1094/PDIS-11-19-2519-PDN Taylor, A., et al. 2019. Plant Dis.6:103. https://doi.org/10.1094/PDIS-12-18-2143-PDN Funding: Science Funds for Young Scholar of Hubei Academy of Agricultural Science (grant no. 2020NKYJJ20), National Modern Agricultural Industrial Technology System (grant no. CARS-21), Technology R&D Program of Enshi (grant no. D20190015), Science Funds for Young Scholar of Institute of Chinese Herbal Medicines, Hubei Academy of Agricultural Sciences (grant no. 2019ZYCJJ03), Key Laboratory of Integrated Management of Crops of Central China, Ministry of Agriculture, P. R. China / Hubei Key Laboratory of Crop Disease, Insect Pests and Weeds Control (grant no.2020ZTSJJ6)