Peeling the Onion: Towards a Better Understanding of Botrytis Diseases of Onion

First Report of Neck Rot Caused by Botrytis aclada on Onion (Allium cepa L.) in Gansu Province, China

Onion (Allium cepa L.) is one of the most important cultivated vegetable crops in many countries, including China (Hanci 2018; Steentjes et al. 2021). Gansu, in the northwest region of China, is a major area of onion production (Zhang et al. 2022). In October 2021, typical symptoms of neck rot were observed on stored onion bulbs (cv. Honghe) in Jiuquan, Gansu Province, China. Further surveys of 20 bags of onion bulbs randomly selected in a storage facility with bulbs harvested from 73 ha indicated that approximately 5% of the bulbs had typical neck rot symptoms. The neck of infected bulbs developed a water-soaked decay, with softened and discolored inner scales with white to gray mycelium in the neck. Infection usually began in the neck and sometimes spread through the entire bulb. In severely affected bulbs, the fleshy scales were span style="font-family:'Times New Roman'">decayed and the bulbs had shrunk, with black sclerotia between the rotting scales in the neck and shoulders of the bulb. Small pieces cut from the margins of lesions were surface-disinfested with 75% ethanol for 10 s, and 1% NaClO for 1 min, rinsed three times in sterile distilled water, dried on sterile filter paper, placed onto potato dextrose agar (PDA), and incubated at 20 ± 1℃ for 5 days in the dark. Ten pure cultures were obtained by single-spore isolation (one from each of 10 bulbs). All 10 isolates produced colonies that initially were white, and became gray to dark in color with gray mycelium that was covered with abundant conidia resembling Botrytis species when cultured on PDA at 20 ± 1℃ for 10 days in the dark. Conidia (n = 100 per isolate) were one-celled, ellipsoid or ovoid, beige to dark brown, and 5.6 to 14.8 × 3.9 to 8.5 μm. Sclerotia were not produced by any of the isolates on PDA after incubation at 20 ± 1°C for 30 days. The internal transcribed spacer (ITS) region of ribosomal DNA, glyceraldehyde-3-phosphate dehydrogenase (G3PDH) gene, heat shock protein 60 (HSP60) gene, and DNA-dependent RNA polymerase II core subunit (RPB2) gene of two representative isolates, JQ21AC03 and JQ21AC09, were amplified and sequenced with primers ITS1/ITS4, G3PDH-F/G3PDH-R, HSP60-F/HSP60-R, and RPB2-F/RPB2-R, respectively (Staats et al. 2005), and deposited in GenBank (ITS: OP604277 and OP604283; G3PDH: OP627512 and OP627515; HSP60: OP627513 and OP627516; and RPB2: OP627514 and OP627517). BLASTn analysis of the resulting sequences showed 98 to 100% similarity with those of Botrytis aclada. A maximum likelihood phylogenetic tree generated by combining the sequenced loci using MEGA11.0, clustered isolates JQ21AC03 and JQ21AC09 with B. aclada with 99% bootstrap support. Based on these results, isolates JQ21AC03 and JQ21AC09 were identified as B. aclada. Pathogenicity of the two isolates on the onion cv. Honghe was confirmed by inoculating 30 heathy onion bulbs per isolate with 100 μL of conidial suspension (1×105 conidia/ml), while control bulbs were inoculated with 100 μL sterile distilled water. All 30 bulbs inoculated with each isolates developed neck rot symptoms after 30 days incubation at 15 ± 1ºC in the dark, with symptoms identical to those observed in the original storage facility, whereas control bulbs remained symptomless. The pathogen was re-isolated from the symptomatic tissue of inoculated bulbs, fulfilling Koch's postulates. hang et al (2008) documented this pathogen causing bulb rot of onions in Wuhan, Hubei Province, 1,700 km from Gansu. To the best of our knowledge, this is the first report of B. aclada causing neck rot on onion bulbs in storage in Gansu Province, which is the main onion production region in China. Considering that onion is the main source of income for growers in Gansu, further studies will be required to understand the epidemiology of this disease and foster appropriate disease management measures to avert disease outbreaks in the future.

Cytotoxic activity of Nep1-like proteins on monocots

Necrosis- and ethylene-inducing peptide 1 (Nep1)-like proteins (NLPs) are found throughout several plant-associated microbial taxa and are typically considered to possess cytolytic activity exclusively on dicot plant species. However, cytolytic NLPs are also produced by pathogens of monocot plants such as the onion (Allium cepa) pathogen Botrytis squamosa. We determined the cytotoxic activity of B. squamosa BsNep1, as well as other previously characterized NLPs, on various monocot plant species and assessed the plant plasma membrane components required for NLP sensitivity. Leaf infiltration of NLPs showed that onion cultivars are differentially sensitive to NLPs, and analysis of their sphingolipid content revealed that the GIPC series A : series B ratio did not correlate to NLP sensitivity. A tri-hybrid population derived from a cross between onion and two wild relatives showed variation in NLP sensitivity within the population. We identified a quantitative trait locus (QTL) for NLP insensitivity that colocalized with a previously identified QTL for B. squamosa resistance and the segregating trait of NLP insensitivity correlated with the sphingolipid content. Our results demonstrate the cytotoxic activity of NLPs on several monocot plant species and legitimize their presence in monocot-specific plant pathogens.

Novel Botrytis and Cladosporium Species Associated with Flower Diseases of Macadamia in Australia

Macadamia (Macadamia integrifolia) is endemic to eastern Australia and produces an edible nut that is widely cultivated in commercial orchards globally. A survey of fungi associated with the grey and green mold symptoms of macadamia flowers found mostly species of Botrytis (Sclerotiniaceae, Leotiomycetes) and Cladosporium (Cladosporiaceae, Dothideomycetes). These isolates included B. cinerea, C. cladosporioides, and unidentified isolates. Amongst the unidentified isolates, one novel species of Botrytis and three novel species of Cladosporium were delimited and characterized by molecular phylogenetic analyses. The new species are Botrytis macadamiae, Cladosporium devikae, C. macadamiae, and C. proteacearum.

Botrytis spp.: A Contemporary Perspective and Synthesis of Recent Scientific Developments of a Widespread Genus that Threatens Global Food Security

This perspective presents a synopsis of the topics contained in the Phytopathology Pathogen Spotlight on Botrytis spp. causing gray mold, including pathogen biology and systematics, genomic characterization of new species, perspectives on genome editing, and fungicide resistance. A timely breakthrough to engineer host plant resistance against the gray mold fungus has been demonstrated in planta and may augment chemical controls in the near future. While B. cinerea has garnered much of the research attention, other economically important Botrytis spp. have been identified and characterized via morphological and genome-based approaches. Gray mold control is achieved primarily through fungicide applications but resistance to various chemical classes is a major concern that threatens global plant health and food security. In this issue, new information on molecular mechanism(s) of fungicide resistance and ways to manage control failures are presented. Finally, a significant leap in fundamental pathogen biology has been achieved via development of CRISPR/Cas9 to assess gene function in the fungus which likely will spawn new control mechanisms and facilitate gene discovery studies.

Visualization of Three Sclerotiniaceae Species Pathogenic on Onion Reveals Distinct Biology and Infection Strategies

Botrytis squamosa, Botrytis aclada, and Sclerotium cepivorum are three fungal species of the family Sclerotiniaceae that are pathogenic on onion. Despite their close relatedness, these fungi cause very distinct diseases, respectively called leaf blight, neck rot, and white rot, which pose serious threats to onion cultivation. The infection biology of neck rot and white rot in particular is poorly understood. In this study, we used GFP-expressing transformants of all three fungi to visualize the early phases of infection. B. squamosa entered onion leaves by growing either through stomata or into anticlinal walls of onion epidermal cells. B. aclada, known to cause post-harvest rot and spoilage of onion bulbs, did not penetrate the leaf surface but instead formed superficial colonies which produced new conidia. S. cepivorum entered onion roots via infection cushions and appressorium-like structures. In the non-host tomato, S. cepivorum also produced appressorium-like structures and infection cushions, but upon prolonged contact with the non-host the infection structures died. With this study, we have gained understanding in the infection biology and strategy of each of these onion pathogens. Moreover, by comparing the infection mechanisms we were able to increase insight into how these closely related fungi can cause such different diseases.