First Report of Fusarium solani Causing Leaf Sheath Rot of Bush lily in China

Bush lily (Clivia miniata) is an important indoor flower. It is the city flower of Changchun City and has important ornamental and medicinal value in China where it is culitvated on an area of 125 hectare. During the summer of 2018, symptoms of a leaf sheath rot disease were observed on bush lily in 103 greenhouses in Changchun city, Jilin Province. The disease incidence ranged from 25 to 60% in 11 surveyed greenhouses. At the early stage, the diseased plants displayed symptoms as initial leaf sheath lesions. Progressively, the whole leaves wilted, and even the plant ultimately died. Once a leaf exhibits leaf sheath lesions, the whole plant’s ornamental value significantly drops. To identify the pathogen, symptomatic leaves were cut into pieces, surface sterilized, placed on potato dextrose agar (PDA) and incubated for 7 days at 25°C in the dark (Cao et al. 2013; the e-Xtra description for details). Fusarium single-spore isolates were obtained from characteristic colonies (Leslie et al. 2006). Two single-spore isolates were selected for further study. The isolates were identified as Fusarium spp. based on microscopic morphology on PDA. Fusarium-like colonies were white to slightly yellow with abundant cottony mycelia. Single or two-celled (single septum) microconidia were reniform or oval, 8.0 to 9.6×4.0 to 6.0m in size. The elongated conidiophores bearing microconidia in monophialides were observed (Summerbell et al. 2002). Macroconidia were abundant, sickle shaped, 18.8 to 34.8×6.4 to 6.8m, with one to three septa (Taylor et al. 2019). For molecular identification, five regions of ITS, EF1-α, RPB1, RPB2 and β-tubulin genes were amplified and sequenced. Sequences of five different regions exhibited at least 97.98% similiarity with the corresponding DNA sequences in F. solani species complex (FSSC) (the e-Xtra description for details). The phylogenetic analysis based on the EF1-α, RPB1, RPB2 and β-tubulin region sequences revealed that the isolated strain in this study was clustered with only F. solani species in the phylogenetic tree for each region. Based on morphological and molecular analysis, the isolated fungal strains were identified as F. solani. Pathogenicity was confirmed by injecting a conidial suspension (106 spores/mL) of the isolated strains in to surface surface-disinfested leaf sheath of 2-year-old potted healthy plants. As a negative control, four plants were injected with sterilized water. All plants were kept in a greenhouse with controlled conditions: 26°C, 50% to 75% relative humidity. The similar rot symptoms were observed on the leaf sheathes in the inoculated plants 30 days after inoculation whereas the control plants remained asymptomatic. The fungi reisolated from the experimental plants were confirmed to be F. solani by morphology and sequences analysis, thus completing Koch’s postulates. To the best of our knowledge, this is the first report of F. solani causing leaf sheath rot of bush lily in China, where this pathogen has been reported to cause rot diseases of other economically important ornamental plants such as Phalaenopsis, Dendrobium according to the U.S. National Fungus Collections (Farr et al. 2020). In recent years, other Fusarium species have been reported to cause rot diseases on bush lily, including F. proliferatum and F. oxysporum (Farr et al. 2020). This study will also provide critical information on the causal agent for growers to implement disease management strategies.

Metabolomic and Transcriptomic Changes Induced by Potassium Deficiency During Sarocladium oryzae Infection Reveal Insights into Rice Sheath Rot Disease Resistance

Rice sheath rot disease caused by Sarocladium oryzae (S. oryzae) infection is an emerging disease, and infection can cause yield losses of 20–85%. Adequate potassium (K) application is a feasible strategy for rice tolerance to S. oryzae infection. However, little is known about the metabolic mechanisms regulated by K that allow rice to cope better with S. oryzae infection. The present study performed a comparative metabolome and transcriptome analysis of rice with different K nutrition statuses before and upon S. oryzae infection. Sarocladium oryzae infection triggered a hydrogen peroxide (H2O2) burst, and K starvation aggravated the accumulation of H2O2 in the flag leaf sheath (FLS), which resulted in lipid peroxidation. Likewise, K deficiency altered the lipid homeostasis of the host plants by hyperaccumulation of 1-alkyl-2-acylglycerophosphoethanolamine. K starvation decreased the content of glycoglycerolipids including monogalactosyldiacyglycerol and digalactosyldoacylglycerol during S. oryzae infection, which destroyed the stability of bilayer membranes. In contrast, sufficient K supply increased antioxidant-related transcript expression (for example, the genes related to glutathione-S-transferase biosynthesis were upregulated), which activated the antioxidant systems. Additionally, upon S. oryzae infection, K starvation amplified the negative impacts of S. oryzae infection on flag leaf photosynthetic potential. These results provide new insight into the role of K in alleviating S. oryzae infection. Adequate K supply decreased the negative impacts of sheath rot disease on rice growth by alleviating lipid peroxidation and maintaining lipid homeostasis.

Identification and First Report of Fusarium andiyazi Causing Sheath Rot of Zizania latifolia in China

Zizania latifolia is a perennial plant native to East Asia. The swollen culm of Z. latifolia is a popular vegetable and traditional herbal medicine consumed in China and some other Asian countries. From 2019 to 2021, a sheath rot disease was found in Zhejiang Province of China. Symptoms mainly occurred in the leaf sheath showing as brown necrotic lesions surrounded by yellow halos. The pathogen fungal isolates were isolated from the affected sheaths. Ten representative isolates were selected for morphological and molecular identification by phylogenetic analyses of the translation elongation factor 1-α (TEF1) and the RNA polymerase II subunit beta (RPB2) gene regions. Based on the combined datasets, the fungal isolates were identified as Fusarium andiyazi. Koch’s postulates were confirmed by pathogenicity test, re-isolation and re-identification of the fungal isolates. To the best of our knowledge, this is the first report of sheath rot caused by F. andiyazi in Z. latifolia in China.

Development of a Rapid Sporulation Method of Fusarium graminearum Using Liquid Cultivation

Fusarium graminearum is an important fungus causing a variety of maize diseases, including stalk rot, ear rot and sheath rot. However, conidia of F. graminearum are not easily obtained under normal culture conditions, which seriously affects the identification and pathogenicity assessment of the isolates and screening of resistance sources. This study was undertaken to develop and utilize a rapid sporulation technique of F. graminearum using liquid cultivation, which could meet the needs of various tests. The results show that the optimum conditions for sporulation of F. graminearum were as follows: culture medium, 0.154 mol/L saline; temperature, 28-30℃; incubation time, 96 h; initial pH, 9-10; illumination, continuous ultraviolet light; and shaking speed, 150 rpm. Using this culture method, conidial concentration of tested F. graminearum strains can reach more than 1.5×105 conidia/ml. Compared with the existing methods using mung bean (MB) and carboxylmethyl cellulose (CMC) as matrix, saline is relatively low expensive, and the culture process is relatively quick. Overall, this study provided a systematic, rapid, and simple method to obtain a large number of conidia of F. graminearum.

Chemical or genetic alteration of proton motive force results in loss of virulence of Burkholderia glumae , the cause of rice bacterial panicle blight.

Rice is an important source of food for more than half the world’s population. Bacterial panicle blight (BPB) is a disease of rice characterized by grain discoloration or sheath rot caused mainly by Burkholderia glumae . B. glumae synthesizes toxoflavin, an essential virulence factor, that is required for symptoms of the disease. The products of the tox operons, ToxABCDE and ToxFGHI, are responsible for the synthesis and the proton motive force (PMF)-dependent secretion of toxoflavin, respectively. The DedA family is a highly conserved membrane protein family found in most bacterial genomes that likely function as membrane transporters. Our previous work has demonstrated that absence of certain DedA family members results in pleiotropic effects, impacting multiple pathways that are energized by PMF. We have demonstrated that a member of the DedA family from Burkholderia thailandensis , named DbcA, is required for the extreme polymyxin resistance observed in this organism. B. glumae encodes a homolog of DbcA with 73% amino acid identity to Burkholderia thailandensis DbcA. Here, we created and characterized a B. glumae Δ dbcA strain. In addition to polymyxin sensitivity, B. glumae Δ dbcA is compromised for virulence in several BPB infection models and secretes only low amounts of toxoflavin (∼15% of wild type levels). Changes in membrane potential in B. glumae Δ dbcA were reproduced in the wild type strain by the addition of sub-inhibitory concentrations of sodium bicarbonate, previously demonstrated to cause disruption of PMF. Sodium bicarbonate inhibited B. glumae virulence in rice suggesting a possible non-toxic chemical intervention for bacterial panicle blight. IMPORTANCE Bacterial panicle blight (BPB) is a disease of rice characterized by grain discoloration or sheath rot caused mainly by Burkholderia glumae . The DedA family is a highly conserved membrane protein family found in most bacterial genomes that likely function as membrane transporters. Here, we constructed a B. glumae mutant with a deletion in a DedA family member named dbcA and report a loss of virulence in models of BPB. Physiological analysis of the mutant shows that the proton motive force is disrupted, leading to reduction of secretion of the essential virulence factor toxoflavin. The mutant phenotypes are reproduced in the virulent wild type strain without an effect on growth using sodium bicarbonate, a nontoxic buffer that has been reported to disrupt the PMF. The results presented here suggest that bicarbonate may be an effective antivirulence agent capable of controlling BPB without imposing an undue burden on the environment.

Symptoms and pathogens diversity of Corn Fusarium sheath rot in Sichuan Province, China

To elucidate the symptoms and pathogens diversity of corn Fusarium sheath rot (CFSR), diseased samples were collected from 21 county-level regions in 12 prefecture-level districts of Sichuan Province from 2015 to 2018 in the present study. In the field, two symptom types appeared including small black spots with a linear distribution and wet blotches with a tawny or brown color. One hundred thirty-seven Fusarium isolates were identified based on morphological characteristics and phylogenetic analysis (EF1-α), and Koch’s postulates were also assessed. The results identified the isolates as 8 species in the Fusarium genus, including F. verticillioides, F. proliferatum, F. fujikuroi, F. asiaticum, F. equiseti, F. meridionale, F. graminearum and F. oxysporum, with isolation frequencies of 30.00, 22.67, 15.33, 7.33, 6.00, 5.33, 3.33 and 1.33%, respectively. Fusariumverticillioides and F. proliferatum were the dominant and subdominant species, respectively. Two or more Fusarium species such as F. verticillioides and F. proliferatum were simultaneously identified at a mixed infection rate of 14.67% in the present study. The pathogenicity test results showed that F. proliferatum and F. fujikuroi exhibited the highest virulence, with average disease indices of 30.28 ± 2.87 and 28.06 ± 1.96, followed by F. equiseti and F. verticillioides, with disease indices of 21.48 ± 2.14 and 16.21 ± 1.84, respectively. Fusarium asiaticum, F. graminearum and F. meridonale showed lower virulence, with disease indices of 13.80 ± 2.07, 11.57 ± 2.40 and 13.89 ± 2.49, respectively. Finally, F. orysporum presented the lowest virulence in CFSR, with a disease index of 10.14 ± 1.20. To the best of our knowledge, this is the first report of F. fujikuroi, F. meridionale and F. asiaticum as CFSR pathogens in China.