First Report of Blossom Blight of Strawberry Caused by Xanthomonas fragariae and Cladosporium cladosporioides in California

Pan-Genomic Analysis Identifies the Chinese Strain as a New Subspecies of Xanthomonas fragariae

Xanthomonas fragariae is classified as a quarantine pathogen by the European and Mediterranean Plant Protection Organization. It commonly induces typical angular leaf spot (ALS) symptoms in strawberry leaves. X. fragariae strains from China (YL19, SHAQP01, and YLX21) exhibit ALS symptoms in leaves and more severe symptoms of dry cavity rot in strawberry crowns. Conversely, strains from other countries do not cause severe dry cavity rot symptoms in strawberries. After employing multilocus sequence analysis (MLSA), average nucleotide identity (ANI), and amino acid identity (AAI), we determined that Chinese strains of X. fragariae are genetically distinct from other strains and can be considered a new subspecies. Subsequent analysis of 63 X. fragariae genomes published at NCBI using IPGA and EDGAR3.0 revealed the pan-genomic profile, with 1,680 shared genes present in all 63 strains, including 71 virulence-related genes. Additionally, we identified 123 genes exclusive to all the Chinese strains, encompassing 12 virulence-related genes. The qRT-PCR analysis demonstrated that the expression of XopD, XopG1, CE8, GT2, and GH121 out of 12 virulence-related genes of Chinese strains (YL19) exhibited a constant increase in the early stages (6, 24, 54, and 96 hours postinoculation [hpi]) of strawberry leaf infected by YL19. So, the presence of XopD, XopG1, CE8, GT2, and GH121 in Chinese strains may play important roles in the early infection process of Chinese strains. These findings offer novel insights into comprehending the population structure and variation in the pathogenic capacity of X. fragariae.

Systemic Colonization of Xanthomonas fragariae Strain YL19 Causing Dry Cavity Rot of Strawberry Crown Tissue in China

Xanthomonas fragariae usually causes angular leaf spot (ALS) of strawberry, a serious bacterial disease in many strawberry-producing regions worldwide. Recently, a new strain of X. fragariae (YL19) was isolated from strawberry in China and has been shown to cause dry cavity rot in strawberry crown. In this study, we constructed a green fluorescent protein (GFP)-labeled Xf YL19 (YL19-GFP) to visualize the infection process and pathogen colonization in strawberries. Foliar inoculation of YL19-GFP resulted in the pathogen migrating from the leaves to the crown, whereas dip inoculation of wounded crowns or roots resulted in the migration of bacteria from the crowns or roots to the leaves. These two invasion types both resulted in the systematic spread of YL19-GFP, but inoculation of a wounded crown was more harmful to the strawberry plant than foliar inoculation. Results increased our understanding of the systemic invasion of X. fragariae, and the resultant crown cavity caused by Xf YL19.

Complete genome sequence of an Israeli isolate of Xanthomonas hortorum pv. pelargonii strain 305 and novel type III effectors identified in Xanthomonas

Xanthomonas hortorum pv. pelargonii is the causative agent of bacterial blight in geranium ornamental plants, the most threatening bacterial disease of this plant worldwide. Xanthomonas fragariae is the causative agent of angular leaf spot in strawberries, where it poses a significant threat to the strawberry industry. Both pathogens rely on the type III secretion system and the translocation of effector proteins into the plant cells for their pathogenicity. Effectidor is a freely available web server we have previously developed for the prediction of type III effectors in bacterial genomes. Following a complete genome sequencing and assembly of an Israeli isolate of Xanthomonas hortorum pv. pelargonii - strain 305, we used Effectidor to predict effector encoding genes both in this newly sequenced genome, and in X. fragariae strain Fap21, and validated its predictions experimentally. Four and two genes in X. hortorum and X. fragariae, respectively, contained an active translocation signal that allowed the translocation of the reporter AvrBs2 that induced the hypersensitive response in pepper leaves, and are thus considered validated novel effectors. These newly validated effectors are XopBB, XopBC, XopBD, XopBE, XopBF, and XopBG.

First Report of Cladosporium Blossom Blight Caused by Cladosporium cladosporioides on Calliandra haematocephala in China

Calliandra haematocephala Hassk., commonly called red powder puff, is widely cultivated as an ornamental plant in Taiwan, Hainan, Guangdong and Fujian in China (CAS, 1988). The flowers are dark crimson with conspicuous stamens, which give them the appearance of powder-puffs. Blossom blight on C. haematocephala was first observed in early January 2019 on plants grown on the university campus as well as in parks in Fuzhou city, with nearly 80% of flowers on individual plants infected. At various locations in the city, disease incidence was 100%. Symptoms appeared as grayish green fungal growth on the stamens with the entire flower eventually turning black and covered with masses of fungal spores. Fifteen single spore isolates obtained from nine necrotic stamen samples were purified and cultured on Potato dextrose agar (PDA) plates at 24 ℃.The resultant fungal colonies were olivaceous-green to olivaceous-brown and had a velvet-like appearance. Conidiophores were smooth-walled, solitary, non-nodulose, and measuring 40 to 340 × 3 to 4 µm (n=50). Ramoconidia were cylindrical-oblong or slightly curved with 0 to 3 septa, and measuring 10 to 25 × 3 to 4 µm (n=50). Conidia were smooth-walled and prolifically produced in long chains. Terminal conidia were aseptate, subglobose, ovoid to limoniform, measuring 3 to 6 × 2 to 2.5 µm (n=50). Intercalary conidia were elliptical to limoniform or subcylindrical, aseptate, measuring 5 to 12 × 2.5 to 3 µm (n=50). On the basis of its morphology, the causal organism was identified as Cladosporium cladosporioides (Bensch et al. 2010). For molecular identification, pure cultures of five single-spore isolates were used for DNA extraction. A fragment in the ITS regions of the fungal rDNA, the ACT and the TEF1-α, was amplified using the primers ITS1/ITS4, ACT-512F/ACT-783R, and EF1728 F/EF1-986R. The DNA sequences obtained from all five isolates were identical. The resulting ITS (MK720012) and ACT (MN013164), and TEFl-α (MK752020) sequences from a representative isolate MRCIM19 were 98-100% identical to the C. cladosporioides accessions (ITS: MH863979, MG228421; ACT: HM148509, JF499878, HM148532; TEFl-α: JF499872). To test pathogenicity, a spore suspension (1×105 conidia/mL) was prepared from a seven- day- old culture of isolate MRCIM19 and 10 mL of the suspension was sprayed onto six flowers on each of three C. haematocephala plants. Sterile distilled water was sprayed onto three flowers of two plants as control. The inoculated flowers were covered with plastic bags which were removed two days post inoculation. Disease symptoms were recorded on each flower at 10 days post inoculation. Based on the morpho-molecular characters, the re-isolated fungus from the inoculated flowers was C. cladosporioides. This fungus was previously reported to cause blossom blight in strawberry in the USA and Korea (Gubler et al. 1999; Nam et al. 2015). Although it has been reported from many plants (Zhang 2003) in China, this is the first report of C. cladosporioides on C. haematocephala worldwide. References Bensch, K. et al. 2010. Stud Mycol. 67:1-94. Chinese Academy of Sciences (CAS), 1988. Flora Republicae Popularis Sinicae Editorial Committee, Beijing Sci. Press., 39: 38. Gubler, W. D. et al. 1999. Plant Dis. 83:400. Nam, M. H. et al. 2015. Microbiol. 43: 354-359. Zhang Z., Ed. 2003. Flora fungorum sinicorum, Vol. 14. Cladosporium, Fusicladium, Pyricularia. Beijing Science Press. 297.

Management of Xanthomonas fragariae with Pre- and Postharvest Treatments to Overcome Trade Barriers for California Strawberries

Xanthomonas fragariae, the causal agent of angular leaf spot (ALS) of strawberry, is a quarantine pathogen in some export markets, causing trade restrictions and economic loss to the California fresh-market strawberry industry. Preharvest chemical management options are limited to copper, and there are no postharvest treatments available that reduce populations of the pathogen if ALS is detected at an export destination. Here, we report high preharvest efficacy for the experimental bactericide amino thiadiazole and the commercial product zinc thiadiazole, alone and in mixtures with low rates of copper or the antibiotic kasugamycin, with average disease incidence reduction of up to 92.8% compared with the control. Although effective against quarantine insect pests of strawberry, postharvest methyl bromide fumigation was ineffective against X. fragariae in diseased plant tissue at a standard commercial rate. Postharvest propylene oxide fumigation, used for decades by the California nut industries for insect and microbial disinfestation, significantly reduced X. fragariae populations in infected leaflet tissues by at least 2.5-log compared with controls at a dose of ≥142 µg/ml for 2 h at 15 to 20°C. Fumigated leaflets showed little to no phytotoxicity at effective rates, and fumigated fruit were not significantly affected in appearance or susceptibility to postharvest gray mold or Rhizopus rot following storage at 2°C for 3 days and at 15°C for an additional 5 days. Together, these new treatments offer potential strategies for establishing a systems approach with preharvest treatments significantly reducing the risk of ALS on plants and, in response to quarantine detections, a postharvest fumigation treatment that reduces viable pathogen populations in existing lesions.

Características físico-químicas e microbiológicas de morango minimamente processado

O presente trabalho teve como objetivo avaliar as alterações físico-químicas e microbiológicas de morangos submetidos ao processamento mínimo. Foram avaliados os efeitos da lavagem com cloreto de cálcio ou polietilenoglicol na microbiota contaminante e na textura dos morangos e da sanificação com ozônio gasoso ou clorado orgânico. Análises de textura, cor, perda de massa e microbiota contaminante foram feitas durante o armazenamento a 5 °C por 12 dias em embalagens envoltas com uma a quatro camadas de filme de cloreto de polivinil (PVC). A adição de até 1,5% de cloreto de cálcio ou de 0,5% de polietilenoglicol na água de lavagem não garantiu a manutenção da textura do morango ao final do período de armazenamento. A ozonização dos morangos por 60 minutos foi mais efetiva para reduzir (p < 0,05) a contagem de mesófilos aeróbios, fungos e leveduras e coliformes do que a ozonização por 30 minutos ou a imersão em solução de clorado orgânico. Os morangos armazenados em embalagens recobertas com três camadas de filme PVC apresentaram aumento na textura e na intensidade de escurecimento e redução na microbiota contaminante. Os principais fungos isolados de morangos minimamente processados durante o armazenamento pertenciam ao gênero Fusarium e à espécie Cladosporium cladosporioides.