First Report of a New Highly Virulent Pathotype of Sunflower Downy Mildew (Plasmopara halstedii) Overcoming the Pl8 Resistance Gene in Europe
Downy mildew of sunflower (Helianthus annuus L.), caused by the oomycete Plasmopara halstedii, is a major constraint to production worldwide. The inbred line RHA 340 has been extensively used to produce resistant hybrids. It harbors the Pl8 gene derived from H. argophyllus Torrey and Gray (Miller and Gulya 1988), providing resistance to all pathotypes of P. halstedii in Europe described to date (Markell et al. 2016). In spring 2017, sunflowers with downy mildew were observed in one commercial field in Cortona (Italy), where a hybrid allegedly carrying Pl8 was grown. In June 2018, plants showing stunting and chlorotic leaves were collected in commercial fields of sunflower hybrids carrying Pl8 in Gers county in France (three fields) and in Cortona (Arezzo county) in Italy (two fields). Disease incidence ranged between 40 and 70%. On the underside of chlorotic lesions, P. halstedii-like sporulation was observed particularly in young leaves. One isolate was recovered from a single plant in each field, and the morphology of sporangiophores and sporangia led to the identification of P. halstedii Farl. Berl. & de Toni (Hall 1989). The identity of the pathogen was molecularly confirmed by amplification of the nuclear DNA coding for the large ribosomal unit (28S rDNA) using LR0R and LR6-O primers (Riethmüller et al. 2002). Sequences between 1,236 and 1,048 bp and 99% identical to P. halstedii (EF553469.1) in the NCBI database were deposited in GenBank (accessions MK249853, MK249854, MK249862, MK249876, and MK294561). The pathotype of the five isolates was determined by inoculation of nine differential sunflower inbred lines according to previously used methodology (Molinero-Ruiz et al. 2008). Additionally, RHA 340, which is used as the donor of resistance in our program, and the two commercial hybrids affected in the prospected fields were included. For each differential and isolate, two replications of 10 seedlings were used. Seedlings with 2- to 5-mm-long radicles were immersed in suspensions of 40 × 103 sporangia/ml for 4 h at 18°C, transplanted in perlite, and grown in a greenhouse (15 to 25°C, 14-h photoperiod) until expansion of the first pair of true leaves. Pathogen sporulation was enhanced by plant incubation for 24 h at 20°C and 100% relative humidity in darkness. Genotype resistance was recorded by the absence of sporulation on the first pair of true leaves, and susceptibility was considered when intense sporulation on cotyledons and leaves occurred. The five isolates were characterized as pathotype 714 and pathogenic to the genotypes carrying Pl8 (sporulation in cotyledons and leaves). Commercial hybrids with Pl8 have been grown throughout Europe and remained asymptomatic since 2012. Pathotype 714 has been reported in the Czech Republic, France, Hungary, and the United States (Viranyi et al. 2015) and is now identified also in Italy. However, the most important outcome of this research is the finding in Europe of this new pathotype, provisionally termed as 714-Pl8, which overcomes Pl8. This work demonstrates the need for updating the system for the identification of P. halstedii pathotypes by including RHA 340 as a publicly available line carrying Pl8. Breeding programs focused in European markets may be compromised from the emergence of these 714-Pl8 populations. Until sources of resistance are identified, monitoring should be implemented to prevent spread.
| Main Authors: | , , , |
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| Format: | artículo biblioteca |
| Published: |
American Phytopathological Society
2020-02
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| Subjects: | Oomycetes, Field crops, Oilseeds and legumes, Epidemiology, Disease development and spread, |
| Online Access: | http://hdl.handle.net/10261/227587 |
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