First Report of Downy Mildew on Blackberry Caused by Peronospora sparsa in Georgia, U.S.A.

HomePlant DiseaseVol. 104, No. 3First Report of Downy Mildew on Blackberry Caused by Peronospora sparsa in Georgia, U.S.A. PreviousNext DISEASE NOTES OPENOpen Access licenseFirst Report of Downy Mildew on Blackberry Caused by Peronospora sparsa in Georgia, U.S.A.M. E. Ali, O. Hudson, S. Waliullah, J. Brock, B. Hayes, J. L. Williams-Woodward, and J. E. OliverM. E. Ali†Corresponding author: M. E. Ali; E-mail Address: ma49268@uga.eduhttp://orcid.org/0000-0002-5871-5055Department of Plant Pathology, University of Georgia, Tifton, GA 31793Search for more papers by this author, O. HudsonDepartment of Plant Pathology, University of Georgia, Tifton, GA 31793Search for more papers by this author, S. WaliullahDepartment of Plant Pathology, University of Georgia, Tifton, GA 31793Search for more papers by this author, J. BrockDepartment of Plant Pathology, University of Georgia, Tifton, GA 31793Search for more papers by this author, B. HayesUniversity of Georgia Cooperative Extension, Mitchell County, Camilla, GA 31730Search for more papers by this author, J. L. Williams-WoodwardDepartment of Plant Pathology, University of Georgia, Athens, GA 30602Search for more papers by this author, and J. E. OliverDepartment of Plant Pathology, University of Georgia, Tifton, GA 31793Search for more papers by this author AffiliationsAuthors and Affiliations M. E. Ali1 † O. Hudson1 S. Waliullah1 J. Brock1 B. Hayes2 J. L. Williams-Woodward3 J. E. Oliver1 1Department of Plant Pathology, University of Georgia, Tifton, GA 31793 2University of Georgia Cooperative Extension, Mitchell County, Camilla, GA 31730 3Department of Plant Pathology, University of Georgia, Athens, GA 30602 Published Online:30 Dec 2019https://doi.org/10.1094/PDIS-09-19-1962-PDNAboutSections ToolsAdd to favoritesDownload CitationsTrack Citations ShareShare onFacebookTwitterLinked InRedditEmailWechat Blackberries (Rubus L. subgenus Rubus) are small, round, aggregate berries comprising a cluster of tiny fruits called drupelets that grow on flowering shrubs or trailing vines (Clark et al. 2007). The acreage of blackberry production in Georgia has increased 10-fold in the last 15 years to 880 acres (NASS 2017). Downy mildew of blackberry, caused by the oomycete pathogen Peronospora sparsa, is a systemic disease that impacts blackberry production in many regions worldwide. Symptoms of downy mildew on blackberry include yellowing on the upper leaf surface and reddish-brown angular lesions with necrotic centers and chlorotic margins. In May 2019, leaf symptoms typical of blackberry downy mildew were observed on blackberry cultivar ‘Ouachita’ plantlets at a nursery in Mitchell County, Georgia. Initial identification was done based on the morphological characteristics of the sporangia and sporangiophores. Round or slightly ovoid sporangia were observed that were colorless to yellowish-brown, consistent with Peronospora sp. (Gubler and Rebollar-Alviter 2017). The identification of P. sparsa was further confirmed using nucleic acid-based molecular methods directly on infected plant tissue. Genomic DNA was extracted from symptomatic leaf tissue and tested for the presence of P. sparsa by targeting the P. sparsa-specific rDNA-ITS region using “P” primer pair P1 (5′-CACGTGAACCGTATCAACC-3′) and P2 (5′-GATAGGGCTTGCCCAGTAG-3′) with an amplicon size of 94 bp (Hukkanen et al. 2006). PCR reactions produced the expected sizes of the fragments for P. sparsa. The amplified products were then sequenced and subjected to BLAST analysis (https://blast.ncbi.nlm.nih.gov/Blast.cgi) for further identification. Sequence analysis revealed that the obtained partial sequence of P. sparsa-specific rDNA-ITS (GenBank submission no. MN477916) shared 100% identity to corresponding sequences from P. sparsa (accession no. Y15816). Samples were also tested for the presence of P. sparsa by real-time PCR using the same set of “P” primers. For real-time PCR, plant internal control primer pair R1 (5′-CAAACGACTCTCGGCAAC-3′) and R2 (5′-CCTCGGCCTAATGGCTT-3′) was used to target the highly conserved region of 5.8S plant ribosomal DNA (Hukkanen et al. 2006). The real-time PCR assay was performed in a Cepheid smart cycler II (Sunnyvale, CA) using iQ SYBR Green Supermix (BioRad Laboratories, Hercules, CA) in a 25-µl reaction according to the manufacturer’s protocol (Harper et al. 2010). All four samples extracted from symptomatic leaves were positive based on the real-time PCR assay. In addition, Koch’s postulates were fulfilled to verify the pathogenicity of the P. sparsa. Leaves from four uninfected blackberry plants (also cv. Ouachita) were surface sterilized, laid under sporulating leaf tissue, and incubated at 17°C with a 12-h photoperiod in a moist chamber. After 16 days, on the previously uninfected leaves, sporangiophores and sporangia consistent with P. sparsa developed. As a control, additional leaves from the same four plants were surface sterilized, placed in a moist chamber, incubated at 17°C with a 12-h photoperiod, and examined 16 days later. No lesions or sporulation developed on the uninoculated controls. To the best of our knowledge, this is the first report of P. sparsa causing downy mildew on blackberry in Georgia. Our findings suggest that Georgia blackberry growers should be aware of this disease and accordingly establish management measures for P. sparsa.The author(s) declare no conflict of interest.References:Clark, J. R., et al. 2007. Page 19 in: Plant Breeding Reviews. Wiley, Hoboken, NJ. https://doi.org/10.1002/9780470168035.ch2 Crossref, Google ScholarGubler, W. D., and Rebollar-Alviter, A. 2017. Page 25 in: Compendium of Raspberry and Blackberry Diseases and Pests, 2nd Ed. APS Press, St. Paul, MN. Google ScholarHarper, S. J., et al. 2010. Phytopathology 100:1282. https://doi.org/10.1094/PHYTO-06-10-0168 Link, ISI, Google ScholarHukkanen, A., et al. 2006. Eur. J. Plant Pathol. 116:225. https://doi.org/10.1007/s10658-006-9054-z Crossref, ISI, Google ScholarNational Agricultural Statistics Service (NASS). 2017. 2017 Census of Agriculture. Retrieved September 19, 2019, from https://www.nass.usda.gov/​Publications/AgCensus/​2017/Full_Report/​Volume_1,_Chapter_2_​US_State_Level/​st99_2_0033_0033.pdf. Google ScholarThe author(s) declare no conflict of interest.DetailsFiguresLiterature CitedRelated Vol. 104, No. 3 March 2020SubscribeISSN:0191-2917e-ISSN:1943-7692 DownloadCaptionPathogenicity of Lasiodiploidia pseudotheobromae in a coffee plant 3 days after inoculation (R. L. Freitas-Lopes et al.). Photo credit: U. P. Lopes. Seedling blight of soybean caused by soilborne pathogens (J. R. Lamichhane et al.). Photo credit: M. I. Chilvers. Metrics Downloaded 3,002 times Article History Issue Date: 3 Mar 2020Published: 30 Dec 2019First Look: 11 Nov 2019Accepted: 2 Nov 2019 Page: 996 Information© 2020 The American Phytopathological SocietyKeywordsblackberryPeronospora sparsadowny mildewoomycete pathogenThe author(s) declare no conflict of interest.