First Report of Phytoplasma ‘Candidatus Phytoplasma aurantifolia’ Associated with Purple Top Diseased Potatoes (Solanum tuberosum) in Guangdong Province, China

Abstract

China is the largest producer of potatoes in the world. In 2014, a total of 96 million metric tons of potatoes were produced on more than 5.6 million hectares. Recognizing its importance, the Chinese central government elevated potatoes in 2010 to the status of the fourth major staple crop, following rice, wheat, and corn. In recent years, phytoplasma-associated diseases are becoming increasingly important in potato production, with an incidence rate of 80 to 100% in some commercial fields, according to disease surveys conducted by our team from 2005 to 2017 in seed and commercial fields of Yunnan Province and Inner Mongolia Autonomous Region. In China, six phytoplasma 16Sr groups, associated with vegetables, cereals, flowers, shrubs, fruit, and ornamental trees, in 29 genera, have been reported (https://plantpathology.ba.ars.usda.gov/cgi-bin/​resource/phytoclass.cgi?strainsearchbox=&countrysearchbox=China&submit_country_​search=search&diseasesearchbox=&hostsearchbox=&group_popup=I&subgroup_popup=​all). We found that phytoplasma group 16SrI, 16SrVI, and 16SrXII were associated with potatoes. Information on phytoplasma associated with potatoes in many potato growing areas remains little known (Cheng et al. 2015). Guangdong Province, especially Huidong County, is one of the major winter potato production areas in China. During a potato disease survey conducted in Huidong County, Guangdong Province (22.9850°N, 114.7200°E) in March 2014, prior to harvest, eight samples collected displayed symptoms of proliferation, upright growth, purpling of apical leaves, shortened and thick-ended stolons, stolons with multiple tubers, and formation of aerial tubers—all characteristics of phytoplasma disease. The incidence of phytoplasma in those fields ranged from 20 to 35%. Four samples were collected from asymptomatic plants at the same time. Total DNA was extracted from tissues (leaves, stems, and roots) of symptomatic and asymptomatic plants using a DNeasy Plant Mini Kit (Qiagen, Valencia, CA) according to the manufacturer’s instructions. A nested polymerase chain reaction (PCR) was performed by using primer pair P1/P7, followed by P1A/16S-SR (Lee et al. 2004). A 1.5-kb PCR fragment was amplified from the DNAs of all symptomatic plants and none of the symptomless plants. The PCR products were cloned into a pCR8/GW/TOPO vector (Invitrogen, Carlsbad, CA) and sequenced by GENEWIZ (South Plainfield, NJ). The potato phytoplasma 16S rDNA sequence (GenBank accession no. KM212951) was analyzed by iPhyClassifier software (https://plantpathology.ba.ars.usda.gov/cgi-bin/resource/iphyclassifier.cgi) (Zhao et al. 2013), and the results showed that the 16S rRNA gene sequence had a 99.7% sequence identity to the reference strain (GenBank accession no. Y10097) for ‘Candidatus Phytoplasma aurantifolia’ (White et al. 1998). The restriction fragment length polymorphism similarity is identical (coefficient 1.00) to the reference pattern of the 16Sr group II, subgroup A (GenBank accession no. L33765). The phylogenetic tree was constructed using representative phytoplasma strains reported from China by the neighbor-joining method with 1,000 bootstrap replicates in the program MEGA 4 (Tamura et al. 2007). The results indicated that the potato phytoplasma 16S rDNA sequence isolated in this study clustered with two 16SrII group strains reported from China (tomato yellows strain SGW and kidney bean little leaf [Dong et al. 2013]) and from Korea (pigeon pea witches’ broom). ‘Ca. P. australiense’ is on the A1 regulated organism list for Canada and Bahrain and quarantined by the United States. To our knowledge, this is the first report of ‘Ca. P. australiense’ associated with potatoes in China. Phytoplasmas from Guangdong Province may be easily spread to spring and summer potato production areas through tubers. This might be one of the factors accounting for the increasingly widespread and important of phytoplasma-associated diseases on potatoes in China. Therefore, further study on identifying alternate host plants, potential vectors, and geographic distribution will be helpful to control this phytoplasmal disease.