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Title: Evidence for the role of an invasive weed in widespread occurrence of phytoplasmal diseases in diverse vegetable crops: implications from lineage-specific molecular markers

item CAI, HONG - Yunnan Agricultural University
item WANG, LIANCHUN - Yunnan Agricultural University
item YANG, ZIXIANG - Yunnan Agricultural University
item WAN, QIONGLIAN - Yunnan Agricultural University
item Wei, Wei
item Davis, Robert
item Zhao, Yan

Submitted to: Crop Protection
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 7/20/2016
Publication Date: 9/1/2016
Citation: Cai, H., Wang, L., Yang, Z., Wan, Q., Wei, W., Davis, R.E., Zhao, Y. 2016. Evidence for the role of an invasive weed in widespread occurrence of phytoplasmal diseases in diverse vegetable crops: Implications from lineage-specific molecular markers. Crop Protection. 89:193-201. doi: 10.1016/j.cropro.2016.07.025.

Interpretive Summary: Parthenium hysterophorus, commonly known as whitetop weed, is an annual or short-lived perennial plant native to Central and South Americas. With a vigorous growth habit and high environment adaptability, the weed is highly invasive and has been naturalized in subtropical and tropical regions of all Continents except for Europe. Farmland invasion of whitetop weed can cause significant production losses. In the present study, working together with researchers in China, ARS scientists found that whitetop weed can host a small bacterium called phytoplasma and, in turn, the phytoplasma-infected weed may serve as a source of infection, spreading the bacterium to other plants, causing diseases in agricultural crops such as cowpea, sword bean, tomato, lettuce, and spinach. Findings from the work further underscore the need to control the noxious weed. The work is important to farmers and extension personnel for pest control, disease diagnosis, and agro-ecosystem management. Since the particular bacterium that can infect whitetop weed and vegetable crops has not been found in the United States thus far, this information is critical to regulatory agencies for preventing the exotic pest from being introduced into the U.S.

Technical Abstract: During the period from 2011 to 2013, several plant diseases repeatedly occurred in vegetable crops grown in Yuanmou County, Yunnan Province, China. Affected plants included cowpea, sword bean, string bean, tomato, lettuce, and water spinach. The diseased plants exhibited symptoms of witches’-broom growth and floral deformations, linking each disease to phytoplasmal infection. Phylogenetic and virtual RFLP analyses of the phytoplasmal 16S rRNA gene sequences amplified from diseased plants revealed that all of the individual strains present in the diverse vegetable plants were affiliated with a single species (‘Candidatus Phytoplasma aurantifolia’) and belonged to the same ribosomal subgroup (16SrII-A). In a survey to identify the possible source of the phytoplasmal infections, a common thread emerged: the areas surrounding the affected vegetable fields were extensively invaded by parthenium weeds (Parthenium hysterophorus), and many individual weed plants exhibited abnormal morphologies typical of infection by phytoplasmas. Since the ecosystems of Yuanmou are insect-rich, and since parthenium weed is known to attract diverse leafhoppers, we hypothesized that parthenium weed poses a substantial risk as a reservoir of phytoplasmas that cause widespread diseases in agricultural crops. To test this hypothesis, we subjected phytoplasma strains present in the vegetable crops and in parthenium weed plants to genotyping. Results from genotyping based on 16S rRNA genes and lineage-specific immunodominant membrane protein genes revealed that the vegetable-infecting phytoplasmas and the parthenium weed phytoplasma belong to the same genetic lineage. The findings support the hypothesized role of the weed as a source of phytoplasmal infection of crops in the Yuanmou geographic region, and they warrant further in-depth studies to assess the impact of farmland invasions by the noxious weed and the possible application of measures for improved weed control.