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Title:   Author  item
                                            WESTPHAL, ANDREAS - PURDUE UNIVERSITYitem
                                            XING, LI JUAN - PURDUE UNIVERSITYitem
                                            Goodwin, Stephen

                                                         - *Steve*
  Submitted to: Plant Pathology
                                    Publication Type: Peer Reviewed Journal
                                    Publication Acceptance Date: 10/29/2010
                                    Publication  Date: 1/30/2011
                                    Citation: Westphal, A., Xing, L., Goodwin, S.B. 2011. Mature watermelon vine decline: evidence for the biological nature of a soil-borne problem. Plant Pathology. 30:111-117.Interpretive Summary: Mature watermelon vine decline (MWVD) is a late-season disease of unknown cause, characterized by vine collapse and discolored, reduced root systems. To test the hypothesis and provide possible identification of a biological cause of MWVD, soil was collected from southern Indiana fields with a history of the disease for experiments in small field plots and greenhouses. Growth of watermelon seedlings was greatest and no disease was present in soils that had been sterilized by heat or fumigant treatments, indicating a biological cause for MWVD. Four of seven cultures obtained from diseased watermelon roots were identified as the asexual fungus Rhizopycnis vagum by morphological analysis and by sequencing of the internal transcribed spacer (ITS) region of the ribosomal DNA. Molecular analysis with specific primers for polymerase chain reaction amplification confirmed that R. vagum was present in roots exposed to untreated MWVD soil but not in control plants grown in fumigated soil. These data indicate that R. vagum is strongly associated with MWVD and seems to affect watermelon preferentially. This is the first report of R. vagum as a potential cause of MWVD in Indiana. This information will be used by plant pathologists to design more effective measures to mitigate losses to watermelon growers caused by this disease. Plant breeders can use this information to select isolates of the fungus for improved identification of potential sources of resistance. The indication of possible host specificity can be used by plant geneticists and pathologists to analyze host-pathogen interactions and possibly to identify new sources of resistance in cantaloupe that could be transferred into watermelon for increased resistance to MWVD in the future.
                                Technical Abstract: Mature watermelon vine decline (MWVD) is a late-season disease of unknown etiology, characterized by vine collapse and discolored, reduced root systems. To test for a biological cause of MWVD, soil was collected from two southern Indiana fields with a history of the disease for microplot and greenhouse experiments. Growth of watermelon seedlings was greatest in microplots treated with methyl bromide, followed by methyl iodide. In greenhouse experiments, single watermelon seedlings were planted into 3-L plastic pots filled with one of three soils: non-amended autoclaved sand (control); 10% MWVD soil in autoclaved sand; and 10% MWVD soil fumigated with methyl bromide. Plant biomass was greater in the control and fumigated treatments. Roots of plants growing in the untreated soil were severely necrotic, while those from the other treatments appeared healthy. Four of seven cultures obtained from necrotic roots were identified as Rhizopycnis vagum by morphological analysis and by sequencing the internal transcribed spacer (ITS) region of the ribosomal DNA. In a second set of tests, heat treatments at 40, 50, 60, 70 or 80 °C were compared to controls consisting of plants grown in MWVD soil that was either autoclaved or left untreated. Watermelon plants in the untreated control and the 40 and 50 °C treatments had reduced top dry weights and decreased fibrous roots with associated discoloration compared to those grown in soil treated at 60 °C or higher. PCR analysis with specific primers confirmed that R. vagum was present in roots exposed to untreated MWVD soil but not in control plants grown in fumigated soil. These data indicate that R. vagum is strongly associated with MWVD and seems to affect watermelon preferentially. This is the first report of R. vagum as a potential cause of MWVD in Indiana.


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