Mature watermelon vine decline: Suppression with fumigants of a soil-borne problem and association with Rhizopycnis vagum

Abstract

Mature watermelon vine decline (MWVD) is a late-season disease of unknown etiology, characterized by vine collapse and discolored, much reduced root systems. While several new disorders in watermelon have been reported recently, this problem appears specific to the Midwest. To test for a biological cause and methods to suppress this disorder, soil was collected from two southern Indiana fields with histories of MWVD for microplot and greenhouse experiments. Watermelons grew best in microplots treated with methyl bromide or methyl iodide. Roots of plants growing in the untreated MWVD soil were severely necrotic; those from the fumigation treatments appeared healthy. In heating experiments, watermelon seedlings were grown in autoclaved soil mixed with 10% MWVD soil that was either untreated or heated at 40, 50, 60, 70 or 80 °C for 30 min. Watermelon plants in the untreated controls and the 40 and 50 °C treatments had reduced top dry weights with decreased fibrous roots and associated discoloration in contrast to treatments of equal to and greater than 60 °C. Representative cultures from necrotic roots were identified as Rhizopycnis vagum by morphological analysis and sequencing the ITS region of the ribosomal DNA. PCR analysis with specific primers confirmed that R. vagum was present in roots from untreated MWVD soil but not in symptomless roots. This is the first report of R. vagum associated with plants suffering from MWVD in Indiana and indicates that this fungus occurs much farther north in the U.S. than known previously.