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Monosporascus root rot/vine decline: a study of double-stranded (DS) RNA and its role in the pathogenesis of Monosporascus cannonballus on muskmelon
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Double-strand (ds) RNA has been shown to induce culture degeneration (slow growth, reduced sporulation, and pigment accumulation) and hypovirulence (on muskmelon) in the fungus, Monosporascus cannonballus. Five M cannonballus isolates with different dsRNA banding patterns, (Az9O-33-, Tx93-449+, Ca9l-17"+, Tx93-529+, and Tx93-314+'-) were examined in repeated greenhouse pathogenicity trials from 1995 to 1996 to determine if a specific set of dsRNA fragments were associated with hypovirulence and/or culture degeneration. There was some variation in virulence levels for isolates Ca9l-17 16+, Tx93-529+, and Tx93-314+/-. In contrast, isolate Tx93449 + was consistently hypovirulent and Az9O-33- was consistently virulent on muskmelon. Pooling greenhouse pathogenicity data from seven trials, isolates Ca9l96+-529+, and Tx93-314+'-caused less stunting than Az9O-33-and were partially 17 , Tx93 hypovirulent. In addition, these isolates had a culture phenotype similar to that of wildtype isolates. In contrast, isolate Tx93-449+ grew significantly slower (LSD, P=0.05) than all other isolates in radial growth assays and accumulated a yellow to brown pigment. Aniline blue staining of fungal mycelia in muskmelon seedlings infected with Tx93-449+ revealed that this isolate took three times longer to penetrate and colonized root tissue compared to the virulent isolate, Az9O-33-. Three hypovirulent isolates (Tx93-449+, Ca9l-17 95+, and Tx9l-18@) were tested for their ability to affect the virulence of an aggressive isolate (Az9O-33-) in controlled pathogenicity tests. Results from two greenhouse trials and one field microplot trial suggest that virulence was reduced when the hypovirulent isolate was added to the inoculum mixture at a ratio 1: IO (virulent:hypovirulent). At this ratio, plants were less stunted than those inoculated with the virulent isolate alone, suggesting that biocontrol of D might be possible. Two isolates (Tx93-529+ and Ca9l-17 96+) were used to make a CDNA library derived from their dsRNAs. Initially, only fungal ribosomal RNA-CDNA clones were generated from Tx93-529+. After introducing additional purification techniques, nine CDNA clones (400 bp to 1500 bp) were obtained from the dsRNAs associated with Ca9l-17 96+ . These clones are currently being sequenced and compared with other known dsRNA mycoviruses. The techniques developed for cloning dsRNAs in M. cannonhallus will be used to derive CDNA clones from the dsRNAs in the hypovirulent isolate, Tx93-449+.
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Includes bibliographical references: p. 68-75.
Issued also on microfiche from Lange Micrographics.
Batten, Jeffrey Samuel (1997). Monosporascus root rot/vine decline: a study of double-stranded (DS) RNA and its role in the pathogenesis of Monosporascus cannonballus on muskmelon. Master's thesis, Texas A&M University. Available electronically from
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