Graduate Thesis Or Dissertation
 

A compound from onion roots inhibitory to Pyrenochaeta terrestris

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  • In vitro tests showed mycelial growth of Pyrenochaeta terrestris was inhibited by root and bulb extracts from the pink root resistant onion cultivar Nebuka, but not by extracts from the susceptible Southport White Globe. The active principle was shown to be fungistatic rather than fungicidal. It is present in both infected and non-infected roots indicating invasion by the pathogen is not required for its production. Root and bulb extracts from Nebuka were passed through a Bio-Gel P10 column and 25 fractions, 10 ml each, were collected. Bioassay tests showed inhibitory activity was confined to two fractions. Active extract applied to Silica Gel-G plates and developed in a mixture of methyl ethyl ketone, pyridine, water and glacial acetic acid (70:15:15:2) indicated the inhibitory compound had an Rf value of 0.44, and reacted positively to ninhydrin, ammoniacal silver nitrate, tetrazotized benzidine and diazotized sulphanilic acid suggesting an amino-phenolic complex. Quantitative estimation of total phenols in extracts from onion cultivars ranging from highly resistant to completely susceptible showed resistant cultivars contained more total phenols than susceptible ones. The inhibitory effect of the compound was destroyed by heating or by exposure to extreme acid or base conditions. Volatile oils liberated by extracts from Nebuka and Southport White Globe onions had little affect on mycelial growth of P. terrestris. Attempts to demonstrate that a fungistatic compound is extruded from roots of Nebuka were unsuccessful.
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