Date of Award

1997

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Plant Pathology and Crop Physiology

First Advisor

Raymond W. Schneider

Abstract

In order to assess the potential of biological control of rice seedling disease, detailed etiological information was required, i.e. when and for how long are seedlings susceptible and which portions of the seed and seedling are susceptible. Contaminant-free seeds were required for these studies. For this purpose, rice seeds were soaked for 2 hrs in freshly prepared 2.6% sodium hypochlorite adjusted to pH 7.0 with a final concentration of 0.5 M potassium phosphate. This treatment was not only very effective in disinfesting seeds but also seemed to promote seedling growth. On the basis of this observation, a study was conducted regarding the direct effect of sodium hypochlorite on rice seedling growth. Seedling growth following seed treatments with HgCl$\sb2$ followed by NaOCl and NaOCl followed by sterile water was significantly greater than seeds treated with sterile water followed by sterile water, HgCl$\sb2$ followed by sterile water, and HgCl$\sb2$ followed by KH$\rm\sb2PO\sb4.$ These results indicated that sodium hypochlorite directly stimulated rice seedling growth apart from an indirect effect related to elimination of microbial contaminants. Infection rates of embryos were significantly higher than those of endosperms for Pythium arrhenomanes, P. myriotylum and P. dissotocum. Susceptibility to all three species was significantly reduced at 2 to 4 days after planting, and seedlings were completely resistant at 8 days after planting. These results indicated that rice seedlings became resistant to infection after a relatively short period of time even though they may still be submerged. Carbon utilization profiles were generated to identify specific carbon substrates that could be incorporated into seed coating formulations in order to provide a competitive advantage to Bacillus spp. used for the biological control agents relative to the pathogenic Pythium species. When incorporated into seed coating formulations, most carbon sources such as L-arabinose and D-galactose, preferentially utilized by the biological control agents, resulted in significantly less disease as compared to appropriate controls in greenhouse and field experiments. This enhanced level of disease control was not associated with higher populations of the introduced bacteria. These results indicated that seedling disease may be controlled with the carbon sources alone.

ISBN

9780591458671

Pages

105

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