The research described in this dissertation examined peach replant disease and two nematode biological control systems using traditional plant pathology methods and molecular microbial ecology methods.;In Chapter 1, the experiments identified microbes associated with peach replant disease in soils with various levels of disease symptoms. To identify bacteria, fungi and oomycetes associated with the replant disease, culture and culture-independent analyses were performed on DNA extracted from plant roots. Among the most abundant bacterial operational taxonomic units, 27 were negatively correlated with peach top weights while 10 were positively correlated. Among the most abundant fungi and oomycetes, negative and positive associations were identified between P. vexans and Trichoderma spp. and peach top weights, respectively, and verified with sequence-selective quantitative PCR analyses.;In Chapter 2, the population dynamics between Dactylella oviparasitica and Heterodera schachtii were investigated. Higher initial D. oviparasitica populations were associated with lower final H. schachtii populations. Regression models showed that the initial densities of D. oviparasitica were only significant when predicting the final densities of H. schachtii J2 and eggs as well as fungal egg parasitism, while the initial densities of J2 were significant for all final H. schachtii measurements. H. schachtii-associated D. oviparasitica populations were greatly reduced in nematodes collected from soil compared to nematodes collected from roots. Finally, phylogenetic analysis of rRNA genes suggested that D. oviparasitica belongs to a clade of nematophagous fungi with a large geographical distribution.;In Chapter 3, three strains of Pochonia chlamydosporia var. chlamydosporia were genetically characterized and examined for their biocontrol efficacies against Meloidogyne incognita. All strains exhibited different patterns with the enterobacterial repetitive intergenic consensus (ERIC) PCR analysis. Strains 1 and 4 were similar in the PCR analyses of beta-tubulin and the rRNA internal transcribed spacer. In greenhouse trials, all strains reduced the numbers of nematode egg masses. Strain 4 reduced almost 50% of the eggs, and reduced the numbers of J2 and root-galling. A newly developed small subunit rRNA-based PCR analysis differentiated strain 4 from the others, and could potentially be used as a screening tool for identifying other effective biocontrol strains of P. chlamydosporia var. chlamydosporia.
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