Cultural and Biological control methods for Phytophthora root rot in Fraser fir
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Date
2009-11-16
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Abstract
Phytophthora root rot of Fraser fir caused by Phytophthora cinnamomi and several other Phytophthora spp. is a severe problem in Christmas tree production. Since fungicides are ineffective in disease control and host resistance is not yet available, cultural control methods are under investigation as a means of reducing disease pressure on infested production sites. Mulching systems with raised beds of pine bark, wood chips, or wood chips blended with compost were tested, along with compost or sulfur as soil amendments, at five sites spanning the western North Carolina growing region. Microbial populations and activity in soils and mulches were characterized over a two year period, using dilution plating with calculation of a log series diversity index for counts of bacteria, fungi, and cellulose-degrading micoorganisms, analysis of fluorescein diacetate hydrolysis for estimation of total microbial activity, and quantification of reducing sugars after incubation with carboxymethyl-cellulose for estimation of total cellulase enzyme activity. Bacterial and fungal counts, microbial activity, and cellulase activity were higher in mulch than in soil at all sites and times (P<0.01), and generally did not differ among mulch types nor among soils. Treatments significantly affected disease ratings and tree survival at three of five sites, with one or more mulch treatments yielding lower disease ratings and greater survival than controls. Tree mortality at each time point varied significantly with cellulase activity in the upper portion of the root zone (P=0.005). Other biological variables did not show significant relationship with disease ratings or mortality.
To further investigate the role of cellulase enzymes in suppression of P. cinnamomi, a commercial formulation of cellulase was used to generate a standard curve which could be used to correlate cellulase activity levels in field samples with the enzyme unit concentrations commonly used in laboratory studies. Two isolates of P. cinnamomi were exposed to a range of enzyme concentrations, and data were collected on biomass and sporangia production. Cellulase exposure reduced sporangia production but did not reduce biomass within the range analogous to that observed in field-applied mulch. In a bioassay with lupine, cellulase applied to soil containing infested root fragments did not reduce disease progress.
Container trials were also used to assess the impacts of a wide range of organic and inorganic amendments on Phytophthora root rot in Fraser fir seedlings, and to examine the contributions of compost, microbial inoculants, and isolates of cellulytic fungi to disease suppression in wood chip mulches. In trials with wood chips blended into soil and trials with seedlings planted directly into wood chip mulch, seedling survival was greater when wood chips were amended with compost, a soil inoculant, or a biocontrol agent. If wood chips were amended with compost, the addition of a cellulytic fungus, including a known biocontrol agent, did not further enhance plant survival, and in most cases did not significantly increase cellulase activity over compost amended wood chips alone.
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Keywords
microbial activity, biocontrol, cellulase, mulch, Abies fraseri, Phytophthora cinnamomi
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Degree
PhD
Discipline
Plant Pathology
