Date of Award

8-2007

Document Type

Thesis

Degree Name

Master of Science (MS)

Legacy Department

Plant and Environmental Science

Advisor

Reighard, Gregory L

Committee Member

Kluepfel , Daniel A

Committee Member

Riley , Melissa B

Abstract

Peach replant disorder (PRD) is a serious problem for peach growers throughout the United States. PRD refers to the pattern of reduced growth and yield observed in young peach trees planted on soil previously cropped with peaches. The conventional treatment for PRD, pre-plant soil fumigation with methyl bromide (a broad-spectrum biocide) adequately controls the disorder. This suggests that PRD results from harmful interaction(s) between the soil biotic community and peach roots. However, the specific causal agents of PRD are still unknown.
Methyl bromide is a known contributor to ozone depletion, and is only available to U.S. growers through Critical Use Exemptions. Therefore, the task of determining the specific cause of PRD has become increasingly important.
The overall objective of this research was to provide new insight into causes and potential management options for PRD. Our specific objectives were: (1) to determine whether prunasin-degrading, cyanide-producing bacteria were present in replant soils and could inhibit young peach tree growth, (2) to study fine root dynamics in treated and untreated replant soils using minirhizotrons, and (3) to compare the ability of traditional and non-traditional replant treatments to improve tree growth on a replant site.
One experiment was based on the theory that prunasin (a cyanogenic glycoside found in peach tissues) provides a substrate for cyanide production in rhizobacteria, causing the growth reductions observed in PRD sites. We isolated prunasin- and amygdalin-degrading bacteria from the rhizosphere of seven-year-old peach trees. These isolates were identified and screened for cyanide production. Peach seedlings were inoculated with a prunasin-degrading, cyanogenic isolate in greenhouse experiments. The isolate was effective at colonizing the rhizosphere of seedling peaches, but no negative growth effects were observed.
Our field experiment compared tree growth and fine root (<1 mm>diameter) dynamics of replanted peach trees under four PRD management strategies (methyl bromide fumigation, Telone C-17 fumigation, soil solarization, or systemic pre-kill of previous peach trees using glyphosate stem injections) and an untreated control, and two peach rootstocks (Guardian¨ and Lovell). Methyl bromide, soil solarization, and to a lesser extent Glyphosate and Telone C-17, increased stem diameter, decreased fine root production and mortality, increased time roots remained white, and increased fine root lifespan. Additionally, we observed differences in fine root dynamics between rootstocks.
While many questions remain concerning PRD and its management, we have gained valuable insight into the bacterial community of our replant site and the fine root dynamics as affected by soil treatments and rootstock selection. It is likely that a single treatment approach for PRD management will not sufficiently replace methyl bromide, but a multifaceted approach may be a viable option for the future.

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Horticulture Commons

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