Semester of Graduation

Summer 2026

Degree

Master of Science (MS)

Department

Plant Pathology

Document Type

Thesis

Abstract

Louisiana wholesale nursery industry was valued at $127 million, and a large proportion of nursery sales consisted of woody ornamentals with a gross farm value of more than $67 million in 2024. Root and crown rot diseases caused by oomycete pathogens are persistent in nursery production systems worldwide, resulting in estimated crop losses of 10-20%. Despite integrating best management practices including use of mefenoxam as first line of defense to avoid disease incidence, growers are struggling to manage root and crown rot in their production systems. In recent years, Louisiana State University Agricultural Center Plant Diagnostic Center (LSUAC-PDC) received an increased number of container-grown woody ornamentals exhibiting root and crown rot symptoms. During 2024-25, a total of 104 Phytophthora spp. isolates from symptomatic woody ornamentals were obtained. The objectives of this study were: 1) to determine the prevalence of Phytophthora or related species associated with root and crown rot in container-grown woody ornamentals using cytochrome c oxidase subunit 1 regions, and 2) to evaluate the sensitivity of these species to fungicides containing mefenoxam and other active ingredients. Results from molecular identification revealed that new emerging Phytopythium helicoides were predominantly associated along with other exiting Phytophthora spp. known to cause root and crown rot diseases. In-vitro fungicide screening assays using active ingredients aluminum tris (Aliette), ametoctradin + dimethomorph (Orvego), mefenoxam (Subdue Maxx), and oxathiapiprolin (Segovis) at 0, 0.1, 1, 10 and 100 µg/ml concentrations of applied product were conducted to test the sensitivity of Pp. cucurbitacearum, P. cinnamomi, Pp. helicoides, P. mediterranea, P. palmivora, P. parvispora and Pp. vexans. Of the four fungicides tested, oxathiapiprolin effectively inhibited all species across all concentrations, whereas mefenoxam was not effective at all against Pp. helicoides, the predominant species identified to be associated with root and crown rot disease. These findings support the hypothesis that there are emerging oomycete species. becoming more prevalent in container-grown woody ornamentals in wholesale nurseries. Additionally, the commonly used active ingredient, mefenoxam, as first choice of growers to combat root and crown rot is becoming insensitive to existing and new prevalent species. Finally, the broad efficacy of oxathiapiprolin against both Phytophthora and Phytopythium spp. highlights its potential importance in updating management strategies for nursery growers.

Date

7-15-2026

Committee Chair

Singh, Raghuwinder

LSU Acknowledgement

1

LSU Accessibility Acknowledgment

1

Download

Available for download on Saturday, July 14, 2029

Share

COinS