Date of Award


Document Type


Degree Name

Doctor of Philosophy (PhD)


Plant and Environmental Science

Committee Chair/Advisor

Dr. Steven N. Jeffers

Committee Member

Dr. William C. Bridges

Committee Member

Dr. S. Bruce Martin

Committee Member

Dr. Sarah A. White


Phytophthora root and crown rot (PRCR) is currently the most important disease on lavender (Lavandula spp.) in the United States. The disease was first described on English lavender (L. angustifolia) in a Maryland nursery in 1991, with Phytophthora nicotianae as the causal agent. Since that time, the disease has been reported on multiple continents and as caused by several species of Phytophthora. This study examined the distribution and pathogenicity of Phytophthora species on lavender in the United States, requested lavender grower feedback regarding their production and concerns, and examined efficacy of selected management options. Lavender is grown in all regions of the United States, with L. angustifolia and L. ×intermedia as the most common species planted. In collaboration with the U. S. Lavender Growers Association, samples of diseased lavender plants were collected from growers across the country over a 5-year period, 2015-2019. PRCR was found to be caused primarily by P. nicotianae, which was found in each of the 24 states from which PRCR positive samples were obtained, making it also the most widespread of the causal agents. Phytophthora palmivora and P. citrophthora were the next most abundant species found associated with symptomatic lavender; P. cinnamomi, P. tropicalis, P. cryptogea, P. sansomeana, P. cactorum, P. drechsleri, and P. megasperma were recovered from plant samples infrequently. Using Koch’s postulates, P. nicotianae, P. palmivora, and P. cinnamomi were documented for the first time as pathogenic on L. ×intermedia; P. cryptogea and P. drechsleri as pathogenic on L. angustifolia; P. nicotianae as pathogenic on L. heterophylla; and P. tropicalis as potentially pathogenic on L. ×intermedia. Additionally, first reports of pathogenicity of P. palmivora and P. citrophthora on L. angustifolia in the United States were documented. Some fungicides that target oomycete plant pathogens were shown to effectively manage P. nicotianae on L. angustifolia under greenhouse conditions. The phosphonate products, containing the active ingredients mono- and di-potassium salts of phosphorous acid or aluminum tris (O-ethyl phosphonate), provided the best protection in repeated trials based on several disease parameters. Attempts to remediate infested field soil using a quaternary ammonia product had some success but did not eradicate the pathogen. This study serves as a foundation for future research on Phytophthora root and crown rot of lavender, the most significant disease affecting this increasingly important specialty crop, in the United States.



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