Phytophthora Root and Crown Rot of Lavender: New Host-Pathogen Relationships Involving Six Species of and Three Species of .

Phytophthora root and crown rot has become a major threat to the lavender industry worldwide. Isolations from symptomatic plants between 2015 and 2019 revealed a number of potential causal agents in the United States. In this study, we tested nine species of and four species of and used Koch's Postulates to prove pathogenicity for six new host-pathogen relationships and confirm two pathogenic relationships for the first time in the United States. A total of 10 experiments were conducted with each consisting of two independent trials. Only host-pathogen combinations that occurred in the field were evaluated. All isolates used in these experiments were recovered from diseased lavender plants or, for one isolate, soil associated with a diseased plant sent to our lab or the Clemson University Plant and Pest Diagnostic Clinic for diagnosis. Experiments were conducted over 3 years, 2017 to 2019, in a research greenhouse under relatively uniform environmental conditions following a standard protocol. Plants were evaluated weekly for foliage symptom severity, and, at the end of each trial, plants were scored for final foliage symptom severity and root rot severity, area under the disease progress curve was calculated, fresh plant mass was weighed, and isolation of pathogens from roots was attempted. These studies successfully demonstrated for the first time pathogenicity of , , and to hybrid lavender ( × ), to sweet lavender (), and and to English lavender (). In addition, a soil isolate of was shown to be potentially pathogenic to . Our results also documented for the first time in the United States pathogenicity of and to . We were not able to confirm pathogenicity for three host-pathogen relationships: on English lavender, on hybrid lavender, and on Spanish lavender (). Results from this study expand the list of species causing root rot on lavender ( species) in the United States and elsewhere.