First Report of <i>Peronospora choii</i> Causing Downy Mildew on <i>Lavandula angustifolia</i> in the United States

In spring 2025, a nursery grower in Wimauma, FL, USA, submitted ‘Avignon Early Blue’ lavender (Lavandula angustifolia) plants (one-month-old, 1L pots) exhibiting stunted growth, leaf chlorosis and necrosis to the Plant Diagnostic Clinic at the University of Florida Gulf Coast Research and Education Center. Disease incidence on this sole cultivar was about 90% (n = ~1500) and severity was about 50%, distributed in three different areas of the nursery, under field conditions. Abundant sporangia and conidiophores that were characteristic of downy mildew pathogens were observed on the abaxial leaf surface. Severely affected leaves developed curling as necrosis progressed. Based on microscopy, the pathogen was identified as a downy mildew, evidenced by coenocytic mycelia dichotomously branched conidiophores, and sporangia. Conidiophores bearing sporangia were harvested from three separate plants (one from each infested area in the nursery) using a sterile needle, and DNA was extracted using the FastDNA kit (MP Biomedicals, Solon, OH). The internal transcribed spacer (ITS), translation elongation factor 1-alpha (EF1A), beta-tubulin (TUB), heat shock protein 90 (HSP90), and cytochrome c oxidase subunit I and ll (COX1, 2) were sequenced according to Hoffmeister et al. (2020). Sequences of the three isolates were deposited in GenBank (ITS: PV954822 to 24; COX1: PV995061 to 63; COX2: PV995064 to 66; EF1A: PV995067 to 69; TUB: PV995070 to 72; HSP90: PV995073 to 75). BLASTn searches revealed that isolates were 100% identical to Peronospora choii (holotype BPI 893223) accession numbers MN450333 (ITS, 1135/1135 bp), MN546902 (EF1A, 867/867 bp), MN546927 (TUB, 919/919 bp), MN547003 (HS90, 907/907 bp), MN546953 (679/679 bp), and MN546979 (496/496 bp). Phylogenetic analysis also revealed isolates grouped with Pe. choii. For morphological characterization, fifty conidiophores and fifty sporangia were collected, mounted on slides, and measured under the microscope. Conidiophores measured in length 363-798 μm (Average=420, Standard Deviation=5.3) and had submonopodial branching. Sporangia were greyish to pale brownish, ovoidal to ellipsoidal, and measured 13-25 μm long (Avg=21, SD=3.3), and 12-22 μm in width (Avg=19, SD=2.4). Oospores were not observed. For pathogenicity, ten ‘Avignon Early Blue’ plants were used, five for inoculation, and five as non-inoculated controls. Symptomatic tissues from the submitted samples that were frozen at –80°C were washed with deionized water containing 0.1% Tween 20 to release the sporangia, and the suspension was adjusted to 104 conidia/ml. Plants were spray inoculated until run-off (~20 ml/plant) and kept inside clear plastic boxes for 72 h. Control plants were sprayed with sterile deionized water plus 0.1% Tween 20. Afterward, plants were kept in a growth room (25°C and 12/12-h light/dark). The experiment was repeated once. Two weeks after inoculation, symptoms were observed in all inoculated plants with abundant sporangia and conidiophores production under the leaves, identical to the symptoms observed on the original sample. The non-inoculated controls remained healthy. To our knowledge, this is the first report worldwide of Pe. choii, a new species arising out of the Pe. belbahrii complex, causing downy mildew on lavender. Little is reported about downy mildew of lavender, but in Israel, it has been attributed to Pe. belbahrii (Ben-Naim et al., 2019; Thines et al., 2020). Peronospora choii is a new species formerly recognized in 2020 by Hoffmeister et al. (2020), which was found on Plectranthus scutellarioides in Tennessee (2015) and Michigan (2007), USA. If downy mildew of lavender caused by Pe. choii were to become a major problem, targeted management efforts such as cultivar screening, optimized fungicide applications, epidemiological research, and surveys of alternative hosts would be essential.