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Phytophthora nicotianae and P. palmivora: Emerging pathogens of hybrid lavender (Lavandula ×intermedia)

Daniel Dlugos: Clemson University

<div>Hybrid lavender (<em>Lavandula</em> ×<em>intermedia</em>) is one of the most popular lavender species grown commercially in the United States due to its hardiness and oil yields. Phytophthora root and crown rot (PRCR), caused by several different species of <em>Phytophthora</em>, is known to occur on English lavender (<em>L. angustifolia</em>) but has not been documented on hybrid lavender. Since 2015, hybrid lavender plants from farms and nurseries across the US have been assayed for PRCR, and <em>P. nicotianae</em> and <em>P. palmivora</em> have been isolated most frequently. To complete Koch’s Postulates, isolates of both species were obtained from infected roots or infested soil around diseased plants. Inocula were prepared from two isolates of each species by growing isolates on vermiculite moistened with V8 broth. Four-month-old plants of <em>L.</em> ×<em>intermedia</em> ‘Grosso’ and ‘Phenomenal’ were not inoculated or were inoculated with individual isolates by mixing inoculum into the top 2 cm of container mix in each pot. Plants were grown for 11 weeks on a greenhouse bench and rated weekly for foliage symptoms. Plants then were harvested and evaluated for root rot, fresh weights were measured, and pathogens were isolated from washed roots. PRCR symptoms developed on all inoculated plants; <em>P. nicotianae </em>and<em> P. palmivora</em> were isolated from these plants and killed some plants in each treatment. Consequently, both <em>P. nicotianae</em> and <em>P. palmivora</em> are pathogenic on hybrid lavender and pose a threat to the lavender industry.</div>