Managing Phytophthora Root and Crown Rot on English Lavender in the Greenhouse with Fungicides

Phytophthora root and crown rot (PRCR) of lavender is an emerging disease that has become one of the most significant threats to the lavender industry worldwide. Primarily caused by Phytophthora nicotianae , the disease impacts multiple species of lavender, and the primary causal agent is spread largely through the nursery industry. This study examined 12 fungicides that target diseases caused by species of Phytophthora and other oomycetes for efficacy against PRCR using artificially inoculated English lavender ( Lavandula angustifolia ) plants in a research greenhouse. Some fungicide treatments significantly reduced disease symptoms while maintaining plant size, but there was considerable variation in efficacy among the fungicide products. The products with the best performance were phosphonates, and a product containing the relatively new active ingredient oxathiapiprolin. An industry standard, mefenoxam, which is the active ingredient in Subdue Maxx, was also effective, but to a lesser degree. The results indicate a strong potential to manage PRCR on lavender with commercially available fungicides, particularly phosphonate products.