Phytophthora root rot, caused by Phytophthora nicotianae Breda de Haan, is one of the destructive diseases of boxwood (Buxus sempervirens L.) and can affect all growth stages of field- and container-grown boxwood plants. Management is a problem and is only possible through an integrated approach. In this study, the efficacy of fungicides, biofungicides, host-plant defense inducers, and fertilizer were evaluated to manage Phytophthora root rot of boxwood. The objective of this experiment was to develop fungicide and biofungicide recommendations for Phytophthora root rot management in boxwood production. Field and greenhouse experiments were conducted in 2019 (Trial 1) and 2020 (Trial 2). The field experiment was arranged in a completely randomized design with four plots per treatment with five single ‘Green Velvet’ boxwood plants per plot. The greenhouse experiment was arranged in a completely randomized design with five single ‘Green Velvet’ container-grown boxwood plants per treatment. Plots/containers were inoculated with P. nicotianae grown on rice grains. Plant growth data such as height and average width were recorded at the beginning and end of the experiments. Total plant fresh weight and root fresh weight were recorded at the end of the experiments. Roots were assessed for root rot disease severity using a scale of 0% to 100% roots affected. Treatments used in both experiments were fungicides—ametoctradin + dimethomorph, fluzapyroxad, mefenoxam, oxathiapiprolin, pyraclostrobin, pyraclostrobin + boscalid; host-plant defense inducers—aluminum tris-drench, aluminum tris-foliar, potassium salts of phosphoric acid; biofungicides—Trichoderma harzianum Rifai strain T-22 + Trichoderma virens strain G-41, Bacillus amyloliquefaciens Priest; fertilizer—water-soluble nitrogen (nitrogen 5%) and soluble potash; and combination of water-soluble nitrogen, soluble potash, and T. harzianum Rifai strain T-22 + T. virens strain G-41. All treatments were drench applied except one of the aluminum tris, which was applied as foliar. The controls were nontreated, inoculated and nontreated, and noninoculated boxwood plants. In the greenhouse experiments, treatments that effectively reduced disease severity were pyraclostrobin, ametoctradin + dimethomorph, and oxathiapiprolin. In the field experiments, treatments such as pyraclostrobin, oxathiapiprolin, mefenoxam, fluzapyroxad, and combination of water-soluble nitrogen (nitrogen 5%), soluble potash, and T. harzianum Rifai strain T-22 + T. virens strain G-41 effectively reduced Phytophthora root rot severity. Oxathiapiprolin and pyraclostrobin are the chemical fungicides that were effective in both field and greenhouse experiments.
Nursery growers rely on fungicides to manage root rot disease of boxwood caused by Phytophthora nicotianae and P. cinnamomi. Repeated use of the same fungicide may lead to the fungicide resistance. In this study, fungicides pyraclostrobin + cyazofamid (Empress + Segway), ametoctradin + dimethomorph (Orvego), ametoctradin + dimethomorph alternated with pyraclostrobin (Orvego alt Empress), ametoctradin + dimethomorph alternated with fluxapyroxad + pyraclostrobin (Orvego alt Orkestra), and oxathiapiprolin (Segovis) were evaluated for their efficacy in managing Phytophthora root rot under greenhouse conditions in Tennessee. One-year-old container-grown boxwood ‘Green Velvet’ plants were inoculated with P. nicotianae or P. cinnamomi. The first applications of fungicide treatments were made preventatively as a drench 48 h before pathogen inoculation. Following inoculation, four applications of fungicide treatments were performed on a 14-day intervals. Initial and final plant height and width were measured. Total plant fresh weight and root fresh weight were measured at the end of the trials, and plants were evaluated for root rot severity (0% to 100% roots affected) and pathogen recovery. All fungicides significantly reduced root rot severity and pathogen recovery of P. nicotianae and P. cinnamomi. Ametoctradin + dimethomorph alternated with pyraclostrobin (Orvego alt Empress) provided similar protection against P. cinnamomi to that of a single application of ametoctradin + dimethomorph (Orvego) or oxathiapiprolin (Segovis). For P. nicotianae, ametoctradin + dimethomorph alternated with pyraclostrobin + fluzapyroxad (Orvego alt Orkestra) was found to be as effective as a single application of either ametoctradin + dimethomorph (Orvego) or oxathiapiprolin (Segovis) in one of the two trials. Effects of fungicides on plant growth such as height, width, total, and root fresh weight were not significant. These findings will be useful to nursery growers in selecting the right fungicide program for the management of root rot disease of boxwood caused by P. nicotianae and P. cinnamomi.