ASHS 2025 Conference

Rose rosette disease (RRD) is a major threat to rose production in nurseries. The causal agent of the disease is a negative strand ssRNA virus called Rose rosette virus (RRV) belonging to the genus Emaravirus. RRD is transmitted by the eriophyid mite Phyllocoptes fructiphilus and grafting. Symptoms of the disease include excessive lateral shoot growth, an abundance of thorns, witches' broom, leaf proliferation, deformed leaves and flowers, mosaic patterns, red pigmentation, and eventually plant death. The aim of this study was to evaluate the effectiveness of novel fungicide treatments, NinjaTM, at various application rates, intervals, and methods in managing RRD. Treatments were applied preventively as a sprench (7-day interval) or drench (14-day interval). Pink Knock Out® rose plants were inoculated with RRD on the same day as fungicide treatments. For inoculation, rose plants were exposed to two mite-infested rose terminals (approximately 4 inches long, with 4-5 leaves), collected from RRD symptomatic shrubs in Rutherford Co., TN. Each plant received about 20 mites. The experiment was arranged in a completely randomized design with ten single-plant replications and conducted twice in a quarantined greenhouse at the Otis L. Floyd Nursery Research Center in McMinnville, TN. At the end of the trials, data on the total number of shoots, number of shoots exhibiting RRD symptoms, open flowers, and phytotoxicity were collected. Two sepals from each plant were randomly selected for mite quantification under a dissecting microscope. The proportion of shoots with RRD was calculated by using the number of affected shoots to the total number of shoots. Rosette severity was rated using a scale of 0-3, where 0= no rosettes, 1= one rosette, 2= two rosettes and 3= three or more rosettes. RRD severity was rated on a scale of 0-3, where 0= no symptoms, 1= one shoot with symptoms, 2= two shoots with symptoms, and 3= three or more shoots with symptoms. Results indicated no significant difference in RRD severity between treated and non-treated control plants. However, plants treated with 11 oz NinjaTM exhibited a significantly lower rosette rating compared to the control plants. Furthermore, both the 8 oz and 11 oz NinjaTM treatments significantly reduced the proportion of shoots with RRD. No significant differences were found in mite count per sepal or open flowers among any of the treated and non-treated control plants. These findings suggest that biochemical pesticide treatment may offer a promising strategy to reduce RRD infection in roses.