Management of Black Spot of Rose with Winter Fungicide Treatment

Fungicides were evaluated for their efficacy against black spot of rose, caused by Diplocarpon rosae, when applied during winter months to hybrid tea rose plants in two studies. One study, conducted at two Auburn University campus sites, consisted of applications of horticultural oil, chlorothalonil plus an antitranspirant, and cyproconazole, along with nontreated plants; one site included triforine applications. Rose canes were treated two and three times between late October and February in 1996-97 and 1997-98, respectively; then plants were examined every 2 to 4 days beginning in mid-February for onset of symptoms of black spot. In both years, winter applications of fungicides (chlorothalonil, cyproconazole, and triforine) delayed disease onset compared with treatments without fungicides (nontreated and oil applications). Decreased plant defoliation, as well as improved plant vigor, were observed through the 1998 growing season following fungicidal winter treatments at campus sites; winter treatments with oil did not suppress disease compared with no treatment. In a second study started in November 1997, at a site near Shorter, AL, three systemic fungicides (myclobutanil, cyproconazole, and triforine) were applied to rose canes during the winter; nontreated plants were included. Decreased disease symptoms and defoliation of rose plants were observed early in the 1998 and 1999 seasons following winter applications of cyproconazole and triforine compared with myclobutanil or nontreated plants. Winter treatments with these fungicides did not consistently provide season-long reduction of black spot. However, season-long plant vigor and flower production on plants were improved at the Shorter site following winter applications of any of the above systemic fungicides compared with nontreated plants.