Powdery mildew is the most common and esthetically damaging disease found in landscape plantings of the ever-popular flowering dogwood.  Although the best defense against this disease is the use of disease-resistant cultivars, previous AAES studies revealed that most of the flowering dogwoods now established in Alabama’s landscape are susceptible to powdery mildew.  However, new AAES research suggests that several fungicides may be useful in controlling the ravages of powdery mildew on susceptible cultivars.

            The powdery mildew fungus, Microsphaera pulchra, causes extensive and unsightly distortion of the leaves and slows shoot growth on flowering dogwood.  In trees that are susceptible to this fungus, the best defense is protective fungicide treatment. To determine the most effective fungicides, AAES trials were established at the Ornamental Horticulture Substation in Mobile on container-grown flowering dogwood to assess the effectiveness of several fungicides that are marketed to homeowners and nursery industry.

            Bare-root flowering dogwood (Cornus florida) were potted in three- or five-gallon containers in a 3:1 pine bark/peat moss mixture, which was amended with 14 pounds of Osmocote 17-7-12, six pounds of dolomitic limestone, two pounds of gypsum, and 1.5 pounds of Micromax per cubic yard.  The flowering dogwood cultivars ‘Cherokee Chief’ and ‘First Lady’ were used in 1997 and 1999, respectively.  The 1997 and 1999 trials were conducted under 40% shade.  The trees were watered daily using overhead impact sprinklers.  Fungicides were applied to the leaves to run-off with a CO₂-pressurized sprayer from June 18 until September 17 in 1997 and from April 12 until October 6 in 1999.  In the 1997 trial, fungicide treatments were applied at two-week intervals. Application rates are shown in Table 2.

            At the completion of each study, the incidence of powdery mildew was visually rated using a scale of 1 to 12 (1 = 0%, 2 = 0 to 3%, 3 = 3 to 6%, 4 = 6 to 12%, 5 = 12 to 25%, 6 = 25 to 50%, 7 = 50 to 75%, 8 = 75 to 87%, 9 = 87 to 94%, 10 = 94 to 97%, 11 = 97 to 100%, and 12 = 100% of the leaves colonized by the powdery mildew fungus).  When the 1999 studies were completed, the height and caliper of each tree also were recorded on September 29.       

            In 1997, considerable powdery mildew development was seen on the leaves of the nonsprayed ‘Cherokee Chief’ flowering dogwoods.  As compared to the nonsprayed trees, all of the fungicides evaluated substantially reduced the incidence of powdery mildew (see Table 1).  However, noticeable differences in the level of disease control were seen among the fungicide treatments.  As indicated by disease ratings of 1.6 to 1.9, very little colonization of the leaves by the powdery mildew fungus was seen on the trees treated with Domain 50W, Eagle 40W, Rubigan A.S., and Phyton 27.   Banner Maxx, which proved slightly less efficacious than the above fungicides, also gave good control of powdery mildew.   Although 3336 4.5F, Bayleton T/O, and Triforine EC reduced incidence of powdery mildew compared to the nonsprayed control, their disease ratings, which ranged from 4.1 to 5.3, were considerably higher than those recorded for the other fungicide treatments.  No symptoms of phytotoxicity were associated with the use of any of the fungicide treatments.  Tree height and caliper were not recorded.

            In 1999, the characteristic signs and symptoms of powdery mildew were noted on 100% of the leaves on the nonsprayed ‘First Lady’ flowering dogwoods (see Table 2).  As was seen in the 1997 study, all of the fungicide treatments reduced the incidence of powdery mildew when compared to the level on the unsprayed dogwoods.  When applied weekly, Compass 50W at half-ounce-per-100-gallon gave better control of powdery mildew than the one- and two-ounce-per-100-gallon rate of same fungicide applied at two-week intervals.   The latter two rates of Compass 50W also proved less effective in controlling this disease than Banner Maxx, Eagle 40W, and Heritage 50W.   As indicated by disease ratings ranging from 1.4 to 2.1, few colonies of the causal fungus M. pulchra were seen on the leaves of the flowering dogwoods treated with Banner Maxx, Eagle 40W, or Heritage 50W. 

            Severe powdery mildew infections slowed the growth of flowering dogwood.  The height and caliper (circumference) recorded for the nonsprayed controls were substantially lower than those for the fungicide-treated trees (see Table 2).  Increases of 10 to 12 inches in height were seen on the flowering dogwood treated with the one- and two-ounce-per-100-gallon rates of Compass 50W.   Trees sprayed with Banner Maxx, Eagle 40W, and Heritage 50W were approximately six to 10 inches higher than the nonsprayed control.  In addition, the leaves were noticeably larger and the canopy was fuller on the fungicide-treated flowering dogwoods.   Gains in tree caliper on the Compass 50W-treated trees ranged from 0.17 to 0.25 inches higher than that recorded for the unsprayed dogwoods.   Similar increases in tree caliper were also obtained with Banner Maxx, Eagle 40W, and Heritage 50W.

            In summary, none of the fungicides completely protected flowering dogwood from powdery mildew, but Banner Maxx and Eagle 40W gave superb disease control in the 1997 and 1999 studies.  Typically, disease development on trees treated with both fungicides was limited to scattered colonies on a few leaves.   In one year, recommended rates of Domain 50W, Rubigan A.S., Phyton 27, and Heritage 50W, as well as weekly applications of the half-ounce-per-100-gallon rate of Compass 50W, proved as equally effective as the above fungicides in controlling powdery mildew on flowering dogwood.  In addition, significant increases in tree height and caliper were obtained in the 1999 study with Banner Maxx, Eagle 40W, Heritage 50W, and the low rate of Compass 50W.

            Despite sizable gains in tree height and caliper on the trees treated with the one-  and two-ounce-per-100-gallon rates of Compass 50W, the level of disease control obtained with both of these treatments was not as high as expected.  Additional studies are needed with Compass 50W to determine the optimum application rate and treatment interval to assure effective disease control. Finally, several fungicides, including 3336 4.5F, Bayleton T/O, and Triforine EC, which are cleared for use on flowering dogwood for the control of powdery mildew, were less effective in controlling this disease than anticipated.

Hagan is Professor of Entomology and Plant Pathology; Olive is Superintendent and Stephenson is Associate Superintendent of the Ornamental Horticulture Station in Mobile; Rivas-Davila is Research Associate of Entomology and Plant Pathology.

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