Volume 46 Number 1 Spring 1999

Double White flowering dogwood.
Meet the Resistance
Several Flowering Dogwood Cultivars Resist Spot Anthracnose and Powdery Mildew

Austin K. Hagan, Brian Harden, Charles H. Gilliam, Gary J. Keever, David Williams, and Joseph Eakes

Over the past four years, powdery mildew has replaced spot anthracnose as the most common disease of flowering dogwood in residential landscapes across Alabama. Although the damage to established flowering dogwood is considered cosmetic, slowed shoot growth along with reduced tree vigor and seedling death has been linked to severe outbreaks of powdery mildew, particularly in nursery stock. AAES research has helped identify cultivars that are resistant to both powdery mildew and spot anthracnose.

Historically, powdery mildew has rarely been seen on flowering dogwood (Cornus florida) in Alabama. In a 1991 to 1993 survey of flowering dogwood in forested sites in North Alabama, powdery mildew was not observed on any trees at any of the numerous locations inspected. In 1994, powdery mildew, which is caused by the fungus Microsphaera penicillata, appeared statewide in forest, landscape, and nursery plantings of flowering dogwood. Similar dramatic increases have been reported throughout the range of flowering dogwood.

Spot anthracnose, caused by the fungus Elsinoe corni, is particularly common on flowering dogwoods grown in full sun. Although the impact of this disease on tree vigor is considered minor, the leaves and bracts of flowering dogwood in the nursery and landscape plantings may be badly defaced and distorted.

Spot anthracnose on Pink Beauty flowering dogwood.

 Severe spot anthracnose damage on bracts of Barton White flowering dogwood.

 Severe leaf curl due to powdery mildew on flowering dogwood.

Despite the popularity of flowering dogwoods, little information has been available concerning the susceptibility of many commercial cultivars to either powdery mildew or spot anthracnose. Results of this AAES study demonstrated that cultivars of flowering dogwood differ considerably in their susceptibility to both diseases. Several, including the white cultivars Welch's Bay Beauty and Weaver's White and the red cultivars Cherokee Brave and Cherokee Chief, have good resistance to both powdery mildew and spot anthracnose.

For the study, bare-root trees approximately two to three feet in height were planted on March 3, 1993, in full sun at a site on the Auburn University campus. A trickle irrigation system with two emitters per tree was installed at planting and the trees were watered as needed. Twice each spring approximately 0.2 pound of 13-13-13 fertilizer was uniformly distributed around each plant. In 1996, all trees were mulched with two to three inches of aged pine bark. Roundup herbicide applied at recommended rates was used for weed control. Alleys between the rows of trees were periodically mown.

Within two years of planting, the causal fungi of both spot anthracnose and powdery mildew were well established in this planting of dogwood. From 1995 through 1997, powdery mildew incidence on the leaves was rated each year in mid- to late May on a scale of 0 to 4 (0 = no disease, 1 = 1 to 25% of the leaves colonized, 2 = 26 to 50%, 3 = 51 to 75%, and 4 = 76 to 100% of the leaves colonized). Each year, the bracts and leaves were rated using this scale for spot anthracnose damage on separate dates, except in 1995 when data were collected for the leaves alone.

Across all cultivars of flowering dogwood, powdery mildew decreased slightly from 1995 to 1996, then sharply increased in 1997 (data not shown). Spot anthracnose-incited discoloration and distortion of the bracts and leaves progressively worsened from 1995 through 1997.

Incidence of powdery mildew varied sharply among cultivars of flowering dogwood and from year to year on individual cultivars (see Table 1). Over a three-year period, the leaves of the cultivar Cherokee Brave remained almost free of powdery mildew. In two of three years, low disease ratings for Cherokee Chief, Springtime, and Cherokee Daybreak indicated that the colonization of the leaves was relatively light and unobtrusive. High powdery mildew ratings were consistently recorded on the cultivars Stokes Pink, Rubra Pink, Pink Beauty, Red Beauty, First Lady, Purple Glory, Dwarf White, and Pink Flame. Significant colonization of the leaves by the powdery mildew fungus was seen in at least one of three years on nearly all the remaining cultivars of flowering dogwood.

Among the 26 cultivars of flowering dogwood in the study, Rainbow suffered the heaviest spotting and distortion of the leaves over the three-year test period. In 1996 and 1997, substantial damage was also seen on the bracts of Rainbow. In 1995, damage to leaves of all the remaining 25 cultivars of flowering dogwood was negligible. Because relatively few cultivars bloomed in 1995, disease ratings for the bracts were not taken.

Over the next two years, a substantial increase of spot anthracnose-induced damage was seen on many cultivars of flowering dogwood (Table 2). In 1996 and 1997, severe damage, as indicated by disease ratings of 2 or above, was recorded for Cherokee Princess, Cloud 9, Ozark Spring, Springtime, Dwarf White, and Barton White. In addition, noticeable damage was seen on Wonderberry, World's Fair, First Lady, Pink Flame, and Rubra Pink. In both years, the cultivars Stokes Pink, Purple Glory, and Cherokee Daybreak suffered little spotting of the leaves, but heavy spot anthracnose-related bract damage was seen.

Cultivars of flowering dogwood that consistently had the least spot anthracnose damage to the bracts and leaves were Cherokee Chief, Cherokee Brave, Weaver's White, Cherokee Sunset, and Welch's Bay Beauty.

In summary, four of the 26 cultivars of flowering dogwood screened demonstrated partial to a high level of resistance to powdery mildew and spot anthracnose. These cultivars would be excellent choices for low maintenance landscapes. Cherokee Brave and Cherokee Chief have red bracts while those of Weaver's White and Welch's Bay Beauty are white. All the flowering dogwoods with pink bracts or variegated leaves suffered considerable damage in at least one year from one or both diseases.

Several flowering dogwoods, particularly the variegated cultivars Cherokee Sunset and Cherokee Daybreak, sustained significant damage from either powdery mildew or spot anthracnose in only one of three growing seasons and may not be particularly disease-prone in residential or commercial landscapes. Conversely, Rainbow, Dwarf White, Ozark Spring, Cloud 9, Barton White, Springtime, Autumn Gold, First Lady, Pink Beauty, Pink Flame, Rubra Pink, Stokes Pink, and Purple Glory were highly susceptible to powdery mildew and/or spot anthracnose. Such disease-susceptible cultivars would require costly fungicide treatments to maintain crop quality in the nursery and in residential and commercial landscapes.

Hagan is Professor, Harden is Graduate Student, and Gilliam, Keever, Williams, and Eakes all are Professors of Horticulture.

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