Volume 45 Number 2 Summer 1998Volume 45 Number 2 Summer 1998
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Austin Hagan, Gary Keever, Charles Gilliam, and David Williams Brilliant fall color, handsome bark, lush green foliage, and large showy flower clusters of white, pink, purple, and dark red have made crapemyrtle a favorite for more than a century with homeowners and landscapers across Alabama. Two diseases, powdery mildew and Cercospora leaf spot, however, often damage crapemyrtles in landscapes and nurseries across the state. An AAES trial has identified cultivars of crapemyrtle that are highly resistant to both powdery mildew and Cercospora leaf spot. Powdery mildew and Cercospora leaf spot are the two most common diseases in Alabama on crapemyrtle. Although neither disease threatens the health of established crapemyrtle, both can detract from their beauty and value in the landscape and nursery.
Disease resistance is an effective, inexpensive, and pesticide-free means of producing and maintaining crapemyrtle plantings. Over the last three decades, personnel with the U.S. National Arboretum along with land-grant universities and private individuals have released a number of improved cultivars in the crapemyrtle taxa Lagerstroemia indica, L. fauriei, and L. indica x fauriei hybrids. In addition to a range of desirable horticultural characteristics, many of these new releases were selected for their resistance to powdery mildew but not to Cercospora leaf spot. A recent AAES field study has identified cultivars of crapemyrtle resistant to both powdery mildew and Cercospora leaf spot. Bare-root crapemyrtle liners were planted in March 1993 in full sun in a Marvyn loamy sand soil on 12-foot centers on rows spaced eight feet apart on the Alabama Agricultural Experiment Station in Auburn. A trickle irrigation system with two emitters per tree was installed at planting and the plants were watered as needed. In March and May of each year, approximately 3.2 ounces of 13-13-13 fertilizer was uniformly spread around the base of each plant. Directed applications of Roundup herbicide were made to control weeds. Escape weeds around the base of each plant were also pulled by hand. Alleys between the rows were periodically mowed. In 1996, all plants were mulched with two to three inches of aged pine bark. During the winter of 1995 and 1996, each crapemyrtle was lightly pruned. Within two years of planting, both powdery mildew and Cercospora leaf spot were well established throughout the planting. Powdery mildew ratings were taken each year in either June or July. The severity of Cercospora leaf spot was assessed in 1995, 1996, and 1997 in late August to mid-September. By mid-May each year, the characteristic symptoms of powdery mildew were seen on the young leaves and shoots of susceptible cultivars. Generally, mildew development peaked between mid-June and late July. Although symptoms of Cercospora leaf spot first appeared on one very susceptible cultivar as early as June, typical spotting of the leaves and premature defoliation did not become noticeable on most cultivars until mid-August to early September. Disease development usually continued through the fall until leaf fall began after the first hard frost. As a group, cultivars of
L. indica proved to be much more susceptible to powdery
mildew than L. indica x fauriei hybrids and L.
fauriei Fantasy (see Table 1). Powdery mildew ratings for
all hybrid cultivars and Fantasy were similar. In two of three
years, the severity of Cercospora leaf spot across all cultivars
of L. indica and the hybrids did not appreciably differ.
Leaf spot ratings for Fantasy were considerably below the average
recorded each year for cultivars of L. indica and L.
indica x fauriei.
Sizable differences in
the incidence of powdery mildew were seen among the L. indica
cultivars (see Table 2). Disease ratings for susceptible cultivars
did vary considerably and none suffered extensive damage every
year of the study. Heaviest mildewing of the leaves and shoots
was seen in two of three years on Country Red. Similarly high
disease levels were recorded in one of three years on Carolina
Beauty, Raspberry Sundae, Wonderful White, Gray's Red, Orbin
Adkins, and Seminole. Moderate but noticeable mildew outbreaks
were noted on an additional 11 cultivars of L. indica. Cherokee
remained mildew-free while low mildew ratings were consistently
recorded for Glendora White.
Moderate to heavy spotting of the leaves along with varying levels of defoliation were seen in at least one year on nearly all L. indica cultivars (Table 2). Each year, Orbin Adkins and Wonderful White suffered the heaviest defoliation and most extensive leaf spotting of the L. indica cultivars. On the cultivars Glendora White and Velma's Royal Delight, leaf spot levels were relatively low and defoliation was limited to the lower quarter of the plant canopy. Overall, powdery mildew levels on the majority of L. indica x fauriei cultivars were very low (see Table 3). Light to moderate mildew outbreaks were seen in one of three years on the cultivars Zuni, Hopi, and Pecos. Among all hybrid crapemyrtle cultivars, only Caddo remained mildew-free all three years. In two of three years, no powdery mildew was found on the foliage of the cultivars Sarah's Favorite, Comanche, Osage, Acoma, Tonto, Souix, Lipan, and Natchez. On the remaining hybrid crapemyrtle cultivars and L. fauriei Fantasy, disease development was very light and unobtrusive. Although most hybrid crapemyrtles
cultivars were resistant to powdery mildew, only a few demonstrated
good resistance to Cercospora leaf spot (Table 3). Symptoms on
the leaf spot-resistant cultivars Tonto, Tuskegee, Tuscarora,
as well as L. fauriei Fantasy, were generally limited
to light, usually inconspicuous spotting of the leaves around
the base of the plant. Minor spotting along with some light defoliation
was seen on Basham's Party Pink, Apalachee, and Wichita. Cultivars
that suffered the most severe defoliation were Acoma and Comanche.
Cercospora leaf spot proved an equal if not a more damaging disease of crapemyrtle than powdery mildew. Cultivars resistant to both diseases were found in all three crapemyrtle taxa. Among the cultivars of L. indica, Glendora White and Cherokee were the most disease resistant. Generally light damage from powdery mildew and Cercospora leaf spot was recorded on the L. indica x fauriei cultivars Tonto, Tuskegee, Tuscarora, Basham's Party Pink, Apalachee, Wichita, and Caddo. The L. fauriei cultivar Fantasy also proved to be highly resistant to both diseases. All of the above cultivars can easily be grown in a nursery or maintain their beauty in the landscape without protective fungicide treatments. Those cultivars that suffered severe damage from one or both diseases would be poor choices to establish in commercial or residential landscapes. |
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