Introduction

Materials and Methods

Results

Discussion

References

Photo gallery of crapemyrtle cultivars

      By mid-May each year, the typical fluffy white colonies of the powdery mildew fungus were seen on the young leaves and shoots of the mildew-susceptible crapemyrtle cultivars. Generally, the occurrence of powdery mildew peaked between mid-June and late July. The fungal colonies persisted on leaves of susceptible cultivars until early fall. Although symptoms of Cercospora leaf spot first appeared on highly susceptible cultivars as early as June, spotting of the leaves and premature leaf shed usually were not noticeable on most cultivars until mid-August to early September. Spotting of the leaves and leaf shed continued through the fall until the first hard frost in late October or early November.
      As a group, the cultivars of crapemyrtle (L. indica) were more susceptible to powdery mildew than were the hybrid crapemyrtles (L. indica x fauriei) and L. fauriei ‘Fantasy’ (Tables 1 and 2). Powdery mildew ratings for all of the hybrid crapemyrtle cultivars and L. fauriei ‘Fantasy’ were similar in all three years. In 1995 and 1996, the severity of Cercospora leaf spot averaged across all cultivars of L. indica and L. indica x fauriei did not differ. Cercospora leaf spot damage levels for L. fauriei ‘Fantasy’ were lower than the averaged ratings for the other two crapemyrtle taxa.
      Considerable differences in the incidence of powdery mildew were seen among the 20 cultivars of L. indica (Table 1). Also, disease incidence differed considerably from year to year on many of these cultivars, especially on those highly susceptible to powdery mildew. In two of three years, heavy colonization of the leaves and shoots, as indicated by disease ratings of 2.0 or above, was seen on ‘County Red’ crapemyrtle (Table 2). High levels of disease also were seen in one of three years on ‘Carolina Beauty’, ‘Raspberry Sundae’, ‘Wonderful White’, ‘Gray’s Red’, ‘Orbin Adkins’, and ‘Seminole’. Noticeable mildewing of the leaves and shoots, which was indicated by disease ratings between 1.0 and 2.0, was seen in at least one year on an additional 11 selections of L. indica crapemyrtle. ‘Cherokee’ remained mildew-free, while low levels of powdery mildew were seen each year on ‘Glendora White’.
      In at least one year, considerable Cercospora leaf spot-related spotting of the leaves and early leaf shed were observed on nearly all cultivars of L. indica (Table 2). By early fall, ‘Orbin Adkins’ and ‘Wonderful White’ consistently suffered severe defoliation and heavy spotting of the few remaining leaves found near the shoot tips. As indicated by disease ratings above 5.0 in at least two years, ‘Raspberry Sundae’, ‘Powhatan’, ‘Majestic Beauty’, ‘Hardy Lavendar’, and ‘Carolina Beauty’ also proved highly susceptible to Cercospora leaf spot. In contrast, leaf spotting and early leaf shed, though noticeable, were limited to the lower quarter of the canopy of ‘Glendora White’, ‘Gray’s Red’, and ‘Velma’s Royal Delight’.
      As expected, the occurrence of powdery mildew on many of the hybrid (L. indica x fauriei) crapemyrtle was generally very low (Table 2). Light to moderate outbreaks of this disease, as indicated by disease ratings between 0.5 and 1.0, were recorded in two years for ‘Zumi’ and in one year for ‘Hopi’ and ‘Pecos’. Of all the cultivars of hybrid crapemyrtle, only ‘Caddo’ remained free of powdery mildew in all three years. In two of three years, the characteristic white colonies of the fungus E. lagerstroemia were not seen on the leaves, flower buds, or shoots of ‘Sarah’s Favorite’, ‘Comanche’, ‘Osage’, ‘Acoma’, ‘Tonto’, ‘Souix’, ‘Lipan’, or ‘Natchez’. On the remaining hybrid crapemyrtle cultivars, as well as on L. fauriei ‘Fantasy’, the incidence of powdery mildew was limited to a single or a few widely scattered colonies of the fungus on the leaves, tender shoots, and flower buds.
      Although most hybrid crapemyrtle selections are highly resistant to powdery mildew, only a handful were resistant to Cercospora leaf spot (Table 2). Of the 22 selections of hybrid crapemyrtle, ‘Tonto’, ‘Tuskegee’, and ‘Tuscarora’ have the highest level of resistance to this disease. In each area,damage on these cultivars and L fauriei ‘Fantasy’ was confined to light, inconspicuous spotting of the leaves around the base of the plants. Light to moderate spotting of the leaves along with a low level of early leaf shed, as indicated by disease ratings of 2.5 to 3.8, was seen on ‘Basham’s Party Pink’, ‘Apalachee’, and ‘Wichita’. Conversely, the heaviest spotting of the leaves and premature defoliation were noted in at least two of three years on ‘Acoma’ and ‘Comanche’. Cercospora leaf spot ratings of 5.0 or higher were recorded in at least one year for another five cultivars of hybrid crapemyrtle.

      Resistance to powdery mildew was one of the major selection criteria used to identify clones of hybrid crapemyrtle for release as part of the U.S. National Arboretum breeding program (5, 6, 7, 8, 9, 10, 11, 12, 13, 14). Beginning with ‘Natchez’ and ‘Muskogee’, a total of 22 hybrid cultivars have been released (7,8,9,10,11,12,13,14 ). Resistance of the cultivars of hybrid crapemyrtle and L. fauriei ‘Fantasy’ to powdery mildew is far superior to that shown by the selections of L. indica. With the exception of ‘Zuni’, ‘Pecos’, and ‘Hopi’, the cultivars of hybrid crapemyrtle proved highly resistant or immune to powdery mildew. Typically, similar results have been obtained when the releases from the U.S. National Arboretum were evaluated in the field (15,16,18).
      Many cultivars of crapemyrtle (L. indica) were moderately or highly susceptible to powdery mildew, while others like ‘Glendora White’, and ‘Cherokee’ were highly resistant to nearly immune to this disease. Of the five mildew tolerant cultivars of L. indica released by the U.S. National Arboretum, light to moderate mildew development was recorded in at least one year on ‘Powhatan’, ‘Seminole’, ‘Potomac’, and ‘Catawba’. As observed in previous trials, ‘Wonderful White’ (18), ‘Carolina Beauty’ (16), and ‘Seminole’ (16) were highly susceptible to powdery mildew.
      Far more cultivars of hybrid crapemyrtle were resistant to powdery mildew than to Cercospora leaf spot. In Louisiana, Holcomb (15) also noted similar differences in the sensitivity of cultivars of hybrid crapemyrtle to Cercospora leaf spot. In both the AAES and Louisiana studies, ‘Tonto’, ‘Tuskegee’, and ‘Tuscarora’ were the most Cercospora leaf spot and powdery mildew resistant. A few cultivars of L. indica along with L. fauriei ‘Fantasy’ also suffered relatively light damage from Cercospora leaf spot and powdery mildew.
      Surprisingly, compared to powdery mildew, Cercospora leaf spot proved to be an equally if not more, damaging disease on two of the three crapemyrtle taxa. Previously, powdery mildew garnered far more attention from professionals than did Cercospora leaf spot. Cultivars resistant to powdery mildew were found in all three taxa of crapemyrtle. Among the cultivars of L. indica, ‘Glendora White’ and ‘Cherokee’ were among the most disease resistant. Hybrid crapemyrtle cultivars highly resistant to powdery mildew were ‘Tonto’, ‘Tuskegee’, ‘Tuscarora’, ‘Basham’s Party Pink’, ‘Apalachee’, and ‘Caddo’.
      The one cultivar of L. fauriei, ‘Fantasy’, also suffered little damage from either disease. When horticultural characteristics and disease resistance were both considered, the superior white-flowered cultivars were ‘Natchez’, ‘Sarah’s Favorite’, and ‘Fantasy’, while ‘Basham’s Party Pink’, ‘Tuscarora’, and ‘Tuskegee’ were the top rated pink-flowered crapemyrtles (3). All of the above crapemyrtles can be easily produced in a nursery or maintained in a landscape without protective fungicide treatments. Those cultivars that were badly damaged by either powdery mildew, Cercospora leaf spot, or in some cases both diseases, may not suffer serious damage in the nursery but would be poor choices for commercial and residential landscapes across Alabama.