Crapemyrtle, Lagerstroemia sp. L. (Myrtales: Lythraceae) is the highest grossing deciduous flowering tree in the United States, with almost $70 million in annual wholesale value in 2019 (National Agriculture Statistics Service 2020). The southeastern region contributes more than 75% of the US crapemyrtle production (National Agriculture Statistics Service 2020). Crapemyrtles are produced in nurseries as small, containerized liners to large trees grown in containers or field nurseries and usually grown for a couple of years before being sold. Crapemyrtles are a popular choice for landscape settings and nursery production (Cabrera 2002; Chappell et al. 2012; Pooler 2007) because of numerous flower color options, long bloom time, showy exfoliating bark, fall leaf color, different plant size from small shrubs to tree form, and adaptability to poor soil and environmental conditions.

There are reports to confirm the existence of up 86 crapemyrtle species, but fewer than 10 are cultivated as ornamentals (Akond et al. 2012; Cabrera 2002; Furtado and Srisuko 1969; Pettis et al. 2004; Pooler 2006; Wang et al. 2011). Lagerstroemia species cultivated in the United States are small to large shrubs or small single to multistemmed trees that usually bloom from midsummer to first frost for up to 120 d with varying shades of white, red, purple, pink, or lavender inflorescence (Cabrera 2002; Qiao et al. 2019). Common crapemyrtle (L. indica L.), is a medium to large shrub that ranges from 3.0 m to 9.0 m in height and 4.5 m to 7.5 m in canopy spread. It is one of the historically prevalent crapemyrtle species that has been grown in the United States as an ornamental for more than 175 years (Dirr 2002). Common crapemyrtle produces large (15.0 to 20.0 cm) panicles of showy pink, red, purple, or white flowers and has ∼5-cm-long subopposite leaves that will turn yellow, orange, or red during the fall season. Japanese crapemyrtle (L. fauriei Koehne) are large shrubs to small trees that often grow 10.0 to 15.0 m in height and 7.5 to 10.0 m in width. Japanese crapemyrtle has mottled bark; produces small, white flowers; and minimal fall leaf color in some climatic zones to no color change during the fall (Creech 1985; Egolf and Andrick 1978). L. subcostata Koehne produces flowers with colors ranging from lavender to pink or white (Wang et al. 2011) and L. limii Merr. has small flowers and has been used in generating interspecific crosses due to high cold hardiness and strong disease tolerance (Pooler 2006).

Currently, more than 200 crapemyrtle cultivars exist, and almost half of these are commercially available from wholesale and retail nurseries (Wang et al. 2011). Since the beginning of the 1960s, the US National Arboretum, universities, and other public and private institutions have contributed to crapemyrtle breeding and selection programs and have released numerous cultivars under taxa L. indica, L. fauriei, L. indica × L. fauriei, L. indica × L. fauriei × L. limii, L. subcostata, and L. limii (Chappell et al. 2012; Dirr 2002; Egolf 1987a, 1987b; Egolf 1990a, 1990b; Einert and Watts 1973; Pooler and Dix 1999; Wang et al. 2011). Since the identification of a seedling from an interspecific cross between L. indica and L. fauriei in 1963, more than 20 interspecific hybrid cultivars have been released (Pounders et al. 2007). Breeders have given substantial consideration to developing cultivars resistant to powdery mildew (Erysiphe lagerstroemia E. West), as it has been reported as one of the most economically important diseases of crapemyrtle, especially in the southern United States. L. fauriei has been used extensively in breeding programs to introduce powdery mildew resistance in L. indica and has resulted in many popular powdery mildew–resistant cultivars.

Cercospora leaf spot, which is caused by the fungus Cercospora lythracearum Heald & F. A. Wolf is another detrimental foliar disease of crapemyrtle (Alfieri 1976; Byers 1997; Chappell et al. 2012). Cercospora leaf spot is characterized by tan to brown circular or semicircular spots randomly distributed throughout the leaves, usually beginning lower in the canopy and spreading upward. Leaf spot development becomes prominent August through October when warm, wet, and humid conditions are prevalent (Chappell et al. 2012). As the disease damage progresses, the size of the individual spots increases and may eventually cover most of the leaf surface. Severe spotting in the leaves can lead to premature leaf drop and affected plants may defoliate before fall color development or the first frost. Cercospora leaf spot control strategies mostly rely on repeated fungicide applications (Baysal-Gurel et al. 2018, 2020, 2021; Hagan 2010; Hagan and Akridge 2007).

The use of cercospora leaf spot–resistant cultivars is the most practical and sustainable approach to manage this disease; however, only a few Lagerstroemia hybrid cultivars (e.g., Apalachee, Fantasy, Osage, Tuscarora, and Tuskegee) have been reported to be resistant to cercospora leaf spot (Chappell et al. 2012). This evaluation was conducted at the Tennessee State University Otis L. Floyd Nursery Research Center located in McMinnville, TN (lat. 35.7°N, long. 85.8°W) on the border of US Department of Agriculture (USDA) Hardiness Zones 6 and 7, hence considered as geographic and climatic transition zone. Plants grown in these zones suffer from cold damage during winter and expose to diseases favored by warm summers. Observations taken in this region can provide beneficial information regarding resistance or susceptibility to cercospora leaf spot on this economically important ornamental plant that can be used in a wider geographical region as far as Zone 5 to Zone 8. The 2004 and 2011 plantings provide the most comprehensive evaluation of crapemyrtle species and cultivars for cercospora leaf spot compared with previous reports (Hagan et al. 1998). The objective of this study was to assess the responses of crapemyrtle cultivars belonging to L. indica, L. fauriei, L. indica × L. fauriei, L indica × L. fauriei × L. limii, a L. subcostata selection, and two unnamed selections of L. limii to cercospora leaf spot in McMinnville, TN.

Materials and Methods

Crapemyrtle cultivars or unnamed selections representing a range of morphological traits were used in this experiment, and each cultivar is described in Table 1 (Chappell et al. 2012; Pounders et al. 2010; Wang et al. 2011). For the 2004 planting, cuttings from the same ortet of a clone were rooted in the summer of 2002 in 26-cell trays at Poplarville, MS, then grown in 11.3-L (#3) nursery containers in McMinnville, TN, during 2003. In Apr 2004, 32 crapemyrtle cultivars of L. indica, L. fauriei, L. indica × L. fauriei, and L. indica × L. fauriei × L. limii were transplanted into field plots with Waynesboro loam soil. A similar timeline for cuttings was used for the 2011 planting, and 12 cultivars or unnamed selections of L. indica, L. indica × L. fauriei, L. indica × L. fauriei × L. limii, L. limii, and L. subcostata were field planted in April 2011. Among the crapemyrtle cultivars evaluated, most were L. indica (n = 14) and L. indica × L. fauriei (n = 13), with a few cultivars of L. fauriei (n = 4), L. indica × L. fauriei × L. limii (n = 2), L. limii (n = 2), and L. subcostata (n = 1). Six cultivars, including Carolina Beauty, Miami, Natchez, Osage, Red Rocket, and Tuscarora were included in both 2004 and 2011 plantings. Plants in both 2004 and 2011 plantings were spaced in rows 4.6 m apart on 3.7 m centers in staggered row. Immediately after planting, plants were pruned to a height of 40 cm. Plants were arranged in completely randomized block designs with three and four replications in the 2004 and 2011 plantings, respectively. Grass alleys between rows were mowed routinely, and vegetation in the row was controlled in a 1.2-m strip with pre- and post-emergence herbicides. Plants were fertilized annually (March to April), with 15N–6.6P–12.5K at a rate of ∼150 g/plant based on University of Tennessee soil test recommendations. During the first 2 years after planting, plants were irrigated using drip irrigation and were not irrigated thereafter. On 8 Apr 2007, five to seven stems from each plant were selected to grow and the remaining stems were pruned to the ground for 2004 planting. Maximum and minimum temperature and total rainfall were monitored during the evaluation seasons using a weather station (WatchDog 2700; Spectrum Technologies, Aurora, IL, USA) (Table 2).

Table 1.

Details of crapemyrtle (Lagerstroemia sp.) cultivars represented in the 2004 and 2011 plantings in McMinnville, TN.

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Table 2.

Weather variables recorded from 2015 to 2017 in a McMinnville, TN.

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Crapemyrtle plants were evaluated weekly for the severity of cercospora leaf spot and defoliation due to cercospora leaf spot from 8 Sep to 22 Sep 2015; 29 Aug to 5 Oct 2016; and 29 Sep to 13 Oct 2017 using a scale of 0% to 100% foliage affected or defoliated, respectively. Season-long disease progress was calculated using the AUDPC formula: Σ{[(x_i + x_i – 1)/2](t_i – t_i–1)}, where x_i is the disease severity rating at each evaluation time and (t_i – t_i–1) is the number of days between evaluations.

Data were analyzed using one-way analysis of variance in SAS software 9.4 (SAS Inc., Cary, NC, USA). The general linear model procedure (PROC GLM) was used to partition variance in cercospora leaf spot severity, AUDPC. and defoliation attributable to species or cultivars. Student’s t test was also used to examine the effects of planting years (2004 and 2011) on cercospora leaf spot severity in cultivars Carolina Beauty, Miami, Natchez, Osage, Red Rocket, and Tuscarora. The data satisfied the assumption of normal distribution and homogeneity. When the effects were significant, the post hoc Fisher’s least significant difference test was performed for means comparisons at α = 0.05. Pearson’s correlation coefficient, r, regression analysis was conducted to determine the relationships between disease severity and defoliation.

Results

Cercospora leaf spot appeared naturally, and characteristic symptoms of the disease were usually observed on susceptible crapemyrtle cultivars by early July. Irregular, yellow-brown spots ∼0.3 cm to 0.6 cm in diameter with no halo appeared on the upper surface of leaves. These leaf spots were first observed on branches closest to the base of the plant and radiated upward through the canopy as the season progressed. In severely infected cultivars, diseased leaves displayed bright red, orange, and yellow pigments (resembling fall coloration) before prematurely senescing in mid-August to early October.

Final disease severity ratings taken from late September to early October across all 3 years when pooled across cultivars within a species or hybrid group showed L. fauriei had the lowest cercospora leaf spot severity and AUDPC in the 2004 planting compared with other crapemyrtle species and hybrid groups (Table 3). Lagerstroemia indica × L. fauriei × L. limii and L. indica had the highest level of disease severity and AUDPC in all 3 evaluation years. Defoliation percentage among crapemyrtle in the 2004 planting was lowest in L. fauriei and L. indica × L. fauriei in 2015 and L. fauriei in 2016 and 2017 (Table 4). Lagerstroemia indica × L. fauriei × L. limii in 2015 and 2016 and L. indica × L. fauriei × L. limii and L. indica in 2017 had the highest defoliation. Among the crapemyrtle evaluated in the 2011 planting, cercospora leaf spot severity, AUDPC, and defoliation were highest in L. indica in all 3 years compared with other species and hybrid groups (Tables 5 and 6). In 2016, the L. subcostata showed significantly lower disease severity than L. indica and L. indica × L. fauriei, and in 2017, the same selection had the lowest disease severity and AUDPC values. In 2017, L. limii and L. subcostata had significantly lower defoliation compared with L. indica and L. indica × L. fauriei.

Table 3.

Cercospora leaf spot (Cercospora lythracearum) disease severity (% foliage affected) and area under disease progress curve (AUDPC) of crapemyrtle (Lagerstroemia sp.) pooled across cultivars within the same species or hybrid group planted in 2004 in McMinnville, TN.

View Table

Table 4.

Mean defoliation (%) due to cercospora leaf spot (Cercospora lythracearum) of crapemyrtle (Lagerstroemia sp.) pooled across cultivars within the same species or hybrid group planted in 2004 in McMinnville, TN.

View Table

Table 5.

Mean cercospora leaf spot (Cercospora lythracearum) disease severity (% of foliage affected) and area under disease progress curve (AUDPC) of crapemyrtle (Lagerstroemia sp.) pooled across cultivars within the same species or hybrid group planted in 2011 in McMinnville, TN.

View Table

Table 6.

Mean defoliation (%) due to cercospora leaf spot (Cercospora lythracearum) of crapemyrtle (Lagerstroemia sp.) pooled across cultivars within the same species or hybrid group planted in 2011 in McMinnville, TN.

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In all 3 evaluation years, there were significant differences in cercospora leaf spot severity among the crapemyrtle cultivars planted in 2004. Cultivars Apalachee, Fantasy, Kiowa, Miami, Townhouse, Tuscarora, Tuskegee, and Woodlander’s Chocolate Soldier in 2015; Apalachee, Fantasy, Kiowa, Townhouse, and Woodlander’s Chocolate Soldier in 2016; and Fantasy, Kiowa, Townhouse, and Woodlander’s Chocolate Soldier in 2017 exhibited the lowest disease severity (<20%) and AUDPC among cultivars in the 2004 planting (Figs. 1–6). Cultivars Fantasy, Kiowa, Townhouse, and Woodlander’s Chocolate Soldier belong to L. fauriei, and the rest of the aforementioned cultivars belong to L. indica × L. fauriei. These cultivars were followed by Tonto, which had a moderate level of disease severity of 20.3%, 26.7%, and 45.0% in 2015, 2016, and 2017, respectively, and AUDPC. ‘Choctaw’, ‘Natchez’, ‘Osage’ and ‘Sarah’s Favorite’, which belong to L. indica × L. fauriei, also showed a moderate level of disease severity (20% to 41%) and AUDPC in 2015. Other cultivars in the 2015 evaluation, such as Catawba, Country Red, Muskogee, Pecos, and Velma’s Royal Delight, had disease severity ranging from 41% to 63%. In the 2016 evaluation, ‘Miami’, ‘Pecos’, ‘Red Rocket’, ‘Tuscarora’, and ‘Tuskegee’ exhibited a moderate level of disease severity of 35% to 47%, all of which belong to L. indica × L. fauriei with the exception of ‘Red Rocket’. In the same year, cultivars Carolina Beauty, Centennial Spirit, Choctaw, Country Red, Muskogee, Natchez, Osage, Sarah’s Favorite, and Velma’s Royal Delight had a disease severity ranging from 51% to 62%. In 2017, cultivars Apalachee, Miami, Natchez, Osage, and Tuskegee showed moderate level of disease severity (26% to 45%). Moreover, ‘Choctaw’, ‘Muskogee’, ‘Red Rocket’, ‘Sarah’s Favorite’, and ‘Tuscarora’ had disease severity between 46% and 70% in 2017. Cultivars Acoma, Burgundy Cotton, Cheyenne, Christiana, Dynamite, Powhatan, Raspberry Sundae, Sioux, Siren, Splash of Pink, and William Toovey consistently showed the highest level of disease severity (>73%) in all 3 years. In addition, cultivar Centennial Spirit in 2015; Catawba in 2016; and Country Red, Pecos, and Velma Royal Delight in 2017 also showed the highest level of disease severity. Cultivars Acoma, Burgundy Cotton, Cheyenne, Christiana, Dynamite, and William Toovey in 2015, and additional Country Red, Sioux, and Splash of Pink in 2016; and additional Carolina Beauty, Catawba, Centennial Spirit, Pecos, Powhatan, Raspberry Sundae, Sioux, Siren, Splash of Pink, and Velma’s Royal Delight in 2017 demonstrated the highest AUDPC.

Mean (± SE) cercospora leaf spot (Cercospora lythracearum) disease severity (% of foliage affected) of crapemyrtle (Lagerstroemia sp.) cultivars planted in 2004 in McMinnville, TN, and evaluated on 22 Sep 2015. Values are the means per plant for three single-plant replicates. Means followed by the different lowercase letters on the top of the bar are significantly different (P ≤ 0.05) according to Fisher’s least significant difference test.

Citation: HortScience 58, 1; 10.21273/HORTSCI16913-22

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Mean (± SE) cercospora leaf spot (Cercospora lythracearum) disease severity (% of foliage affected) of crapemyrtle (Lagerstroemia sp.) cultivars planted in 2004 in McMinnville, TN, and evaluated on 22 Sep 2015. Values are the means per plant for three single-plant replicates. Means followed by the different lowercase letters on the top of the bar are significantly different (P ≤ 0.05) according to Fisher’s least significant difference test.

Citation: HortScience 58, 1; 10.21273/HORTSCI16913-22

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Mean (± SE) cercospora leaf spot (Cercospora lythracearum) disease severity (% of foliage affected) of crapemyrtle (Lagerstroemia sp.) cultivars planted in 2004 in McMinnville, TN, and evaluated on 22 Sep 2015. Values are the means per plant for three single-plant replicates. Means followed by the different lowercase letters on the top of the bar are significantly different (P ≤ 0.05) according to Fisher’s least significant difference test.

Citation: HortScience 58, 1; 10.21273/HORTSCI16913-22

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Mean (± SE) area under disease progress curve (AUDPC) for cercospora leaf spot (Cercospora lythracearum) disease of crapemyrtle (Lagerstroemia sp.) cultivars planted in 2004 in McMinnville, TN, and evaluated on 8 Sep, 15 Sep, and 22 Sep 2015. Values are the means per plant for three single-plant replicates. Means followed by different lowercase letters on the top of the bar are significantly different (P ≤ 0.05) according to Fisher’s least significant difference test.

Citation: HortScience 58, 1; 10.21273/HORTSCI16913-22

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Mean (± SE) area under disease progress curve (AUDPC) for cercospora leaf spot (Cercospora lythracearum) disease of crapemyrtle (Lagerstroemia sp.) cultivars planted in 2004 in McMinnville, TN, and evaluated on 8 Sep, 15 Sep, and 22 Sep 2015. Values are the means per plant for three single-plant replicates. Means followed by different lowercase letters on the top of the bar are significantly different (P ≤ 0.05) according to Fisher’s least significant difference test.

Citation: HortScience 58, 1; 10.21273/HORTSCI16913-22

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Mean (± SE) area under disease progress curve (AUDPC) for cercospora leaf spot (Cercospora lythracearum) disease of crapemyrtle (Lagerstroemia sp.) cultivars planted in 2004 in McMinnville, TN, and evaluated on 8 Sep, 15 Sep, and 22 Sep 2015. Values are the means per plant for three single-plant replicates. Means followed by different lowercase letters on the top of the bar are significantly different (P ≤ 0.05) according to Fisher’s least significant difference test.

Citation: HortScience 58, 1; 10.21273/HORTSCI16913-22

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Mean (± SE) cercospora leaf spot (Cercospora lythracearum) disease severity (% of foliage affected) of crapemyrtle (Lagerstroemia sp.) cultivars planted in 2004 in McMinnville, TN, and evaluated on 5 Oct 2016. Values are the means per plant for three single-plant replicates. Means followed by the different lowercase letters on the top of the bar are significantly different (P ≤ 0.05) according to Fisher’s least significant difference test.

Citation: HortScience 58, 1; 10.21273/HORTSCI16913-22

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Mean (± SE) cercospora leaf spot (Cercospora lythracearum) disease severity (% of foliage affected) of crapemyrtle (Lagerstroemia sp.) cultivars planted in 2004 in McMinnville, TN, and evaluated on 5 Oct 2016. Values are the means per plant for three single-plant replicates. Means followed by the different lowercase letters on the top of the bar are significantly different (P ≤ 0.05) according to Fisher’s least significant difference test.

Citation: HortScience 58, 1; 10.21273/HORTSCI16913-22

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Mean (± SE) cercospora leaf spot (Cercospora lythracearum) disease severity (% of foliage affected) of crapemyrtle (Lagerstroemia sp.) cultivars planted in 2004 in McMinnville, TN, and evaluated on 5 Oct 2016. Values are the means per plant for three single-plant replicates. Means followed by the different lowercase letters on the top of the bar are significantly different (P ≤ 0.05) according to Fisher’s least significant difference test.

Citation: HortScience 58, 1; 10.21273/HORTSCI16913-22

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Mean (± SE) area under disease progress curve (AUDPC) for cercospora leaf spot (Cercospora lythracearum) disease of crapemyrtle (Lagerstroemia sp.) cultivars planted in 2004 in McMinnville, TN, and evaluated on 29 Aug, 21 Sep, and 5 Oct 2016. Values are the means per plant for three single-plant replicates. Means followed by different lowercase letters on the top of the bar are significantly different (P ≤ 0.05) according to Fisher’s least significant difference test.

Citation: HortScience 58, 1; 10.21273/HORTSCI16913-22

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Mean (± SE) area under disease progress curve (AUDPC) for cercospora leaf spot (Cercospora lythracearum) disease of crapemyrtle (Lagerstroemia sp.) cultivars planted in 2004 in McMinnville, TN, and evaluated on 29 Aug, 21 Sep, and 5 Oct 2016. Values are the means per plant for three single-plant replicates. Means followed by different lowercase letters on the top of the bar are significantly different (P ≤ 0.05) according to Fisher’s least significant difference test.

Citation: HortScience 58, 1; 10.21273/HORTSCI16913-22

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Mean (± SE) area under disease progress curve (AUDPC) for cercospora leaf spot (Cercospora lythracearum) disease of crapemyrtle (Lagerstroemia sp.) cultivars planted in 2004 in McMinnville, TN, and evaluated on 29 Aug, 21 Sep, and 5 Oct 2016. Values are the means per plant for three single-plant replicates. Means followed by different lowercase letters on the top of the bar are significantly different (P ≤ 0.05) according to Fisher’s least significant difference test.

Citation: HortScience 58, 1; 10.21273/HORTSCI16913-22

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Mean (± SE) cercospora leaf spot (Cercospora lythracearum) disease severity (% of foliage affected) of crapemyrtle (Lagerstroemia sp.) cultivars planted in 2004 in McMinnville, TN, and evaluated on 13 Oct 2017. Values are the means per plant for three single-plant replicates. Means followed by the different lowercase letters on the top of the bar are significantly different (P ≤ 0.05) according to Fisher’s least significant difference test.

Citation: HortScience 58, 1; 10.21273/HORTSCI16913-22

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Mean (± SE) cercospora leaf spot (Cercospora lythracearum) disease severity (% of foliage affected) of crapemyrtle (Lagerstroemia sp.) cultivars planted in 2004 in McMinnville, TN, and evaluated on 13 Oct 2017. Values are the means per plant for three single-plant replicates. Means followed by the different lowercase letters on the top of the bar are significantly different (P ≤ 0.05) according to Fisher’s least significant difference test.

Citation: HortScience 58, 1; 10.21273/HORTSCI16913-22

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Mean (± SE) cercospora leaf spot (Cercospora lythracearum) disease severity (% of foliage affected) of crapemyrtle (Lagerstroemia sp.) cultivars planted in 2004 in McMinnville, TN, and evaluated on 13 Oct 2017. Values are the means per plant for three single-plant replicates. Means followed by the different lowercase letters on the top of the bar are significantly different (P ≤ 0.05) according to Fisher’s least significant difference test.

Citation: HortScience 58, 1; 10.21273/HORTSCI16913-22

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Mean (± SE) area under disease progress curve (AUDPC) for cercospora leaf spot (Cercospora lythracearum) disease of crapemyrtle (Lagerstroemia sp.) cultivars planted in 2004 in McMinnville, TN, and evaluated on 29 Sep, 6 Oct, and 13 Oct 2017. Values are the means per plant for three single-plant replicates. Means followed by different lowercase letters on the top of the bar are significantly different (P ≤ 0.05) according to Fisher’s least significant difference test.

Citation: HortScience 58, 1; 10.21273/HORTSCI16913-22

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Mean (± SE) area under disease progress curve (AUDPC) for cercospora leaf spot (Cercospora lythracearum) disease of crapemyrtle (Lagerstroemia sp.) cultivars planted in 2004 in McMinnville, TN, and evaluated on 29 Sep, 6 Oct, and 13 Oct 2017. Values are the means per plant for three single-plant replicates. Means followed by different lowercase letters on the top of the bar are significantly different (P ≤ 0.05) according to Fisher’s least significant difference test.

Citation: HortScience 58, 1; 10.21273/HORTSCI16913-22

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Mean (± SE) area under disease progress curve (AUDPC) for cercospora leaf spot (Cercospora lythracearum) disease of crapemyrtle (Lagerstroemia sp.) cultivars planted in 2004 in McMinnville, TN, and evaluated on 29 Sep, 6 Oct, and 13 Oct 2017. Values are the means per plant for three single-plant replicates. Means followed by different lowercase letters on the top of the bar are significantly different (P ≤ 0.05) according to Fisher’s least significant difference test.

Citation: HortScience 58, 1; 10.21273/HORTSCI16913-22

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Among cultivars in the 2011 planting, Arapaho, L. limii 1 and L. limii 2 selections, L. subcostata selection, Miami, Osage, and Tuscarora had significantly lower disease severity and AUDPC compared with Carolina Beauty, Red Rocket, Rhapsody in Pink, and Twilight in all 3 years (Figs. 7 and 8). The L. subcostata selection evaluated in 2017 exhibited the lowest disease severity and AUDPC. Cultivars Red Rocket and Rhapsody in Pink in all 3 years, Carolina Beauty in 2016 and 2017, and Twilight in 2017 had the highest disease severity and AUDPC. ‘Natchez’ had significantly higher disease severity and AUDPC than cultivars Arapaho, L. limii 1, L. limii 2 selections, L. subcostata selection, Miami, Osage, and Tuscarora and significantly lower than Red Rocket, Rhapsody in Pink, and Twilight in at least 2 of the 3 evaluation years.

Mean (± SE) cercospora leaf spot (Cercospora lythracearum) disease severity (% of foliage affected) of crapemyrtle (Lagerstroemia sp.) cultivars planted in 2011 in McMinnville, TN, and evaluated on 22 Sep 2015, 5 Oct 2016, and 13 Oct 2017. Values are the means per plant for three single-plant replicates. Means followed by the different lowercase letters on the top of the bar are significantly different (P ≤ 0.05) according to Fisher’s least significant difference test. Mean comparisons were performed within a year.

Citation: HortScience 58, 1; 10.21273/HORTSCI16913-22

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Mean (± SE) cercospora leaf spot (Cercospora lythracearum) disease severity (% of foliage affected) of crapemyrtle (Lagerstroemia sp.) cultivars planted in 2011 in McMinnville, TN, and evaluated on 22 Sep 2015, 5 Oct 2016, and 13 Oct 2017. Values are the means per plant for three single-plant replicates. Means followed by the different lowercase letters on the top of the bar are significantly different (P ≤ 0.05) according to Fisher’s least significant difference test. Mean comparisons were performed within a year.

Citation: HortScience 58, 1; 10.21273/HORTSCI16913-22

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Mean (± SE) cercospora leaf spot (Cercospora lythracearum) disease severity (% of foliage affected) of crapemyrtle (Lagerstroemia sp.) cultivars planted in 2011 in McMinnville, TN, and evaluated on 22 Sep 2015, 5 Oct 2016, and 13 Oct 2017. Values are the means per plant for three single-plant replicates. Means followed by the different lowercase letters on the top of the bar are significantly different (P ≤ 0.05) according to Fisher’s least significant difference test. Mean comparisons were performed within a year.

Citation: HortScience 58, 1; 10.21273/HORTSCI16913-22

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Mean (± SE) area under disease progress curve (AUDPC) for cercospora leaf spot (Cercospora lythracearum) disease of crapemyrtle (Lagerstroemia sp.) cultivars planted in 2011 in McMinnville, TN, and evaluated on 8 Sep, 15 Sep, and 22 Sep 2015; 29 Aug, 21 Sep, and 5 Oct 2016; 29 Sep, 6 Oct, and 13 Oct 2017. Values are the means per plant for three single-plant replicates. Means followed by different lowercase letters on the top of the bar are significantly different (P ≤ 0.05) according to Fisher’s least significant difference test. Mean comparisons were performed within a year.

Citation: HortScience 58, 1; 10.21273/HORTSCI16913-22

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Mean (± SE) area under disease progress curve (AUDPC) for cercospora leaf spot (Cercospora lythracearum) disease of crapemyrtle (Lagerstroemia sp.) cultivars planted in 2011 in McMinnville, TN, and evaluated on 8 Sep, 15 Sep, and 22 Sep 2015; 29 Aug, 21 Sep, and 5 Oct 2016; 29 Sep, 6 Oct, and 13 Oct 2017. Values are the means per plant for three single-plant replicates. Means followed by different lowercase letters on the top of the bar are significantly different (P ≤ 0.05) according to Fisher’s least significant difference test. Mean comparisons were performed within a year.

Citation: HortScience 58, 1; 10.21273/HORTSCI16913-22

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Mean (± SE) area under disease progress curve (AUDPC) for cercospora leaf spot (Cercospora lythracearum) disease of crapemyrtle (Lagerstroemia sp.) cultivars planted in 2011 in McMinnville, TN, and evaluated on 8 Sep, 15 Sep, and 22 Sep 2015; 29 Aug, 21 Sep, and 5 Oct 2016; 29 Sep, 6 Oct, and 13 Oct 2017. Values are the means per plant for three single-plant replicates. Means followed by different lowercase letters on the top of the bar are significantly different (P ≤ 0.05) according to Fisher’s least significant difference test. Mean comparisons were performed within a year.

Citation: HortScience 58, 1; 10.21273/HORTSCI16913-22

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In the 2004 planting, L. fauriei ‘Fantasy’, ‘Kiowa’, ‘Townhouse’, and ‘Woodlander’s Chocolate Soldier’ and L. indica × L. fauriei ‘Apalachee’, ‘Tonto’, and ‘Tuskegee’ were consistently among the least defoliated cultivars in all 3 evaluation years, with less than 3%, 3%, and 15% defoliation in 2015, 2016, and 2017, respectively (Table 7). Other cultivars, such as Choctaw, Miami, Natchez, Osage, Sarah’s Favorite, and Tuscarora, were among the least defoliated cultivars in 2015 and 2016 (<10%) but were more than 20% defoliated in 2017. ‘Burgundy Cotton’, ‘Cheyenne’, ‘Christiana’, ‘Dynamite’, and ‘William Toovey’ consistently exhibited the highest defoliation (>80%) in all 3 years.

Table 7.

Mean defoliation due to cercospora leaf spot (Cercospora lythracearum) of crapemyrtle (Lagerstroemia sp.) cultivars planted in 2004 in McMinnville, TN.

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Similar to disease severity results from the 2011 planting, ‘Arapaho’, the two L. limii selections, the L. subcostata selection, and ‘Miami’ had the lowest defoliation percentage in all 3 years, with less than 3%, 8%, and 18% in 2015, 2016, and 2017, respectively (Table 8). Additional cultivars Natchez, Osage, and Tuscarora in 2015; Natchez, Osage, Red Rocket, and Tuscarora had similarly lowest defoliation in 2016. ‘Arapaho’, ‘Osage’, ‘Tuscarora’, and the two L. limii selections had less than 1% defoliation in 2015, ∼1% to 3% in 2016, and up to 50% defoliation in 2017. ‘Rhapsody in Pink’ was the most severely defoliated among the cultivars and selections planted in 2011, with >85% defoliation in all 3 evaluation years. In both the 2004 and 2011 plantings, defoliation evaluated in 2015 and 2017 had a strong, positive correlation with cercospora leaf spot severity (r ≥ 0.84; Table 9). In 2016, defoliation and cercospora leaf spot severity had a moderate, positive correlation in both the 2004 (r = 0.68) and 2011 (r = 0.64) plantings.

Table 8.

Mean defoliation due to cercospora leaf spot (Cercospora lythracearum) of crapemyrtle (Lagerstroemia sp.) cultivars planted in 2011 in McMinnville, TN.

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Table 9.

Coefficients of linear correlation (r) for the relationship between final annual cercospora leaf spot (Cercospora lythracearum) disease severity (% of foliage affected) and defoliation (%) of crapemyrtle (Lagerstroemia sp.) cultivars in the 2004 and 2011 plantings in McMinnville, TN.

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Cultivars Carolina Beauty, Miami, Natchez, Osage, Red Rocket, and Tuscarora were included in both 2004 and 2011 plantings and evaluated from 2015 to 2017. In 2015, ‘Carolina Beauty’, ‘Osage’, and ‘Tuscarora’ that had been planted in 2004 had significantly higher cercospora leaf spot severity compared with those planted in 2011 (Table 10). In the 2016 evaluation, ‘Tuscarora’ showed significantly higher disease severity in 2004 planting than in 2011 planting, whereas ‘Red Rocket’ exhibited significantly higher disease severity in the 2011 planting than in the 2004 planting. In 2017, ‘Miami’ had significantly higher cercospora leaf spot severity in the 2004 planting than in the 2011 planting.

Table 10.

Effects of planting years (2004 and 2011) on cercospora leaf spot (Cercospora lythracearum) disease severity (% of foliage affected) of crapemyrtle (Lagerstroemia sp.) cultivars in each evaluation year.

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Discussion

In this 3-year evaluation, 32 cultivars of Lagerstroemia sp. planted in 2004 and 12 cultivars planted in 2011 were evaluated for cercospora leaf spot severity, AUDPC, and defoliation in USDA Hardiness Zone 6b from Sep to Oct 2015 to 2017. L. fauriei cultivars Fantasy, Kiowa, Townhouse, and Woodlander’s Chocolate Soldier; L. indica × L. fauriei cultivar Apalachee; two L. limii selections; and an L. subcostata selection were highly resistant to cercospora leaf spot. L. indica × L. fauriei × L. limii ‘Arapaho’ and L. indica × L. fauriei cultivars Choctaw, Miami, Natchez, Osage, Sarah’s Favorite, Tonto, Tuscarora, and Tuskegee were moderately resistant to cercospora leaf spot. Most of the cultivars of L. indica and L. indica × L. fauriei × L. limii Cheyenne were highly susceptible to cercospora leaf spot, had the highest AUDPC, and highest defoliation in all the evaluation years.

Previous literature describes the susceptibility of L. indica and resistance of L. fauriei to powdery mildew, but only one study (in addition to the current study) has assessed cercospora leaf spot severity on hybrid cultivars in the southern United States, where much of the crapemyrtle production occurs. In a previous crapemyrtle cultivar screening study conducted in Alabama, Hagan et al. (1998) reported L. fauriei ‘Fantasy’ had high resistance to cercospora leaf spot, L. indica cultivars were generally highly susceptible, and L. indica × L. fauriei hybrids were intermediate or similar to L. indica in susceptibility to cercospora leaf spot. Our results generally agree with this pattern of resistance to cercospora leaf spot and furthermore indicate that cercospora leaf spot tolerance is a consistent trait across four L. fauriei cultivars, not unique to the cultivar Fantasy. The other three cultivars of L. fauriei that showed strong resistance to cercospora leaf spot in this study were not evaluated by Hagan et al. (1998). Hagan et al. (1998) reported that among hybrid cultivars, Apalachee was highly resistant to cercospora leaf spot and Acoma was highly susceptible. Our results corroborate this finding, as cercospora leaf spot severity in ‘Apalachee’ was similar to that of the L. fauriei cultivars in 2 of the 3 evaluation years, and ‘Acoma’ was consistently among the most severely infected.

Although hybrids of L. indica and L. fauriei were generally more resistant to cercospora leaf spot than cultivars of L. indica, the relative level of resistance among hybrids varied and did not necessarily coincide with previously reported powdery mildew resistance (Chappell et al. 2012; Hagan et al. 1998). For example, ‘Acoma’ has been reported to have excellent resistance to powdery mildew yet was moderately to highly susceptible to cercospora leaf spot in the current and previous studies (Egolf 1986; Hagan et al. 1998). Other L. indica × L. fauriei cultivars that had moderate to high resistance to cercospora leaf spot in the current study (e.g., Apalachee, Tuscarora, and Tuskegee) are also highly resistant to powdery mildew. Whereas past crapemyrtle breeding efforts have focused on using L. fauriei primarily to develop powdery mildew–resistant hybrid selections (Dix 1999), cercospora leaf spot resistance should be included in the selection criteria in future L. indica × L. fauriei hybridizations.

Both L. limii selections and the L. subcostata selection planted in 2011 showed resistance to cercospora leaf spot. The genetic similarity among L. limii, L. subcostata, and L. fauriei (Rinehart et al. 2015; Wang et al. 2022) supports the idea that these three species may be similarly adapted to certain diseases. The L. limii and L. subcostata selections evaluated in this research may be useful breeding stock to not only confer cercospora leaf spot resistance but to also diversify the genetic base of cultivated crapemyrtle, which is currently almost entirely L. indica and L. fauriei. Additional L. limii and L. subcostata taxa need to be evaluated for cercospora leaf spot resistance to determine whether this trait is consistent within these species or specific to the selections included in this research.

Fluctuating final disease severity values between evaluation years may have been a consequence of environmental factors. The total rainfall during the evaluation months (August to October) was higher in 2017 (33.6 cm) than 2015 (24.4 cm) and 2016 (11.2 cm), and most likely played a role in increasing cercospora leaf spot severity in 2017 compared with the previous years. High humidity and wet leaf surfaces promote conidia germination and thus cercospora leaf spot development (Weiland and Koch 2004).

In at least 1 of the 3 evaluation years, ‘Carolina Beauty’, ‘Miami’, ‘Osage’, and ‘Tuscarora’ planted in 2004 had higher cercospora leaf spot severity compared with those planted in 2011. A possible explanation for higher cercospora leaf spot severity in the 2004 planting is the higher amount of pathogen inoculum present in the immediate vicinity of the 2004 plants or fallen leaves. Larger plants result in a shorter distance between trees, which would likely facilitate infection. Moreover, there were more L. indica cultivars in the 2004 vs. the 2011 planting that were more prone to cercospora leaf spot.

The strong, positive correlation between cercospora leaf spot severity and defoliation among the evaluated cultivars suggests that cercospora leaf spot likely contributed to premature leaf senescence. This observation further indicates the destructive association between cercospora leaf spot and the defoliation in Lagerstroemia sp. that affects to the overall well-being of the plants.

In this study, no crapemyrtle cultivar was 100% resistant to cercospora leaf spot; however, minor defoliation and the few leaf spots that were on the most resistant cultivars were confined to the lower branches and had minor impact on the esthetic value of the plant. Some of the cultivars or selections evaluated in this study, such as the L. subcostata and L. limii selections, have not been evaluated in other geographic locations before the current study. Results from this research may aid breeders, nursery producers, and landscapers in selecting crapemyrtle species and cultivars with cercospora leaf spot resistance.