Materials and Methods

Study sites.

A block of 41 commercially available Lagerstroemia cultivars in Blairsville, GA, USA (34.8761°N, 83.9584°W) at the Georgia Mountain Research and Education Center and two blocks of Lagerstroemia selections at the University of Georgia Horticulture Farm in Watkinsville, GA, USA (33.8629°N, 83.4088°W) were observed. Two or three replications of each cultivar were included in that planting at Blairsville.

At the Blue Ridge Mountain site in Blairsville [549 m elevation, USDA hardiness zone 7b (USDA 2023)], the plot comprised four rows of trees, 10 to 15 years of age, spaced 4.6 m apart. Cultivars were randomized within the rows.

The Watkinsville area in the Piedmont region of Georgia comprised a breeding program of Lagerstroemia (indica × fauriei) × subcostata, L. limii, L. indica, L. indica × subcostata, L. indica × limii, L. limii × indica, and L. [(indica × fauriei) × subcostata) × limii)] plants. Plants were established between 2010 and 2019 at the University of Georgia Horticulture Farm in Watkinsville [220 m elevation, USDA hardiness zone 8a (USDA 2023)]. Selections of Lagerstroemia (indica × fauriei) × subcostata were replicated once, whereas all others were single-plant evaluations. The L. subcostata breeding lines were from a seed source in Taiwan, and the L. limii selections were received as seed from South Korea.

Crape myrtle plants were observed bimonthly for leaf spotting and defoliation due to Cercospora leaf spot in Blairsville from 7 Jun 2021 to 11 Nov 2021 and from 12 Jul 2022 to 21 Oct 2022 and in Watkinsville from 1 Jul 2021 to 18 Nov 2021 and 16 Jun 2022 to 27 Oct 2022.

Disease evaluations.

A disease rating scale of 0 to 9 was created based on the Horsfall-Barratt scale (Horsfall and Barratt 1945) to rate the amount of disease observed on the crape myrtles. The ratings were a quality scale corresponding to the percentage of leaves with spots and defoliated leaves, such that 0 = 0% of leaves affected, 0 = 0%, 1 = 1% to 5%, 2 = 5% to 10%, 3 = 10% to 15%, 4 = 15% to 20%, 5 = 20% to 25%, 6 = 25% to 40%, 7 = 40% to 60%, 8 = 60% to 80%, 9 = 80% to 100% (Horsfall and Barratt 1945). Cultivars were given a rating of low, medium, or high resistance to Cercospora leaf spot based on the area under the disease progress curve (AUDPC). High resistance was defined as less than 150 AUDPC in 2021 and 40 AUDPC in 2022, moderate resistance between 150 and 350 AUDPC in 2021 and 40 and 150 AUDPC in 2022, and low resistance was defined as above 350 AUDPC in 2021 and 150 AUPDC in 2022 in the Blairsville plot (Table 1).

Table 1. Disease resistance rankings as indicated by area under the disease progress curve (AUDPC) values of 43 commercial crape myrtle (Lagerstroemia) cultivars to Cercospora leaf spot (Psuedocercospora lythracearum). Data collected from Blairsville, GA, USA study location unless noted.

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Data in Table 2 were analyzed using a t test, and data in Tables 3 and 4 were analyzed using a one-way analysis of variance and in the statistical programming language R (R Core Team 2021). The plugin epifitter (Alves and Del Ponte 2021) was used to calculate the AUDPC $Ak\hspace{0.17em}=\hspace{0.17em}{\displaystyle \sum _{i=1}^{N_i-1}\frac{\left(y_i+y_i+1\right)}{2}}\left(t_{i+1}-t_i\right)$. Tukey’s honestly significant difference test was used for mean comparison (alpha = 0.05). The R package multcompView (Graves et al. 2024) was used to visualize mean comparisons in Table 3.

Table 2. Area under the disease progress curve (AUDPC) of Lagerstroemia indica (n = 22) and L. indica × fauriei (n = 20) cultivars naturally infected with Cercospora Leaf Spot (Psuedocercospora lythracearum) in 2021 and 2022 in Blairsville, GA, USA (USDA Zone 7b).

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Table 3. Area under the disease progress curve (AUDPC) for Lagerstroemia species and hybrids naturally infected with Cercospora leaf spot (Pseudocercospora lythracearum) in Watkinsville, GA, USA (USDA Zone 8a).

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Table 4. Average area under the disease progress curve (AUDPC) for dark-foliage Lagerstroemia indica cultivars and their crosses by L. subcostata infected naturally with Cercospora Leaf Spot (Psuedocercospora lythracearum) in Watkinsville, GA, USA in 2021 and 2022.

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Results

Cercospora leaf spot appeared naturally in late June in Watkinsville and late July in Blairsville. The disease intensified from initial spot development to peak at different times depending on the cultivar or seedling selection at each study location. Crape myrtle accessions are highly susceptible to Cercospora leaf spot and had peak leaf spot incidence in late August to September in Watkinsville and mid-September in Blairsville before defoliating shortly after. In each year, highly resistant selections retained most of their leaves until the first freeze each year, after which they defoliated.

Discussion

For 2 years 42 cultivars in Blairsville, GA, USA, and 13 groups of species and hybrids in Watkinsville, GA, USA, were evaluated for season-long leaf spotting and premature defoliation. Cercospora leaf spot susceptibility was quantified by calculating the AUDPC, a measure of disease severity over time. L. indica × fauriei cultivars had significantly lower AUDPC values than L. indica cultivars in Blairsville. Before the current study, two studies have assessed the relationship between L. indica × fauriei hybridization and Cercospora leaf spot. Parajuli et al. (2023) found that L. indica × fauriei hybrids generally had a lower AUDPC values in L. indica cultivars. Still, there was wide variation among the susceptibility of L. indica × fauriei cultivars. Our study supports this finding, although we observed less season-long disease development within L. indica × fauriei cultivars. Parajuli et al. (2023) observed that L. indica × fauriei ‘Acoma’ was among the most susceptible cultivars, whereas in our study, only moderate susceptibility was observed. Here, we found no L. indica × fauriei cultivars with high susceptibility, but only low and moderate susceptibility. Additionally, our study observed no L. indica cultivars with high resistance to Cercospora leaf spot. L. indica ‘Dynamite’ was observed to be the most susceptible cultivar evaluated, confirming the same observation from Parajuli et al. (2023). Parajuli et al. (2023) also observed that pure L. fauriei cultivars were resistant to Cercospora leaf spot, and this trait was consistent among all observed plants. Hagan et al. (1998) also observed that L. fauriei was resistant to Cercospora leaf spot. Our study did not include pure L. fauriei selections. Hagan et al. (1998) observed no correlation between L. indica × fauriei cultivars and resistance, with L. indica × fauriei cultivars showing similar levels of disease compared with L. indica cultivars. Our results disagree with the conclusion of Hagan et al. (1998) that L. indica × fauriei cultivars are not more resistant to Cercospora leaf spot than L. indica. The difference in results between our study and Parajuli et al. (2023) and Hagan et al. (1998) could be due to evaluation methods and location. Hagan et al. (1998) evaluated each cultivar once per year in late August or early September in the lower Coastal Plain of Alabama, USA, whereas Parajuli et al. (2023) evaluated cultivars throughout the entire progression of disease development using AUDPC to determine plant susceptibility on the Cumberland Plateau of Tennessee, USA.

Additionally, study location could impact the timing of disease onset and intensification due to differing levels of rainfall and humidity between locations. During warm, wet conditions, leaf spottings and defoliation may rapidly intensify from August to October (Chappell et al. 2012). Although there were no differences in total rainfall between locations (∼48 cm) in 2021, rainfall was 64% less in Blairsville in 2022 compared with Watkinsville (Supplemental Tables 1 and 2). Between 2021 and 2022, total rainfall decreased 165% in Blairsville and 59% in Watkinsville. In Aug 2021, in Blairsville, there was 34 cm of rainfall compared with 8.9 cm in Aug 2022 (Supplemental Table 1). Parajuli et al. (2023) also observed that Cercospora leaf spot disease could vary yearly based on rainfall distribution during the growing season.

In addition to Cercospora leaf spot, crape myrtles are also affected by the fungal disease powdery mildew, caused by Erysiphe australiana (McAlpine) Braun and Takamatsu. Previously, the high resistance of L. fauriei and L. indica × fauriei to powdery mildew was reported (Chappell et al. 2012; Egolf 1986; Hagan et al. 1998). However, based on our findings and Parajuli et al. (2023), L. indica × fauriei hybridization does not impart as much resistance to Cercospora leaf spot as powdery mildew. Similar results were reported by Hagan et al. (1998).

There are some trade-offs associated with cross-breeding L. indica by L. fauriei. The main disadvantages are reduced flower size, inflorescence size, and less vibrant flower color, as L. fauriei has small petals with pale colors. The valuable traits associated with crossing L. indica × L. fauriei are bronze exfoliating bark along with resistance to powdery mildew and Cercospora leaf spot. The most susceptible L. indica plants still have horticultural value because the flowers of many cultivars are larger with more vibrant color than those of the L. indica × fauriei cultivars during the flowering months before Cercospora leaf spot is severe. Further improvements can be made to flower size and color by continued interspecific hybridization among other Lagerstroemia species.

Thirteen groups of Lagerstroemia species and hybrids were evaluated in Watkinsville for Cercospora leaf spot susceptibility in 2021 and 2022. Few significant differences exist between these groups, with two notable standouts, L. limii × indica in 2021 being very susceptible and L. (indica × fauriei) × subcostata being very resistant in 2022. The lack of significant differences was likely due to the wide variation in AUDPC values observed in each group. For example, the L. indica × subcostata group contained CANR-1 with an AUDPC of 693, a plant with severe disease completely defoliated by September, and CANR-7 with an AUDPC of 112, a plant that retained most of its leaves until the first frost each year. Variation was seen among almost every group except for L. indica, L. limii × indica, and L. indica × limii. These three groups had consistently high Cercospora leaf spot susceptibility among all individuals. Parajuli et al. (2023) found few significant differences between different species, finding that L. indica was significantly more susceptible than all other evaluated hybrids and species, including L. subcostata L. limii, and L. indica × fauriei × limii. Our study was unable to confirm these relationships. Parajuli et al. (2023) observed resistance in their L. limii population. Resistance to Cercospora leaf spot was not seen in our L. limii population, suggesting that resistance may not be consistent across an entire species.

Two dark-foliage cultivars, Ebony Embers and Ebony Flame, susceptible to Cercospora leaf spot were crossed in 2018 with L. subcostata selections and established in Watkinsville. These F₁ individuals were selected based on powdery mildew resistance and dark foliage. The L. indica × subcostata selections from this cross had significantly better resistance to Cercospora leaf spot than both ‘Ebony Embers’ and ‘Ebony Flame’. On the basis of this finding and the discovery that L. (indica × fauriei) × subcostata hybrids were resistant to Cercospora leaf spot in 2022 support the idea that hybridization with L. subcostata is a source of disease resistance in crape myrtle cultivars. This is further supported by Parajuli et al. (2023), who observed resistance to Cercospora leaf spot in L. subcostata selections, and by Rinehart et al. (2015) and Wang et al. (2023), who observed a genetic similarity between L. fauriei and L. subcostata.

No crape myrtle selections of L. indica, L. indica × fauriei, and L. (indica × fauriei) × subcostata proved immune to Cercospora leaf spot, with all selections showing varying levels of leaf spottings and premature defoliation. Study results may guide breeders in selecting species and landscapers in selecting cultivars with resistance to Cercospora leaf spot. Previous studies have been conducted in the northern and southern United States, but this is the first study with disease comparisons in the Piedmont and Blue Ridge Mountain regions of Georgia, USA.