Bulletin 673 March 2010

Alabama Agricultural Experiment Station Richard Guthrie, Director Auburn University, Auburn, Alabama

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Cercospora Leaf Spot and Growth of Crapemyrtle as Inﬂuenced by Nitrogen Rate
A. K. Hagan

Introduction Brilliant fall color, handsome bark, and showy ﬂowers have made crapemyrtle (Lagerstroemia indica L., L. indica x faurei) a ﬁxture in Southern landscapes (32,33). While the diseases powdery mildew (Erysiphe lagerstroemia) and Cercospora leaf spot (Cercospora lythracearum) are not serious health threats to established crapemyrtle (1,2,11), both can greatly detract from the beauty of crapemyrtle in landscape plantings as well as the value of ﬁeld and container-grown nursery stock (16). Cercospora leaf spot, which is characterized by tan to dark brown spots randomly scattered on yellow to red discolored leaves (Figure 1), can ruin the fall color display of crapemyrtle (16) (Figure 2); however, impact of this disease as well as that of powdery mildew (Figure 3) on the growth and ﬂowering of crapemyrtle has not been assessed. Recently, damaging outbreaks of Cercospora leaf spot, spot anthracnose, and powdery mildew have been associated with reduced tree growth and/or trunk diameter on ﬂowering dogwood (14,18,25). Similarly, growth of hybrid tea and shrub roses partially defoliated by black spot (4,19) or Cercospora leaf spot (19) was greatly reduced. Bowen et al. (4) also noted that the number of ﬂowers on hybrid tea roses declined as the severity of black spot increased. Nitrogen (N) rate can inﬂuence the severity of diseases of landscape shrubs and trees. Increasing N rates intensify ﬁre blight severity on apple (30), Phytophthora dieback on rhododendron (21), and powdery mildew on ﬂowering dogwood (14). In contrast, resistance of black walnut to anthracnose increased with supplemental N applications (26). Severity of Alternaria leaf spot as well
Hagan is a professor in the Department of Entomology and Plant Pathology, Auburn University.

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Figure 1, top row above, Cercospora leaf spot. A) Brown leaf spots and reddish leaf discoloration on ‘Carolina Beauty’ crapemyrtle and B) ‘Wonderful White’ crapemyrtle defoliated by Cercospora leaf spot. Figure 2, middle row above, ‘Carolina Beauty’ crapemyrtle. A) Absence of a fall color display on a defoliated, non-fungicide treated tree compared with B) vibrant reddish-orange fall color display due to increased leaf retention on a Heritage 50WDG-treated tree. Figure 3, left, Powdery mildew. A) Early sign is circular white colonies of causal fungus on leaves that B) may intensify until entire leaf surface and shoots are colonized and C) the diseased plants turn almost white.

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as leaf spot diseases incited by plant pathogenic bacteria in the genera Xanthomonas and Pseudomonas on several tropical herbaceous and woody plants fell with increasing N rates (6). With ﬂowering dogwood, increasing N rates may reduce dogwood anthracnose intensity when weather patterns are less than ideal for disease development (3). Intensity of Cercospora leaf spot and to a lesser extent spot anthracnose on ﬂowering dogwood declined when N rates increased from 37.7 pounds to 300 pounds N per acre per year (14). Few crapemyrtle cultivars are resistant to powdery mildew and Cercospora leaf spot (16,22); therefore, most may be damaged by one or both of these diseases. While fungicides are a control option for both diseases (10,12), considerable time and resources would be needed to maintain a preventive fungicide program in a landscape or nursery setting. While not as effective, adjusting nitrogen rates may be an alternative to fungicides for managing these diseases on crapemyrtle. The objectives of this study were to evaluate the impact of N rate on the development of powdery mildew and Cercospora leaf spot on crapemyrtle in a simulated landscape planting and to determine the effects of N rate and fungicide applications on plant growth. Materials and Methods Plant management. On February 5, 2002, 72 ‘Carolina Beauty’ crapemyrtles (Lagerstroemia indica) were planted into a Benndale (A) ﬁne sandy loam at the Brewton Agricultural Research Unit in Brewton, Alabama. According to the results of a pre-plant soil fertility assay, extractable concentrations of phosphorus, potassium, magnesium, and calcium were 66, 66, 140, and 520 pounds per acre, respectively (7). Prior to planting, 1.5 tons per acre of dolomitic limestone and 450 pounds per acre of 5-10-5 analysis fertilizer were broadcast and incorporated. A drip irrigation system with a single emitter per tree was installed before planting and the trees were watered as needed. A 9-foot2 area of approximately 1 inch of aged pine bark mulch was maintained around the base of each tree and kept weed-free. Centipedegrass (Eremochola ophiuroides) alleys separating each row of trees were periodically mowed but not fertilized during the study period. Separate applications of 3.6 ounces of murate of potash (0-0-60 K2O) and super-phosphate (0-46-0 P204) were made over the mulched area around each tree on March 3, 2003, and March 9, 2004. Directed applications of 1 pound per acre of Gallery® DF plus 2 quarts per acre of Surﬂan® T/O were made to the mulched area on March 5, 2003, November 3, 2003, April 22, 2004, and March 18, 2005 for pre-emergence weed control. Hand weeding and spot applications of Finale® 1E at 2 ﬂuid ounces per gallon were used to control escaped weeds.

Design. The experimental design was a split plot arranged in six replications with nitrogen (N) rate as the main plot and Heritage 50WDG fungicide treatment as the split plot. Ammonium nitrate (33N-0P2O5-0K2O) was applied at 18.8, 37.5, 75, 150, 300, and 600 pounds N per acre per year for each of the six replications. A quarter of each N rate was evenly distributed over a 2-foot2 area around the base of each plant in March, April, May, and June of each year. Heritage 50WDG was applied at a rate of 4 ounces per 100 gallons of spray volume to one of two ‘Carolina Beauty’ crapemyrtle in each replicate, while the second tree was not treated with the fungicide. The fungicide was applied to drip with a CO2-pressurized sprayer at two-week intervals from May 2 to July 10, 2003, May 5 to July 14, 2004, and May 4 to July 29, 2005. Applications were timed to control powdery mildew. Data collection and analysis. Cercospora leaf spot intensity was rated using the Horsfall and Barratt rating scale (23) where 0 = 0 percent, 1 = 0 to 3 percent, 2 = 3 to 6 percent, 3 = 6 to 12 percent, 4 = 12 to 25 percent, 5 = 25 to 50 percent, 6 = 50 to 75 percent, 7 = 75 to 87 percent, 8 = 87 to 94 percent, 9 = 94 to 97 percent, 10 = 97 to 100 percent, and 11 = 100 percent of diseased and/ or prematurely shed leaves. Cercospora leaf spot-induced defoliation was also rated using the above scale. Ratings for Cercospora leaf spot were recorded on July 16, August 19, September 13, October 2, October 15, October 28, and November 15, 2003; July 29, August 24, September 30, October 27, and November 9, 2004; and June 27, July 27, August 24, October 10, October 25, November 5, and November 16, 2005. Powdery mildew incidence was also assessed using the Horsfall and Barratt rating scale (23) on the above dates. Disease ratings were not recorded in 2002. Tree height and canopy diameter were measured on January 14, 2004, January 31, 2005, and January 12, 2006. Growth index (GI) was calculated using the following formula: GI = (height + width 1 + width 2)/3. Within one to two weeks after their measurement, each tree was lightly pruned primarily to remove the spent bloom clusters. In each year, area under the disease progress curve (AUDPC) was calculated for total leaf spot intensity and defoliation attributed to Cercospora leaf spot on crapemyrtle (28). Outliers of AUDPC values were removed from data sets. AUDPC and GI values were transformed to normalize data before conducting analysis of variance (ANOVA) then back-transformed for presentation. ANOVA was done according to a split plot design arrangement of treatments using the GLM procedure. The main factor was N rate and the split plot was fungicide treatment. Signiﬁcance of interactions was ﬁrst evaluated. Means were separated using Fisher’s protected least signiﬁcant difference at P≤0.05.

Cercospora Leaf Spot and Growth of Crapemyrtle as Inﬂuenced by Nitrogen Rate — 2 —

Results N rate and powdery mildew. While ‘Carolina Beauty’ crapemyrtle is susceptible to powdery mildew (16), development of the disease was minimal throughout the study period. In 2003 and 2004, no signs of powdery mildew were noted on the leaves, shoots, ﬂower peduncle, or ﬂower bud scales. On June 27, 2005, light powdery mildew development on the ﬂower peduncle and bud scales was seen on some trees. Powdery mildew incidence was not inﬂuenced by N rate (data not shown). N rate and Cercospora leaf spot. In 2003 and 2004, N rate did not have a signiﬁcant impact on Cercospora leaf spot total disease intensity or premature defoliation on crapemyrtle. The non-signiﬁcant N rate x fungicide interaction for total leaf spot density and defoliation values in 2003 and 2004 showed that the impact of N rate on disease development on the fungicide and non-fungicide treated trees was similar. In 2005 a signiﬁcant decline in total leaf spot intensity and defoliation was linked with increasing N rates (Figure 4). At 300 and 600 pounds of N per acre, total leaf spot intensity and defoliation values were signiﬁcantly lower than at 18.8, 37.5, and 75 pounds N per acre. While total leaf spot intensity and defoliation values at 75 and 150 pounds N per acre were similar, trees receiving the higher N rate suffered less leaf spotting and premature defoliation compared with those receiving 18.8 and 37.5 pounds N per acre. As indicated by a nonsigniﬁcant interaction for total leaf spot intensity and defoliation, inﬂuence of N rate on Cercospora leaf spot development on the fungicide and non-fungicide treated trees was similar. Previous studies have shown that increasing N rates are associated with a reduction in the incidence of Cercospora leaf spot on ﬂowering dogwood (14), walnut anthracnose (26), and Alternaria and bacteria-incited diseases of tropical herbaceous and woody foliage plants (6). In the current study, the non-fungicide-treated crapemyrtles receiving the three highest N rates had 50 pecent less leaf spotting and premature defoliation than the crapemyrtles receiving the three lowest N rates on the late October and November rating dates in 2005 (Figures 5 and 6). Heritage fungicide, Cercospora leaf spot intensity, and crapemyrtle growth. Heritage 50WDG, which was applied from mid-spring through early summer in all study years, greatly reduced the level of Cercospora leaf spot-related leaf spotting and premature defoliation into November (Figure 6). Overall, Heritage 50WDG reduced the level of leaf spotting due to Cercospora leaf spot by 92 percent, 79 percent, and 93 percent in 2003, 2004, and 2005, respectively, compared to non-fungicide treated trees (Table 1). In all three years, similar reductions in Cercospora leaf spot-related defoliation were also obtained.

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Cercospora leaf spot AUDPC values

1400 1200 1000 800 600 400 200 0 18.8 lb

AB ABC BC C

Total Leaf Spot Defoliation

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Figure 4, top left. Areas under disease progress curves for Cercospora leaf spot intensity and defoliation in response to N rate in 2005. Values were recorded between July 23 and November 16, 2005. 1 Means for the total leaf spot or defoliation values that are followed by the same letters are not signiﬁcantly different according to Fisher’s protected least signiﬁcant difference (LSD) test (P≤0.05).

37.5 lb

75 lb

150 lb

300 lb

600 lb

N rate per acre

Figure 5, left. A) Inﬂuence of N rate on Cercospora leaf spot intensity and B) defoliation during the 2005 growing season.

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Cercospora Leaf Spot and Growth of Crapemyrtle as Inﬂuenced by Nitrogen Rate — 3 —

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Despite sizable reductions in the level of disease-related leaf spotting and premature leaf loss with Heritage 50WDG, growth indices (GI) for the non-fungicide and fungicide-treated crapemyrtle were similar in 2004, 2005, and 2006 (Table 2). Although the control of Cercospora leaf spot with Heritage 50WDG did not result in an increase in tree growth, fungicide-treated trees in October and November, particularly at the lower N rates, had a denser leaf canopy compared with the adjacent non-fungicide treated crapemyrtle (Figure 6A and B). In contrast, little visible difference in leaf canopy density on the Heritage 50WDG- and the non-fungicide-treated trees was seen at the highest N rate (Figure 6C). While applications of Heritage 50WDG ended in mid- to late-July when Cercospora leaf spot usually begins, this fungicide noticeably slowed disease spread into November (Figure 7). Previously, Heritage 50WDG controlled Cercospora leaf spot on ﬁeld-grown ‘Wonderful White’ crapemyrtle (12) and on container-grown bigleaf hydrangea (17). In all years, the GI for the non-treated and fungicide-treated crapemyrtle were similar. This result was especially surprising in 2004, when the level of leaf spotting and defoliation at the November rating dates was noticeably higher than in 2003 and 2005. In a previously published study, damaging outbreaks of Cercospora leaf spot on ﬂowering dogwood were correlated with reduced tree height and trunk diameter (14). In a concurrent study, control of leaf spot, powdery mildew, and spot anthracnose with several fungicides on established ‘Rubra’ ﬂowering dogwood did not translate into inTable 1. Cercospora leaf spot intensity and defoliation on Heritage 50WDG fungicide-treated and non-fungicide-treated crapemyrtle ———2003——— ———2004——— ———2005——— Treatment Leaf spot1 Defoliation Leaf spot Defoliation Leaf spot Defoliation No fungicide 3565 a2 2018 a 5115 a 3972 a 1905 a 683 a Treated3 275 b 87 b 1063 b 704 b 129 b 23 b
1 Cumulative leaf spot intensity and defoliation values calculated from total leaf spot and defoliation ratings recorded between July 16 to November 15, 2003, July 29 to November 9, 2004, and July 23 to November 16, 2005. 2 Means in each column that are followed by the same letters are not signiﬁcantly different according to Fisher’s protected least signiﬁcant difference (LSD) test (P≤0.05). 3 Heritage 50WDG fungicide was applied at approximately two-week intervals between May 2 to July 10, 2003, May 5 to July 14, 2004, and May 4 to July 29, 2005.

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Figure 6. Cercospora leaf spot-incited defoliation on October 25, 2005, on ‘Carolina Beauty’ crapemyrtle that received A) 18.8 pounds N per acre per year, B) 37.5 pounds N per acre per year, and C) 600 pounds N per acre per year.

Table 2. Impact of Heritage 50WDG fungicide on the growth of ‘Carolina Beauty’ crapemyrtle ——Growth Index1—— Treatment 2004 2005 2006 No fungicide 1722 196 202 Heritage 50WDG-treated 167 197 200
Growth Index = (height + width 1 + width 2)/3. Means did not differ signiﬁcantly according to Fisher’s protected least signiﬁcant difference (LSD) test P≤0.05.
2 1

Cercospora Leaf Spot and Growth of Crapemyrtle as Inﬂuenced by Nitrogen Rate — 4 —

100 90 80 70 60 50 40 30 20 10 0 A u g u st 1 S e p te m b e r 1 O c to b e r 1 N o ve m b e r 1

CLS CLS CLS CLS

In te n s ity N o n F u n g ic id e T re a te d In te n s ity F u n g ic id e T re a te d D e fo lia tio n N o n F u n g ic id e T re a te d D e fo lia tio n F u n g ic id e T re a te d

2003

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In te n s ity N o n F u n g ic id e T re a te d In te n s ity F u n g ic id e T re a te d D e fo lia tio n N o n F u n g ic id e T re a te d D e fo lia tio n F u n g ic id e T re a te d

2004

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CLS CLS CLS CLS

In te n s ity N o n F u n g ic id e T re a te d In te n s ity F u n g ic id e T re a te d D e fo lia tio n N o n F u n g ic id e T re a te d D e fo lia tio n F u n g ic id e T re a te d

2005

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Figure 7. Impact of fungicide treatment on Cercospora leaf spot (CLS) intensity and defoliation on ‘Carolina Beauty’ crapemyrtle in 2003, 2004, and 2005. (Six fungicide applications were made each year, beginning the ﬁrst week of May and ending the second week of July, except in 2005 when seven fungicides applications were made, ending the fourth week of July).

creased shoot elongation or trunk diameter (13). In contrast, control of severe powdery mildew on container-grown ‘First Lady’ ﬂowering dogwood with one of several fungicides resulted in a sizable increase in canopy spread and trunk diameter (18). In addition, increased shoot growth and ﬂower bud numbers on hybrid tea and shrub roses have resulted from the control of black spot and Cercospora leaf spot with protective fungicides (4,19). N rate and crapemyrtle growth. Since the N rate GI values for the nontreated and fungicide-treated crapemyrtle, as indicated by a non-signiﬁcant N rate x fungicide interaction, were similar, growth data for the non-treated and fungicide-treated crapemyrtle from the January 2004, 2005, and 2006 rating dates were pooled. In all three years, crapemyrtle growth was not impacted by N rate. Failure to observe differences in the growth of ‘Carolina Beauty’ crapemyrtle (L. indica) related to increasing N rates over the three-year study period was unexpected. Unfortunately, relatively little information is available concerning the growth response of container- or ﬁeld-grown crapemyrtle to N inputs. Cabrera and Devereaux (5) noted that containerized crapemyrtle are known among nursery producers as being heavy feeders, i.e. the growth response of crapemyrtle is proportional to N fertilization rate. For containerized ‘Tonto’ crapemyrtle (L. indica x fauriei) grown in a sphagnum peat moss, vermiculite, and sand media (2:1:1 ratio by volume), shoot biomass, leaf area, and canopy diameter rose as N rate increased from 15 to 60 mg per liter and these parameters declined at higher N rates due to tree sensitivity to high soluble salt levels (5). In contrast, little difference in stem cross-section area was noted over a two-year period for established ‘Natchez’ crapemyrtle (L. indica x fauriei) receiving no supplemental N compared with 89 pounds N per acre per year (24). Recently, Hagan et al. (15) noted that trunk diameter and height of ﬁeld-grown ﬂowering dogwood was not impacted by N rate until three and four years, respectively, after transplanting and initial N application. Delays ranging from 18 months to four years after transplanting in responding to N inputs have previously been reported for bare-root (31), containerized (9), balled and burlaped (8,9) and established landscape trees (27). In contrast, N applications at-planting resulted in signiﬁcant increases in height and shoot length within six months following the transplanting of container-grown southern magnolia (20). Treatment interference due to shared root zones, high fertility of native soils, and sod competition may increase the difﬁculty in assessing shade tree growth response to N inputs (29). Summary While we were able to demonstrate that elevated N rates slowed the development of Cercospora leaf spot on crapemyrtle in one of three years, effective-

Cercospora Leaf Spot and Growth of Crapemyrtle as Inﬂuenced by Nitrogen Rate — 5 —

ness of using N as a tool for managing this disease, particularly on established trees in residential or commercial landscapes is doubtful. The annual application of 40 to 70 pounds N per acre per year, which is currently recommended for landscape plantings of crapemyrtle (33), is well below 300 to 600 pounds per acre per year, which noticeably reduced leaf spotting and premature defoliation due to Cercospora leaf spot. While the Heritage 50WDG fungicide proved highly effective in controlling Cercospora leaf spot and greatly enhanced leaf retention and ultimately fall color of ‘Carolina Beauty’ crapemyrtle, control of this disease did not translate into enhanced plant growth. Given the concerns about N contamination of surface and ground water as well as health risks and expense of a protective fungicide program, the most effective control is the establishment of a Cercospora leaf spot and powdery mildew resistant crapemyrtle cultivar such as ‘Apalachee’, ‘Basham’s Party Pink’, ‘Caddo’, ‘Fantasy’, ‘Sarah’s Favorite’, ‘Tonto’, ‘Tuskegee’, and ‘Tuscarora’ (16). We also showed that N rate had little effect on crapemyrtle growth. While container-grown crapemyrtle may be a heavy feeder of N, ‘Carolina Beauty’ crapemyrtle, with its low N requirements, ﬁts the description of a low maintenance landscape plant. Literature Cited 1. Alﬁeri, S. A. 1969. Powdery mildew on crapemyrtle. Fla. Dept. of Agric. Cons. Serv. Plant Path. Cir. 83. 2. Aﬂieri, S. A. 1976. Cercospora leaf spot of crapemyrtle, Lagerstroemia indica. Fla. Dept. of Agric. Cons. Serv. Plant Path. Cir. 171. 3. Anderson, R. L., J. Rauschenberger, J. L. Knighten, S. E. Dowsett, and C. Henson. 1992. Effectiveness of cultural techniques, with emphasis on fertilization to control dogwood anthracnose. p. 53-56. In Results of the 1992 dogwood anthracnose impact assessment and pilot test in the Southeastern United States. J. L. Knighten and R. L. Anderson, eds. USDA For. Serv. South Region Prot. Rep R8-PR 24. 4. Bowen, K. L., B. Young, and B. Behe. 1995. Management of black spot in rose in the landscape in Alabama. Plant Disease 79:250-253. 5. Cabrera R. I. and D. R. Devereauz. 1998. Effects of nitrogen supply on growth and nutrient status of containerized crape myrtle. J. Environ. Hort. 16:98-104. 6. Chase, A. R. 1991. Greenhouse ornamental crops, pest management systems for diseases. p. 709-728. In Handbook of Pest Management, 2nd ed. Vol.3. D. Pimental, ed. CRC Press, Boca Raton, FL. 7. Donohue, S. J. 1992. Reference soil test methods for the southern region of the United States. Univ. of Georgia. Sou. Coop. Ser. Bull. 374.

8. Ferrini, F. and M. Baietto. 2006. Response to fertilization of different tree species in the urban environment. Arboriculture & Urban Forestry 32:93-99. 9. Gilman, E. F., T. H. Yeager, and D. Kent. 2000. Fertilizer rate and type impacts magnolia and oak growth in a sandy landscape soil. J. Arboriculture. 26:177-182. 10. Hagan A. K. 2001. Crapemyrtle diseases. Pages 114-116, In Diseases of Woody Ornamentals and Trees in Nurseries. R. Jones and M. Benson, eds. APS Press. St. Paul, MN. 11. Hagan, A. K. 2004. Common diseases of crapemyrtle. Ala. Coop. Ext. Sys. Cir. ANR-1047. 12. Hagan, A. K. and J. R. Akridge. 2007. Fungicides compared for the control of Cercospora leaf spot on crapemyrtle, 2006. Plant Disease Management Reports OT017. 13. Hagan, A. K. and J. R. Akridge. 2007. Synthetic and biorational fungicides compared for the control of three foliar diseases of ﬂowering dogwood. J. Environ. Hort. 25:157-165. 14. Hagan, A. K., J. R. Akridge, and K. L. Bowen. 2008. Nitrogen rate and ﬂowering dogwood. I. Inﬂuence of nitrogen fertilization rate on the occurrence of spot anthracnose, powdery mildew, and Cercospora leaf spot and impact of these diseases on tree growth. J. Environ Hort. 26:197-203. 15. Hagan, A. K., J. R. Akridge, and K. L. Bowen. 2008. Nitrogen rate and ﬂowering dogwood. II. Impact of nitrogen fertilization rate on ﬂower bud set and tree growth. J. Environ. Hort. 26:204-209. 16. Hagan, A. K., C. H. Gilliam, G. J. Keever, and J. D. Williams. 1998. Susceptibility of crapemyrtle cultivars to powdery mildew and Cercospora leaf spot in a simulated landscape planting in Alabama. J. Environ. Hort. 16:143146. 17. Hagan, A. K., J. W. Olive, J. Stephenson, and M. E. Rivas-Davila. 2004. Impact of application rate and interval on the control of powdery mildew and Cercospora leaf spot on bigleaf hydrangea with azoxystrobin. J. Environ. Hort. 22(2): 58-62. 18. Hagan, A. K., J. W. Olive, J. Stephenson, and M. Rivas-Davila. 2005. Comparison of fungicides for the control of powdery mildew on dogwood. J. Environ. Hort. 23:179-184. 19. Hagan, A. K., M. E. Rivas-Davila, J. R. Akridge, and J. W. Olive. 2005. Resistance of shrub and groundcover roses to black spot and Cercospora leaf spot, and impact of fungicide inputs on the severity of both diseases. J. Environ. Hort. 23:77-85.

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20. Hensley, D. L., R. E. McNeil, and R. Sundheim. 1988. Management inﬂuences on growth of transplanted Magnolia grandiﬂora. J. Arboriculture 14:204-207. 21. Hoitink, H. A., M. E. Watson, and W. R. Farber. 1986. Effect of nitrogen concentration in juvenile foliage of rhododendron to Phytophthora dieback severity. Plant Disease 70:292-294. 22. Holcomb, G. E. 1997. Reaction of crapemyrtle cultivars to Cercospora leaf spot and powdery mildew. Biological and Cultural Test for Control of Plant Diseases. 12:56. 23. Horsfall, J. G. and R. W. Barratt. 1945. An improved grading system for measuring plant disease. Phytopathology 35:655 (abstr.). 24. Lindstrom, O. M., R. B. Beverly, and H. A. Mills. 2002. Neither bloom period nor postbloom nitrogen application reduces cold hardiness in crapemyrtle. Communications in Soil Science and Plant Analysis 33:3837-3844. 25. Mmbaga, M. T. and R. J. Sauve. 2004. Management of powdery mildew in ﬂowering dogwood in the ﬁeld with biorational and synthetic fungicides. Canadian J. Plant Sci. 84:837-844. 26. Neely, D. 1981. Application of nitrogen fertilizer to control anthracnose on black walnut. Plant Disease 65:580-581. 27. Perry, E. and G. W. Hickman. 1998. Correlating foliar nitrogen levels with growth of two landscape tree species. J. Arboriculture 24:149-153. 28. Shaner, G. and R. E. Finney. 1977. The effect of nitrogen fertilization on the expression of slow-mildewing resistance in wheat. Phytopathology 67:10511056. 29. Stuve, D. K. 2002. A review of shade tree nitrogen fertilization research in the United States. J. Arboriculture 28:252-263. 30. Tattar, T. A. 1989. Diseases of Shade Trees. Acad. Press. San Diego, CA. 31. Van der Werken, H. 1981. Fertilization and other factors enhancing the growth rate of young shade trees. J. Arboriculture 7:33-37. 32. Wade, G. L. and J. Williams Woodward. 2001. Crape myrtle culture. Georgia Coop. Ext. Serv. Leaﬂet 331. 33. Williams, D., K. Tilt, and S. Valenti-Windsor. 1998. Common crapemyrtle. Ala. Coop. Ext. Sys. Cir. ANR-1083.
Web publication, March 2010 Auburn University is an equal opportunity educational institution/employer. http:www.auburn.edu http:www.aaes.auburn.edu
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