The Effect of Moisture and Sunlight on the Severity of Dogwood Anthracnose in Street Trees

Materials and Methods

In 1989 and 1990, white flowering dogwoods located in the suburban community of East Brunswick, New Jersey were evaluated for visual symptoms of dogwood anthracnose. Trees were approximately sixteen years old and were planted approximately 25 ft apart at or near the curb on both sides of the street. The meandering streets ran nearly three quarters of the way around a block. There were no sidewalks. The trees were situated in a sandy - sandy loam soil. Since the subsoil in this area was also very sandy, the soil had a poor water holding capacity that was not conducive to the growth of shallow-rooted plants such as dogwood without sufficient rainfall or supplemental irrigation.In September of 1989, 23 trees were rated for leaf spot severity and twig dieback due to dogwood anthracnose on a 1 to 4 scale, where 1 = healthy; 2 = slight disease; 3 = moderate disease; and 4 = severe disease. For each tree, the amount of sunlight received (full sun, partial sun, or full shade), type of ground cover (mulched bed or lawn), and irrigation status (irrigated by property owners to avoid moisture stress or non-irrigated) were recorded. The amount of water applied by each homeowner was variable and could not be exactly ascertained. The water was not applied to the tree foliage.

In September of 1990, a similar disease evaluation was expanded to 76 trees in the same area. In this evaluation, however, the type of ground cover was not recorded. Trees exposed to full or partial sun were classified as “sunny” and trees in full shade were classified as “shaded.” Data for both years were analyzed by analysis of variance (7). Maximum daily temperatures and rainfall for the growing seasons of 1988, 1989, and 1990, obtained from a weather station approximately three miles from the study site, are presented in Figure 1.

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Figure 1.

Daily rainfall and maximum daily air temperatures during the growing seasons of 1988, 1989, and 1990, recorded at a weather station three miles from the study site in East Brunswick, NJ.

Results and Discussion

In both 1989 and 1990, leaf spot severity and twig blight due to dogwood anthracnose was most severe on non-irrigated trees (Table 1). In addition, the disease was more pronounced on trees in shaded locations than on trees in sunny sites (Table 1). Whether a dogwood was situated in a mulched bed or in a lawn did not appear to influence disease severity (Table 1).

Dogwood anthracnose was particularly severe in 1989, and this may be partially explained by the preceding environmental conditions. During the 1988 growing season, the weather was extremely hot (there were 30 days where the daily maximum air temperature exceeded 90°F) (Figure 1). In addition, there were two extended periods of drought during June and July (14 and 24 days, respectively). Dogwoods, because of their shallow root systems, are intolerant of severe drought stress (4). Periods of drought greaterthan 22 days can significantly reduce vigor (19), especially when accompanied by excessive heat. In this study, the street trees that were in poor condition due to moisture and heat stress in 1988 and prior years may have had reduced ability to resist infection. In contrast, irrigated trees that were protected from moisture stress showed fewer visual symptoms of anthracnose in 1989 and 1990 (Table 1).

The extensive leaf spotting observed in 1989 may also have been partially attributable to heavy rainfall during the spring of that year. Frequent rainfall, accompanied by cool temperatures (Figure 1), created an environment especially favorable for both spore germination and leaf infection. It was anticipated that the disease would be more serious on dogwoods in 1990 than in 1989 because of the continued frequent rains during the spring and summer of 1990. Even though environmental conditions in 1990 were again conducive to leaf infection, anthracnose was less severe than in 1989 and the trees appeared more vigorous. The increase in vigor and reduction in disease observed was possibly due to the adequate rainfall and lack of heat stress in 1989 and 1990.

Several studies concerning the effect of sunlight on dogwood anthracnose development have produced conflicting results (1,11,12,21). In our study, trees in partial or full sun were not as severely affected by dogwood anthracnose as heavily shaded trees. Trees that are not shaded typically have better air circulation and reduced humidity in the outer canopy, two conditions that tend to reduce disease development. Chellemi and Britton (2) reported that dogwood anthracnose was less severe in the exterior canopy of exposed trees than in the interior canopy or within the canopies of understory trees. In full sun, leaves on the northern exposure of a tree or within the canopy develop anthracnose first (12,20). Hartman et al (6) reported that dogwood anthracnose was most prevalent in heavily shaded landscape trees growing under stressful conditions.

Full sun may directly affect disease development by helping to reduce the period of free moisture on leaf surfaces, thereby creating an environment that is unfavorable for spore germination and leaf infection (4). Sunlight also heats the leaf surface, which reduces the number of leaf spots and the production of fungal fruiting bodies (acervuli) (11, 12). In our study, both the direct affects of sunlight and the reduction in humidity typical of exposed plantings may have contributed to the reduced disease severity in sunny sites.

Summary

In this study, supplemental irrigation was associated with reduced disease severity in street plantings. In addition, dogwoods placed in full or partial sun with adequate moisture were less diseased than trees grown in the shade. Dogwoods are shallow-rooted trees, and control recommendations for dogwood anthracnose often include irrigation during periods of moisture stress to increase plant vigor. Since anthracnose infection is enhanced when dogwood leaf surfaces are moistened, however, care should be taken to utilize irrigation practices that do not impact the foliage.