Academic journal article The American Midland Naturalist

Dogwood Anthracnose (Discula Destructiva): Effects of and Consequences for Host (Cornus Florida) Demography

Academic journal article The American Midland Naturalist

Dogwood Anthracnose (Discula Destructiva): Effects of and Consequences for Host (Cornus Florida) Demography

Article excerpt

ABSTRACT.-Dogwood anthracnose is a disease caused by Discuht destructiva, a fungus of probable exotic origin that is a serious threat to natural populations of Cornus florida in the eastern United States. This epidemic provides an interesting opportunity for the study of the ecology of a highly virulent pathogen and its, host. We present evidence that dead dogwoods tend to have larger neighboring conspecifics than do living dogwoods, that smaller trees have higher levels of foliar infection than larger trees and that in trees of reproductive size, higher levels of anthracnose infection are associated with lower fruit production. In contrast to our data on mortality, proximity to conspecifics did not account for significant variation in the severity of foliar anthracnose infection among live dogwoods. These patterns of infection suggest that D. destructiva could severely impair the ability of this important component of the forest understory to regenerate itself.

INTRODUCTION

According to most theoretical treatments, transmission rates of infectious agents are expected to increase with the density of hosts (e.g., Anderson and May, 1979; May and Anderson, 1979). This should be especially true for passively dispersed diseases of plants. Burdon and Chilvers (1982) surveyed 69 studies on disease spread through crop plants, 57% of the reviewed cases had positive correlations between planting density and disease incidence. Of studies with a positive correlation, 85% were of fungal pathogens. Fewer studies have looked explicitly at other demographic factors of a native host population, such as age or size-structure. In a study of the effects of fungal pathogens on Capsella bursapastoris infections at the seedling stage had a much greater impact on the host population than infections occurring later in the host's life cycle (Alexander and Burdon, 1984). This study examines demographic factors (host density and size structure) that might affect the spread of dogwood anthracnose through a natural host population. We also document potential changes in fecundity produced by sublethal levels of infection.

Dogwood anthracnose is a fungal disease with potentially lethal consequences for two species of dogwood native to North America, Cornus florida and C nuttallii (Hibben, 1990). Purple-rimmed lesions appear on the foliage and the infection spreads through the petiole, into the branch and finally into the trunk. The cankers that develop on the trunk can eventually girdle and kill the tree. Infection incidence and the severity of the symptoms are affected by it host of interacting environmental factors including rainfall and vapor pressure deficit (Chellemi and Britton, 1992; Britton, 1993; Smith, 1994), acid precipitation (Anderson et at., 1993; Britton et al., 1996) and the light environment (Hibben and Daughtrey, 1988; Chellemi and Britton, 1992; Dudt and Shure, 1993).

Several studies of the effects of anthracnose on natural Comus florida populations have reported massive mortality. In Catoctin Mountain Park, Maryland, the number of live dogwoods per ha was reduced from a mean of 682 stems in 1984 to 79 in 1988, an 88% loss in 5 y (Sherald et aL, 1996). Mortality slowed considerably after 1988, with only 29% of the trees alive in 1988 dying by 1994. A similarly high level of mortality (86%), presumably due to dogwood anthracnose, was observed between 1977 and 1987 censuses in Connecticut forests (Anagnostakis and Ward, 1996). The devastation has been less extreme over similar periods of time in other areas. For example, dogwood mortality across 210 monitoring plots in affected areas of Virginia, the Carolinas, Georgia, Alabama, Tennessee and Kentucky rose from 0% in 1988 to 17% in 1992 (Knighten and Anderson, 1993).

We examined patterns of anthracnose infection in a natural population of flowering dogwoods, Cornus florida. We test for effects of the number and proximity of neighboring conspecifics on the levels of infection, the relationship between infection level and plant size and the effects of infection levels on fruit production. …

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