Differential aluminum and calcium concentrations in the tissues of ten Cornus species

In both ornamental and forested environments, dogwood anthracnose has caused widespread dieback and decline of Cornus florida L., flowering dogwood, since the late 1970s. Early observations of this disease were more or less simultaneous with the peak of acid rain in North America. Aluminum is known to be toxic to some plants at low concentrations, and soil availability of A1, and hence plant uptake, may be increased by acid rain. Experimental treatment with simulated acid precipitation has been shown to increase the severity of anthracnose infection. In a preliminary study, we observed surprisingly high levels of A1 in C. florida wood. Suspecting a possible link between A1 and anthracnose, we hypothesized that A1 concentrations would be higher in anthracnose-susceptible Cornus species. We also hypothesized that anthracnose-infected C. florida would have higher levels of foliar A1 than uninfected trees. Finally, we hypothesized that if there was indeed a link between acid rain and anthracnose via A1 uptake and toxicity, that younger wood should have higher concentrations of A1 than older wood, reflecting increased soil availability of A1. To investigate the first hypothesis, we collected tissue samples from ten Comas species from locations across North America for chemical analysis. The four large-bracted species, C. florida, C, nuttallii Aud., C. kousa (Buerger ex Miq.) Hance and C. canadensis L., accumulated A1 at concentrations an order of magnitude greater than any of the small-bracted species we studied. However, C. florida and C. nuttallii are known to be highly susceptible to infection by dogwood anthracnose, while C. koasa and C. canadensis are considered resistant to infection. To investigate the second hypothesis, we collected leaves from infected and uninfected trees growing on the same site; A1 concentrations in leaves from uninfected trees were actually 432 ppm higher than those from infected trees, suggesting that A1 toxicity is not likely a factor in the severity of anthracnose infection, To investigate the third hypothesis, we examined tissue chemistry of both old and young wood. We found that A1 concentrations in C. florida wood decreased from older wood to younger wood, contrary to what we would expect if A1 availability (and hence plant uptake) had increased as a consequence of acid rain.