Phytophthora disease of alder is a fairly unknown but lethal disease of black alder. There has not been much research on this disease in Europe or Slovenia, especially not about the impact of the disease on tree growth. We sampled trees in two pure stands of black alder in Polanski Log. In the infected stand, we obtained tree cores of 24 seemingly healthy (OZ) trees and 20 infected (O) trees. In the uninfected stand we obtained 15 tree cores. Some tree parameters have been measured or assesed for each sampled tree and the presence of phytophthora disease symptoms were registered for the infected trees. We found significant differences in the average tree ring widths in the infected (O) and healthy (OZ & Z) trees. The widest average tree ring was found for Z trees, and the narrowest for O trees. Reduced radial increment in O trees has been reflected in the last 9 years. Healthy OZ trees in the infected stand experienced reduced radial increment for several years before visible symptoms of infection even occur. In the infected stand, there was 25% infection rate and 11% mortality of trees.
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