Impact of ash yellows mycoplasmalike organisms on radial growth of naturally infected white, green, and velvet ash

Abstract

Growth of white ash (Fraxinusamericana L.), green ash (Fraxinuspennsylvanica Marsh.), and velvet ash (Fraxinusvelutina Torr.) as related to infection by ash yellows mycoplasmalike organisms (MLOs) was assessed by diagnosing and grouping trees as MLO infected or healthy and comparing their previous annual radial growth over periods of 9–22 years. White ash and green ash 17–30 years old were studied on two sites per species in central New York State. Velvet ash 30–90 years old was studied in Zion National Park, Utah. MLOs were detected by fluorescence microscopy. Widths of growth rings were measured on increment cores or cross sections of main stems. Growth trends of diseased and healthy groups of white ash and green ash diverged when ash stands were 12–21 years old. In the fifth year after divergence, the unweighted average growth of diseased green ash across two plots was 70% of that of healthy trees. For white ash, the corresponding average was 61%. In white ash tested annually for MLO infection, growth reduction preceded MLO detection by an average of 1 year. Velvet ash did not sustain MLO-associated growth reduction. This species may be tolerant of infection by ash yellows MLOs.