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Juvenile Susceptibility of Ponderosa Pine to Dwarf Mistletoe. Lewis F. Roth, Professor of Botany and Plant Pathology, Oregon State University, Corvallis 97331; Phytopathology 64:689-692. Accepted for publication 27 November 1973. DOI: 10.1094/Phyto-64-689.

Susceptibility of the new growth of a population of juvenile ponderosa pines to infection by western dwarf mistletoe decreases with increasing tree age; i.e., the rate of infection per infection site declines with age of the host up to about 50 yr. Consequently, other things being equal, mistletoe buildup after a stand is thinned should be slower in older stands than in those less than 50. Two rather common forms of extreme mistletoe susceptibility in older trees are reported: (i) susceptibility to infection, and (ii) susceptibility to damage. In the first form, excessive numbers of mistletoe plants accumulate, lateral endophytic mistletoe growth is limited and host damage results from sheer numbers. Trees of the second category may be indifferently susceptible to infection, but may suffer severely from even a few mistletoe plants because of extensive lateral spread of the endophyte.

Additional keywords: Arceuthobium, disease resistance, pine management.