VIEW ARTICLE

Ecology and Epidemiology

Simulation Model for Spread and Intensification of Western Dwarf Mistletoe in Thinned Stands of Ponderosa Pine Saplings. M. A. Strand, Department of Plant Pathology, University of California, Davis, CA 95616; L. F. Roth, Department of Botany and Plant Pathology, Oregon State University, Corvallis, OR 97331. Phytopathology 66:888-895. Accepted for publication 11 November 1975. DOI: 10.1094/Phyto-66-888.

A mathematical model was formulated to describe the population increase of western dwarf mistletoe (Arceuthobium campylopodum) in a young ponderosa pine stand. The model predicts numbers and locations of new infections on evenly spaced (2-6 m) trees of uniform height (3.0 m initially). Submodels were used to estimate probabilities of occurrence of events which lead to infection. Rules of probability were used to combine the submodels. Two routes to a new infection were considered: reinfection, the infection of a branch on an infected tree by inoculum from the tree; and contagion, infection by inoculum from an outside source. The simulation predicted 2.83 new infections at the end of 5 years for a typical tree in a stand with spacing of 2.7 m and an average initial infection level of 1.2 infections per tree. For stands with similar initial infection levels, the expected new infections were about halved for every 1.5 m increase in spacing. Where the initial infection level was 2.4 mistletoe plants per tree, 6.46 new infections were expected after 5 years for trees spaced 2.7 m apart. Trees spaced 2.7, 3.9, and 5.4 m apart received about 55%, 22%, and 12%, respectively, of their new infections through contagion.