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Measurement of Expanding Oak Wilt Centers in Live Oak. D. N. Appel, Associate professor, Department of Plant Pathology and Microbiology, Texas A&M University, College Station 77843; R. C. Maggio(2), E. L. Nelson(3), and M. J. Jeger(4). (2)Associate professor, Department of Forest Science, Texas A&M University, College Station 77843; (3)Graduate research assistant, Computer Science Department, Texas A&M University, College Station 77843; (4)U.K. Overseas Development Natural Resources Institute, London, U.K. Phytopathology 79:1318-1322. Accepted for publication 28 July 1989. Copyright 1989 The American Phytopathological Society. DOI: 10.1094/Phyto-79-1318.

Local, tree-to-tree spread of Ceratocystis fagacearum was monitored in live oak (Quercus fusiformis) with sequential, color infrared aerial photography during 19821987. A computer-based system for analyzing expanding foci was developed to measure rates of crown defoliation and mortality, as well as expansion distances. Four foci expanded radially an average of 1116 m/yr, with longer maximum distances of expansion (up to 40 m/yr) commonly occurring. One focus that expanded most rapidly increased from 0.3 to 3.6 ha over 5 yr, affecting 10,774 m2 of crown cover. This was initially the smallest focus, and it had the greatest live oak density. The largest initial focus had a lesser oak density and increased from 1.5 to 6.6 ha, affecting 11,396 m2 of crown cover. Crown survival in 1987 ranged from 4 to 26% for trees that originally showed symptoms in 1982. A strong linear correlation between the area of affected crown cover and total area occupied by each focus was noted. The rapid rates of focus expansion were attributed to a high potential for root grafting and the occurrence of common root systems among clonally propagated live oaks.

Additional keywords: Ceratocystis fagacearum, epidemiology, Quercus fusiformis.