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Ecology and Epidemiology

Temporal Analysis of Two Viruses Increasing in the Same Tobacco Fields. L. V. Madden, Associate professor, Department of Plant Pathology, Ohio Agricultural Research and Development Center (OARDC), The Ohio State University (OSU), Wooster 44691; T. P. Pirone(2), and B. Raccah(3). (2)Professor, Department of Plant Pathology, University of Kentucky, Lexington 40546; (3)Senior research scientist, Virus Laboratory, Agricultural Research Organization, The Volcani Center, Bet Dagan, Israel. Phytopathology 77:974-980. Accepted for publication 30 December 1986. Copyright 1987 The American Phytopathological Society. DOI: 10.1094/Phyto-77-974.

Virus epidemics caused by tobacco etch virus (TEV) and tobacco vein mottling virus (TVMV) were monitored in six experimental fields (about 3,300 plants) from 19831985. Fields were arranged in pairs, with one field in each pair treated with insecticides to control aphid colonization. Proportions of plants with symptoms due to infection by TEV (y1) and TVMV (y2) were determined at least weekly. Disease progression was quantified by fitting the Lotka-Volterra linked differential equations to the individual virus data and the logistic equation to the combined virus data. The Lotka-Volterra equations, which have parameters representing rates of increase (r1 and r2), maximum disease levels (K1 and K2), and inhibitory effects of incidence of one virus on the increase of the other (a12 and a21), provided excellent fits. The estimated rate of increase for TVMV (r2) was significantly (P = 0.05) different from that of TEV (r1) in nine cases. Rates ranged from 0.09 to 0.28 per day. Maximum disease levels were as high as 1.0 in some 1984 fields and as low as 0.01 in 1983. Estimated K1 often was less than or equal to K2. The competition coefficients were not significantly different from 0 in > 70% of the epidemics, indicating that neither virus, at the population level, had a consistent inhibitory effect on increase of the other. The logistic equation also precisely described the combined virus data. Except for one field, rates (r) of combined virus increase were between 0.13 and 0.26 per day.

Additional keywords: comparative epidemiology, differential equations, disease progress curves, Nicotiana tabacum, quantitative epidemiology.