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Plant Mortality Distribution and Crop Losses in Flue-Cured Tobacco. Charles S. Johnson, Assistant Professor of Plant Pathology, Southern Piedmont Agricultural Experiment Station, Virginia Polytechnic Institute and State University, Blackstone, VA 23824. Plant Dis. 75:390-394. Accepted for publication 5 October 1990. Copyright 1991 The American Phytopathological Society. DOI: 10.1094/PD-75-0390.

Relationships between crop losses in flue-cured tobacco (Nicotiana tabacum) and the distribution of plant mortality due to black shank, caused by Phytophthora parasitica var. nicotianae race 0, or injection of glyphosate were investigated in Virginia during 19861987. The number of plants compensating for the yield of adjacent dead plants decreased as plant mortality became increasingly clustered. Plot yield and gross economic returns increased linearly with the number of compensating plants per plot. In 1986, relationships between plant mortality distribution and plot yield or gross economic returns were unaffected by cause of death (P. p. nicotianae vs. a 41% solution of glyphosate) or inoculation date (4 vs. 6 wk after transplanting). In 1987, injection of P. p. nicotianae or glyphosate 4 wk after transplanting had no effect on the relationships. Although yield and gross economic returns were reduced in 1987 by inoculation 6 wk after transplanting, yield and economic returns increased faster with decreased clustering of mortality when P. p. nicotianae, rather than glyphosate, was injected into treated plants. Compensating plants produced higher yields than individual plants from control plots, but yields from compensating plants were not affected by the number of neighboring dead plants. Lower grade indices were sometimes noted for compensating plants. Lower quality characteristics from compensating plants were also sometimes associated with the number of adjacent dead plants. Relationships between quality characteristics of compensating plants and adjacent plant mortality were more consistent for cv. K 326 than for cv. K 394 and when injections involved P. p. nicotianae rather than glyphosate. Although the 26% variation in plot yields resulting from changes in plant mortality distribution was statistically significant, it may not be large enough to require inclusion of mortality distribution in models for predicting yield losses resulting from reduced stands of flue-cured tobacco. The highly significant relationships between plot yield and gross economic returns and the number of compensating plants (rather than characteristics of individual compensating plants) suggest that reasonably accurate crop loss estimates could be based on stand counts performed during critical periods in the growing season.

Keyword(s): compensation, epidemiology.