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The Influence of Diurnal Temperatures on the Postharvest Susceptibility of Poinsettia to Botrytis cinerea. P. M. Pritchard, Former Graduate Research Assistant, Michigan State University, East Lansing 48824. M. K. Hausbeck, Assistant Professor, and R. D. Heins,Professor, Michigan State University, East Lansing 48824. Plant Dis. 80:1011-1014. Accepted for publication 13 May 1996. Copyright 1996 The American Phytopathological Society. DOI: 10.1094/PD-80-1011.

The influence of day/night (DT/NT) temperatures of 16/16, 19/19, 22/22, 16/19, 19/22, 16/22, 19/16, 22/19, and 22/160C during poinsettia production on postharvest bract and foliage susceptibility to Botrytis cinerea was investigated. Plants were inoculated with 2.7 x 105 B. cinerea conidia per ml of water following a 3- or 6-week temperature treatment and incubated at 20C. Area under the disease progress curve (AUDPC) data indicated that the postharvest susceptibility of poinsettia bracts and foliage to B. cinerea, measured by the proportion of bracts and foliage infected and the proportion with sporulating B. cinerea, was not influenced by the difference in DT and NT but increased as DT or NT during production increased. As plants matured, as indicated by thermal time, AUDPC values increased (P = 0.001) more for the proportion of bracts infected (R2 = 0.73) than for the proportion of bracts with sporulating B. cinerea (R2 = 0.86) and the proportion of foliage with sporulating B. cinerea (R2 = 0.74). Results suggested that commercial growers using higher NT than DT to limit poinsettia height are not increasing the postharvest susceptibility of their crop to B. cinerea. However, the increased susceptibility of maturing poinsettias suggests disease management strategies should be intensified during crop finishing and postharvest handling.