Severity of Soybean Stem Canker Disease Affected by Insect-Induced Defoliation. J. S. Russin, Department of Entomology, Louisiana Agricultural Experiment Station, Louisiana State University Agricultural Center, Baton Rouge 70803. M. B. Layton, D. J. Boethel, E. C. McGawley, J. P. Snow, and G. T. Berggren. Department of Entomology, and Department of Plant Pathology and Crop Physiology, Louisiana Agricultural Experiment Station, Louisiana State University Agricultural Center, Baton Rouge 70803. Plant Dis. 73:144-147. Accepted for publication 12 September 1988. Copyright 1989 The American Phytopathological Society. DOI: 10.1094/PD-73-0144.

Greenhouse studies using the soybean (Glycine max) cultivar Bragg showed a significant (P <0.0001) negative relationship (Y = 102.636 – 1.066X + 0.006X²) between lengths of stem cankers incited by Diaporthe phaseolorum var. caulivora and defoliation caused by larvae of the soybean looper, Pseudoplusia includens. Reductions in length were greatest when cankers developed at times when defoliation was just completed. Plants allowed 22 or 43 days to recover from 30% defoliation grew sufficient new foliage so that leaf areas did not differ significantly from those of nondefoliated plants. Consequently, lengths of stem cankers on defoliated and nondefoliated plants also did not differ significantly at these times. Nitrogen provided to plants by fertilizing with NH₄NO₃ or by treating seeds with commercial Bradyrhizobium japonicum inoculant before planting increased plant growth and decreased stem canker lengths.

Keyword(s): insect-disease interactions, integrated pest management.