Phomopsis seed decay in soybean with dense pubescence.

Phomopsis seed decay is a major cause of poor soybean [Glycine max (L.) Merr.] seed quality in regions where the climate is warm and humid during and after seed maturation. Increasing pubescence density on soybean plants may increase retention of moisture in pods and seeds, and thereby influence seed infection by Phomopsis longicolla Hobbs. The objective of this study was to evaluate the effect of increased pubescence density on seed infection by P. longicolla and germination. Near-isogenic lines of two soybean culti vars, Clark and Harosoy, differing in pubescence density (normal vs. dense) were evaluated for seed-borne disease in 12 environments across four years (1985-88) in Maryland, Pennsylvania and Ohio. There were no significant differences between normal and dense pubescence isolines for seed infection by P. longicolla in nine envi ronments and seed germination in 11 environments. In three envi ronments where significant pubescence effects on seed infection were found, infection varied depending on the genetic background into which the dense pubescence gene has been introduced. Dense pubescence was generally associated with delayed maturity (nine of 12 environments) and increased plant height (eight of 12 environ ments). There was also slightly greater lodging in the plants with dense pubescence than in the normal pubescence isolines (four of 12 environments). Growing dense pubescence soybean in the east ern United States should not increase the potential for seed disease problems due to P. longicolla. Additional index words: Glycine max, seed germination, seed-borne diseases, Phomopsis spp., Diaporthe phaseolorum, Phomopsis longicolla, trichome, isogenic lines, morphology.