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Effect of Osmopriming Sugar Beet Seed on Exudation and Subsequent Damping-off Caused by Pythium ultimum. R. M. Osburn, Former graduate research assistant, Department of Plant Pathology, University of California, Berkeley 94720, Present address: Gustafson, Inc., 1400 Preston Rd., Suite 400, Plano, TX 75075; M. N. Schroth, Professor, Department of Plant Pathology, University of California, Berkeley 94720. Phytopathology 78:1246-1250. Accepted for publication 20 April 1988. Copyright 1988 The American Phytopathological Society. DOI: 10.1094/Phyto-78-1246.

Control of preemergence damping-off of sugar beets, caused by Pythium ultimum, with NaCl- or polyethylene glycol (PEG)-osmoprimed seed planted in naturally infested field soil was related to a reduction of the rate and incidence of seed colonization by the pathogen. Incidence of colonization of untreated seed was 23.3 and 86.7% by 12 and 24 hr, respectively, whereas colonization of NaCl- and PEG-osmoprimed seed was reduced to 0 and 3.3%, respectively, after 12 hr, and 13.3 and 16.7%, respectively, after 24 hr. Reduced colonization by P. ultimum was related to decreased exudation from osmoprimed seed upon imbibition with water. The amount of carbohydrate exuded from NaCl- and PEG-osmoprimed seed was reduced 98.1 and 91.7%, respectively, after 60 min, and 41.5 and 55.0%, respectively, after 48 hr of incubation in water compared with untreated seed. The rate of exudation from osmoprimed seed over 48 hr was correlated with the rate of germination, whereas untreated seed did not germinate during the 48-hr incubation period. Disease reduction with NaCl-osmoprimed seed was nullified when the seeds were incubated in exudates before planting. Bacterial population densities of 10⁸–10⁹ cfu ml^–1 occurred in the NaCl or PEG osmopriming solutions after 6 days of seed treatment, and residual population densities on seed were 10⁴–10⁵ cfu seed^–1. The bacterial populations have potential to affect the incidence of damping-off. The incidence of damping-off with NaCl-osmoprimed bacteria-free seed was 20–30% less than with unprimed seed, whereas reductions ranged from 20–54% when bacteria were present on the primed seed.