|Abstract or Summary
- Damping-off of table beets is of widespread occurrence in the
Willamette Valley of Oregon. Pythium ultimum Trow., Pythium
debaryanum Hess., and Rhizoctonia solani Kuhn. were found to be the
important pathogenic fungi causing the disease.
Trichoderma viride, Penicillium spp. Streptomyces spp. and
certain isolates of bacteria, all saprophytic organisms associated
with diseased beet seedlings, beet rhizospheres and infested soil,
were strongly antagonistic toward Pythium ultimum in agar culture
and were selected for control tests in soil under greenhouse conditions.
Infesting beet seeds with spores of Trichoderma or Penicillium,
afforded good protection of seedlings from pre-emergence damping-off but did not prevent damping-off of older seedlings in infested
field soil. Beet seedlings suffered less pre-emergence damping-off
in soil amended with K₂HPO₄ and urea; cornmeal, sucrose and casein,
on the other hand, increased incidence of the disease.
Simultaneous infestation of sterilized greenhouse soil with
Pythium and Trichoderma or Pythium and Penicillium one week before
seeding resulted in good control of damping-off. The antagonists
were less effective when mixed with sterilized soil in which
Pythium had been pre-established, or when mixed with non-sterilized
greenhouse soil simultaneously with the pathogen. The best results
again were obtained in soil amended with KH₂PO₄ and urea. In sterilized
soil, bacterial antagonists had no effect on damping-off.
Streptomyces spp. were inconsistent in controlling the disease.
Treatment of soil with fungicides markedly affected the micro-flora in infested soil, and recolonization by certain species varied
with different chemicals. Following treatment with 50, 100, or 200
lbs. Lanstan, or with 50 lbs. Ceresan per acre, Pythium initially
disappeared from the soil but recolonized rapidly. Greater concentrations
of Ceresan either completely suppressed Pythium or resulted in
very slow recolonization by the pathogen. In soil treated with Dithane,
Maneb, or Bayer 47531, both Pythium and Trichoderma decreased
progressively during the period of the test. In soils treated
with Lanstan or Ceresan Trichoderma initially was reduced but subsequently
increased to populations greater than in untreated soil.