- Twelve chemicals (Busan 72, Chloropicrin, Dacthal, Dexon,
Duter, EP 161, Lanstan, Methyl bromide, Phaltan, Vapam, Vidden D,
Vorlex) were tested to determine their effect on populations of certain
important soil-borne microorganisms including the onion pink root
fungus Pyrenochaeta terrestris. The soil used was from a field in
Malheur County, Oregon that was known to be heavily infested with
P. terrestris. Fumigants were added to the soil in glass jars which
were immediately sealed with plastic electrician's tape. Fungicides
and the herbicide were added to soil in polyethylene bags and were
mixed very thoroughly with the soil. These experiments were done
entirely in the laboratories and greenhouse at the Oregon State
University Campus in Corvallis.
The fumigants (Chloropicrin, EP 161, Lanstan, Methyl
bromide, Vapam, Vidden D, Vorlex) were more effective than the fungicides (Busan 72, Dexon, Duter, Phaltan) in reducing the number
of viable propagules of all of the fungi studied. In these tests the
herbicide Dacthal was relatively ineffective against any of the microorganisms.
EP 161, Methyl bromide and Vorlex gave best control of
Pythium spp. (mostly P. ultimum) and the rate of recovery in soils
treated with these fumigants were slow. The fungicide Dexon usually
reported to be highly toxic to P. ultimum did not control this fungus in
Chloropicrin, Methyl bromide, Vapam and Vorlex were most
effective against Fusarium spp. (mostly F. roseum and F. oxysporum).
Chloropicrin permanently inactivated most of the actinomycetes
and recovery of this group of microorganisms was very slow
in soils treated with either Chloropicrin or Methyl bromide. In soils
treated with Vapam or Vorlex, on the other hand, populations of
actinomycetes became much greater than in the untreated soil.
The heavily sporulating, carbohydrates reducing fungi were
almost completely eliminated by Chloropicrin, Methyl bromide and
Vorlex. EP 161, and Vapam were almost as effective but Lanstan
and Vidden D were ineffective. Penicillium spp. were encountered
with greatest frequency, following by Mortierella, Mucor, Alternaria,
Rhizopus, Trichoderma, Aspergillus, Cephalosporium, Stephyllium,
Epicoccum, Chaetorniurn, Sclerotinia and Monilia in that order. EP 161, Methyl bromide and Vapam were most effective
against Pyrenochaeta terrestris and the rate of recovery in soils
treated with these materials was very slow. Chloropicrin, Vapam
and Vorlex were almost as effective, while Lanstan and Vidden D
were relatively ineffective.