Abstracts Biological Sciences

The influence of soil fumigants and soil fungicides on some important soil-borne plant pathogens

by Marlene Mei-yu Liu

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 these tests. 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.