Fungicide seed dressings: their effects on emergence of soybean and nodulation of pea and soybean

Abstract

A range of fungicides was tested for their efficacy as seed treatments for the control of seedling diseases of soybeans. Thiram, captan and captafol at rates of 2000 ppm a.i. w/w or 3000 ppm a.i. w/w provided the most consistent protection in field trials conducted for 3 years at three separate sites. Of these three fungicides, thiram had the least effect on nodulation by Rhizobium applied to the seed, but an inhibiting effect did occur in some years. Several other fungicides had little effect on nodulation but were inconsistent in controlling seedling diseases. In a glasshouse pot trial, the fungicides did not reduce nodulation even though some were consistently toxic in the field experiments. In field trials with peas and soybeans, the inhibitory effect of a toxic fungicide (captan) on nodulation by seed-borne Rhizobium was avoided by using alternative methods of inoculation which separated Rhizobium from the treated seed. A water suspension of peat-based rhizobial inoculant, or sand particles coated with peat inoculant introduced to the furrow together with the seed, were effective methods of inoculation. A large-particle inoculant carrier (heat-killed soybean seed) was unsuitable. For control of soybean seedling disease we recommend the use of thiram fungicide at a rate of 2000 ppm a.i. w/w, and where practicable the use of alternative methods of Rhizobium inoculation to separate the inoculum from the treated seed.