Control of yellow rust ( Puccinia striiformis) on winter wheat by ergosterol inhibitors at full and reduced dosages

Abstract

The efficacy of broad-spectrum fungicides was investigated at normal and reduced dosages on yellow rust ( Puccinia striiformis) in 1989 and 1990, when severe attacks of yellow rust occurred on winter wheat in Denmark. Eleven ergosterol-inhibiting (EBI) fungicides were tested, most of which gave 98–100% control of yellow rust; only prochloraz and tridemorph gave considerably lower, and inadequate, levels. Several control strategies using reduced dosages of broad-spectrum fungicides gave excellent to good control of yellow rust, when one full dose was divided into two, three or four applications, depending on disease pressure. Control of low levels of rust attack can be carried out using down to 25% of the normal dose. When using such low dosages on susceptible varieties, where outbreaks have occurred, additional treatments should be carried out after a maximum of 3 weeks. This was particularly important during stem elongation to ensure protection of newly developed leaves. A clear dose response was seen when application was carried out after growth stage 39–45, indicating a longer residual effect for the highest dosages. A computer-developed advisory program, which includes a model for control of yellow rust, was included in the trials. The models' recommendation of reduced and adjusted dosages gave excellent control compared with standard applications using four sprayings. A pot trial, carried out in a greenhouse, using reduced dosages of triadimenol, tebuconazole and propiconazole applied curatively at different times after inoculation, confirmed the good possibilities of obtaining total control when using down to 12.5% of the normal dose of demethylation-inhibiting products. As in the field trials, correct timing was more important than applied dose rates.