Sensitivity of eyespot to prochloraz in England in 1996

Abstract

A survey was undertaken in 1996 to investigate sensitivity to prochloraz of UK populations of the cereal eyespot pathogen, Pseudocercosporella herpotrichoides. Sixty-three wheat and barley samples were received and, from over 434 eyespot lesions found, isolation of the pathogen was attempted from 278 lesions. Most attempted isolations were contaminated by Fusarium spp., so only 18 isolates of P. herpotrichoides were obtained for testing, from 3 fields in southwestern England.

All were of the Rye-pathotype. There was some variation in ED50s for prochloraz, but every isolate had an ED50 below 0.50 mg/l, which is within the range previously found in the UK. ED50s were also determined for flusilazole and propiconazole. There was a much stronger correlation between sensitivity to flusilazole and to propiconazole than between either of these fungicides and prochloraz.

Due to the very limited number of pathogen isolates available for testing, no conclusions can be drawn regarding the sensitivity of the UK eyespot pathogen population to prochloraz.

AICC review by Emma Saxton (Hampshire Arable Systems)

Introduction

Assessing the severity of eyespot infections and the associated risk of lodging and yield reductions is difficult. The control of eyespot using Prochloraz is, also, often disappointing; only 30 - 40%, making crop protection difficult. Prochloraz is our most effective fungicide. In Northern France increased sensitivity to Prochloraz has been recorded.

Therefore, this survey was needed to check the sensitivity to Prochloraz of UK populations. The aim was to collect 50 samples of eyespot from both wheat and barley during late April and May from across the UK. A low incidence of eyespot in spring '96 meant that further collections were required to achieve an adequate sample number.

Techniques

Eyespot lesions were selected for isolation. Isolation was not attempted where there was a clear contamination with other stem-base diseases. The isolated legions were cultured onto agar plates and tested for sensitivity to fungicide concentrations. The isolate numbers were far below target, so ED50 (50% reduction in fungal growth) values for Flusilazole and Propiconazole were obtained in addition to those for Prochloraz.

Results

The production of eyespot isolations from stem base samples was very low due the contamination by other stem base pathogens. Only rye pathotypes were isolated and these were all from Southwest England. ED50s results were all below 0.5mg/l. Therefore these isolates had normal sensitivity levels as found by previous UK work, not concurring with the French findings. However, the low number of isolates tested (all from the Southwest and all rye type) mean no conclusions can be drawn about changes in sensitivity of UK eyespot populations.

Agronomist Analysis

Eyespot only occurs at low levels (below commercial thresholds), so field spraying is very rarely justified. The poor level of field control that Prochloraz offers means that any reduced sensitivity could render control impossible with current fungicides. Therefore sensitivity should be monitored. Unfortunately, the output from this project was rather inconclusive.