5.5.26
GENETICS OF RESISTANCE TO STEROL DEMETHYLATION (DMI) FUNGICIDES IN THE CEREAL EYESPOT PATHOGEN TAPESIA YALLUNDAE

PS DYERl, JA LUCAS2 and JF PEBERDYl

lDepartment of Life Science, School of Biology, University of Nottingham, Nottingham NG7 2RD, UK; 2IACR-Long Ashton Research Station, University of Bristol, Long Ashton, Bristol BS18 9AF, UK

Background and objectives
A diverse group of systemic fungicides have been developed which prevent fungal growth by inhibiting a variety of different enzymes of the sterol biosynthesis pathway (SBis). Of these, the sterol 14 (x-demethylase inhibitor (DMI) fungicides represent one of the largest and most important groups of modern fungicides and they have remained highly effective in most field applications despite many years of intense agricultural use and their single-site mode of action. However, field resistance to certain DMis has been reported and recently field isolates of the cereal eyespot pathogen Tapesia yallundae have been detected with greatly decreased sensitivity to 'prochloraz' (trade name 'Sportak'), an imidazole DMI fungicide [1]. This may be the first indication of a possible breakdown in control of eyespot disease by DMI fungicides. The aim of the present study was to determine the genetic basis of the increased resistance to prochloraz in T. yallundae , assessingwhether it is predominantly monogenic or polygenic in nature.

Materials and methods
Isolates from the UK, France and New Zealand were screened for resistance to prochloraz by selection on MYG agar plates supplemented with prochloraz at 2 mg/L. Sexual crosses were then set up between susceptible and the most resistant isolates using an in vitro crossing protocol [2]. Ascospores from a series of crosses were collected and the IG50 values (level at which growth was inibited by 50 %) of the offspring were determined by Petri dish assays measuring radial growth on a range of concentrations of prochloraz.

Results and conclusions
Isolates with decreased sensitivity to prochloraz (IG50 vajues > 0,5 mg/L) were detected only from field sites in North France and a single site in New Zealand. Analysis of the IG50 values of progeny obtained from a cross between a susceptible isolate and one with intermediate resistance (IG50 = 0.5 mg/L) revealed a 1:1 segregation of resistant:sensitive phenotypes, indicative of a single gene for resistance. A similar IG50 value peak was detected in analyses of crosses between susceptible and highly resistant isolates (IG50 > 2 mg/L). However, the distribution of IG50 values showed a longer 'tail' indicative of a polygenic component. Backcrosses have now been set up to verify these initial observations. The significance of these results to the potential occurrence of field resistance to prochloraz in T. yallundae are discussed.

References
1. Dyer PS, Lucas JA. 1995. Plant Pathology 44, 796-804.
2. Dyer PS, Nicholson P, Lucas JA, Peberdy JF, 1996. Mycological Research 100, 1219-1226.