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Effects of geographical location on phoma stem canker and yield of oilseed rape crops in the UK

Student Report No. 14

Effects of geographical location on phoma stem canker and yield of oilseed rape crops in the UK

by

Jenna Stonard

Plant Pathology and Microbiology Department, Rothamsted Research

 

Abstract

Phoma stem canker, caused by Leptosphaeria maculans and L. biglobosa, is the most important disease of winter oilseed rape in the UK.  Experiments at different sites in England examining the effects of fungicide timing on severity of phoma stem canker epidemics have produced conflicting results.  The effects of geographic location may have contributed to these differences.

Ascospore release was monitored in three growing seasons (2005/06-2007/08) at Rothamsted.  The onset of ascospore release was affected by temperature and rainfall and subsequent release by rainfall.  Using quantitative PCR (qPCR), maximum L. maculans ascospore release was observed earlier in the season than that of L. biglobosa

The national winter oilseed rape disease survey (1990 - 2006) showed regional differences in cultivar choice and patterns of fungicide application.  Field experiments, using 42-45 different oilseed rape cvs/breeding lines, were done at Rothamsted during four growing seasons (2003/04, 2005/06, 2006/07, 2007/08).  The severity of phoma stem cankers differed between cvs. 

Regional variation in the distribution of L. maculans and L. biglobosa in stem cankers on commercial crops was determined in June/July 2001-2003 and 2006.  DNA of both L. maculans and L. biglobosa was identified in 65% of samples.  Geostatistical mapping and qPCR revealed the greatest amounts of L. maculans DNA in basal stem cankers in southern England (where epidemics are most severe) whilst that of L. biglobosa was greatest northern England.  Similar patterns were observed in geostatistically mapped predictions of the time between onset of phoma leaf spot (autumn) and of phoma stem canker (spring).  Basal stem cankers were associated with L. maculans; upper stem lesions were associated with L. biglobosa

Geographic variation in L. maculans population structure was surveyed in the 2006/07 growing season.  In total 120 isolates were analysed (collected from sites in Hertfordshire, Bedfordshire, Norfolk, Lincolnshire and Gloucestershire) using a plant set with known resistance (Rlm) genes.  All isolates were virulent on cvs with Rlm2, Rlm3 and Rlm9 whilst 88% were not virulent on the cv. with Rlm7.  Nineteen raceswere identified; the populations at Hertfordshire and Bedfordshire sites were closely related.  Genetic variation in these populations was assessed using minisatellites; all isolates were unique. 

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