Lack of varietal purity undermines disease resistance

In northern New South Wales in 2008 and 2009 significant levels of stripe rust were reported in commercial Sunvale crops, a variety released in 1995 with moderate resistance (MR) to the disease.

However, in GRDC-funded National Variety Trials (NVT) in 2010, a season very conducive to the disease, Sunvale plots had very low levels of stripe rust development, consistent with its MR rating.

This led Dr Steven Simpfendorfer of the NSW Department of Primary Industries, Tamworth, and Dr Anke Martin and Professor Mark Sutherland from the University of Southern Queensland, Toowoomba, to wonder whether the discrepancy in the commercial versus NVT reaction of Sunvale was related to seed purity, variety mix-ups or some other factor.

To investigate this, Dr Simpfendorfer and his team collected 23 commercial Sunvale seed lots from the 2010 harvest and determined the varietal purity of these samples and three ‘pure’ seed samples using DNA analysis. All seed lots were also sown in small plots to obtain detailed observations of the rust reaction of the plants.

He said that based on DNA analysis, 16 of the 23 commercial varieties (approximately 70 per cent) contained at least five per cent contamination with other varieties, and in most cases at least one of the contaminants was susceptible to stripe rust (MS to very susceptable (VS)). Field plot evaluations for rust reaction were consistent with the DNA analysis, showing a higher stripe rust incidence in the plots contaminated with susceptible varieties.

Dr Simpfendorfer says it is not surprising that high levels of impurity were found in commercial Sunvale crops given that it is a 16-year-old variety. Contamination of commercial Sunvale seed lots has introduced more stripe-rust-susceptible varieties into these crops, which is unfairly tarnishing the resistance rating of this variety and jeopardising industry confidence in stripe rust resistance breeding as a whole.

 

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