Comparison of growthroom screening techniques for the determination of physiological resistance to sclerotinia stem rot in Brassica napus

Abstract

Screening Brassica napus L. lines for resistance to sclerotinia stem rot caused by Sclerotinia sclerotiorum (Lib.) de Bary in the field is usually unreliable. Six growthroom screening methodologies were evaluated and compared to accurately, reliably, and efficiently measure 10 B. napus lines for resistance to sclerotinia stem rot. These lines were planted in 2001 and 2002 in a nursery infested with S. sclerotiorum sclerotia and evaluated for disease incidence and tested in four growthroom experiments. The growthroom methodologies consisted of: (i) leaves of the B. napus lines inoculated with fragments of S. sclerotiorum mycelium suspended in a potato dextrose broth (Myspray technique); (ii) B. napus petals bearing S. sclerotiorum mycelium (Mypetal technique); (iii) B. napus petals bearing S. sclerotiorum ascospores (Ascpetal technique); (iv) leaf petioles dipped in a solution of oxalic acid (Oxalate technique); and (v, vi) both leaves and stems inoculated with agar plugs with S. sclerotiorum mycelium (Plug Leaf and Plug Stem techniques). Infection was low and highly variable in the field trials with CVs of 205.9% and 133.6%, respectively, and with low correlation between them (R = 0.6164, p = 0.1405). The Plug Leaf technique had low correlations among growthroom trials and resulted in inconsistencies among cultivar rankings between the trials. The Plug Stem technique had high levels of variation and was not considered to be accurate. The Ascpetal technique, although regarded as more accurate, was found to be highly variable and was cumbersome to use. The Myspray technique had the greatest variability of all the techniques with a CV of 57.3% and low correlation between the growthroom trials. Myspray technique was significantly correlated with field data (R = 0.9150, p = 0.0039) and with the Ascpetal technique (R = 0.7945, p = 0.0060). The Oxalate test was the most efficient of all techniques but did not correlate with any other technique. The Mypetal technique was chosen to best assess physiological resistance since it had the least variation in its results and cultivar ranks between growthroom trials. It was accurate in determining resistant lines observed in the field, and it was efficient to employ.