Dalbó, M.A., G.N. Ye, N.F. Weeden, W.F. Wilcox, and B.I. Reisch. 2001. Marker-assisted selection for powdery mildew resistance in grapes. J. Amer. Soc. Hort. Sci. 126(1):83-89.

The efficiency of marker-assisted selection for powdery mildew (PM) resistance in grapes was studied using molecular markers associated with a major QTL for this trait. Initially, genetic maps were constructed from a segregating population of the cross 'Horizon' x Illinois 547-1 (Ill. 547-1; V. rupestris x V. cinerea). A major QTL from Ill. 547-1, the resistant parent, explained 41% of the variation. One RAPD marker (CS25b) and one AFLP marker (AfAA6), obtained by Bulked Segregant Analysis, showed the highest association with PM resistance in the mapping population. Segregation of the QTL was followed in different crosses by CAPS markers developed from these two markers. An ASAP marker that segregates as present/absent, was also developed from the CS25b locus. PM resistance was evaluated visually on a 1 to 5 scale in four different seedling populations. Two populations originated from crosses using Illinois 547-1 as the resistant parent. Two other populations were from crosses with NY88.0514.03, a resistant seedling from the original 'Horizon' x Ill. 547-1 mapping population. Segregation ratio distortions were observed in some crosses. In these cases, the allele associated with the QTL for PM resistance was less frequent than the alternate allele. In all crosses, the markers were closely associated with resistance. If selection were based on markers, the percentage of susceptible individuals (classes 4 and 5) would drop from 24.5-52.1% to 2.6-18.2%. Selection efficiency was greatest in crosses where segregation distortion was most intense.

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