Biplot Analysis of Diallel Data

Ascochyta blight of pea (Pisum sativum L.) is a fungal disease caused by Mycosphaerella pinodes (Berk. & Bloxham) Verstergren, Phoma medicaginis Malbr. & Roum. var. pinodella (L.K. Jones) Boerema, and Ascochyta pisi Lib. that can result in significant reductions to pea yield and quality. To characterize the genetics of resistance and to identify molecular markers for use in plant breeding, quantitative trait loci (QTLs) affecting Ascochyta blight resistance were mapped in F2:3 and F2:4 families produced from a cross between resistant breeding line 3148-A88 and susceptible cultivar Rovar. A linkage map containing 96 loci on 11 linkage groups was constructed for 133 families from this cross. Resistance of progeny lines to natural Ascochyta blight epidemics was examined in field trials at Medina, Western Australia, in 1997, 1998, and 1999. Disease severity was assessed on stems, leaves, and pods by means of separate rating scales. Because pea shows increased susceptibility to Ascochyta blight as it matures, plant reproductive stage was assessed at the time of disease scoring in the 1998 and 1999 trials. Thirteen QTLs were detected for Ascochyta blight resistance on seven linkage groups. Eight of these QTLs were detected in multiple environments or by multiple trait scores. One QTL for plant developmental stage was detected. Linkage of Ascochyta blight resistance QTLs to candidate genes including disease response genes and resistance gene analogs and of the QTL for plant reproductive stage to a pea homolog of the Arabidopsis thaliana CONSTANS gene controlling flowering time in response to photoperiod is discussed.