Comparison of ALS inhibitor resistance and allelic interactions in shattercane accessions

Reports of unacceptable shattercane (Sorghum bicolor) control with acetolac tate synthase (ALS)-inhibiting herbicides prompted the investigation of 29 fields in central and south-central Nebraska for ALS-resistant (ALSr) shatt ercane. These fields were located in three distinct geographical areas desi gnated C, G, and P. Shattercane from 13 fields spanning all three areas was resistant to 80 g ai ha(-1) (2 x field rare) primisulfuron. Accessions C a nd G were more resistant than accession P to primisulfuron and nicosulfuron . Accessions C and G were susceptible to imazethapyr, whereas accession P w as resistant. The ALS resistance was associated with alterations in the ALS enzyme. Primisulfuron I-50 values for ALS from ROX (forage sorghum), C, G, and P were 7, 8,510, 8,870, and 714 nM, respectively; nicosulfuron I-50 va lues were 647, 4,110, 4,070, and 1,460 nM, respectively; and imazethapyr I- 50 values were 5,440, 13,100, 11,800, and 51,700 nM, respectively Based on cross-resistance and enzyme sensitivities, ar least two biotypes are repres ented in the three accessions of ALSr shattercane. Shattercane individuals from accessions C, G, and P were intercrossed to determine if the ALSr gene s in each of the accessions were at independent loci. All the F-2 populatio ns were resistant to 80 g ai ha(-1) primisulfuron, suggesting that the ALSr alleles in the three accessions are at the same locus or possibly linked l oci. When the C, G, and P accessions were crossed with the wild type (WT), comparisons between the F-1, susceptible, and resistant populations showed that primisulfuron resistance was expressed as a dominant, partially domina nt, and additive trait for the C, G, and P accessions, respectively. The di fferences in ALSr allelic interactions indicate that primisulfuron resistan ce developed independently in each of the three accessions.