Florida hydrilla populations have shown an alarming increase in resistanceto fluridone, an herbicide used extensively for controlling invasive UShydrilla populations. A rapid PCR and sequencing method was developed toidentify and screen hydrilla genomic DNA for three previously identifiedphytoene desaturase (pds) gene mutations that conferresistance to fluridone. Ninety hydrilla accessions were screened forfluridone resistant genotypes including 46 accessions from the US and 44accessions from 15 other countries. In Florida, hydrilla from five of ninesites tested was heterozygous for wild-type and herbicide-resistant alleles.Additionally, a new resistant population was identified from Lake Seminolein Georgia, the first genetically confirmed strain of resistant hydrillaoutside of Florida. All resistance-conferring mutations were located on thesame homologous haplotype of US dioecious hydrilla. All other hydrillasamples tested possessed only wild type alleles, including monoeciousstrains that had been exposed to fluridone. Management implications arediscussed.
