New races of the downy mildew pathogen of spinach, identification of molecular markers for disease resistance, and molecular diversity of spinach germplasm

Spinach downy mildew, caused by Peronospora farinosa f. sp. spinaciae, is the most economically important disease of spinach worldwide. The most effective management tool for the control of downy mildew is the use host resistance mediated by major genes. Since 1997, we have identified six new races (5, 6, 7, 8, 9, and 10) of the downy mildew pathogen based on qualitative disease reactions on a set of host differentials. In addition, a modified greenhouse screening procedure has been developed to quantify differences in field resistance observed among various cultivars. The established screening procedure and the race nomenclature have been widely adopted in the industry in both the U.S. and Europe. An open-pollinated isogenic line was also developed by crossing a parental line with resistance to race 6 of the downy mildew pathogen to the highly susceptible recurrent parent 'Viroflay'. Evidence through four backcross generations shows that the race 6 disease resistance is inherited as a single dominant gene. This material can now be used to develop a open-pollinated line with fixed homozygous resistance to race 6. Concurrently, AFLPs (Amplified Fragment Length Polymorphisms) were used to identify a marker closely linked to resistance to race 6 and the AFLP marker has been converted into a SCAR (sequence characterized amplified region) marker. This marker could expedite screening for resistance to race 6 of the downy mildew pathogen. The AFLP technique was also used to assess genetic diversity in the USDA spinach germplasm collection. From 61 AFLP primer combinations evaluated initially, 9 were selected to differentiate 49 spinach genotypes from diverse geographic origins. Results showed that the technique was valuable for assessing genetic relationships among spinach genotypes; genetic similarity (GS) ranged from 0.58-0.95 for spinach germplasm accessions. Overall variation among spinach germplasm accessions genotypes selected from different backgrounds indicates that the genetic diversity in spinach is high when compared to other crop species. These data indicate that these spinach genotypes are very diverse and are a valuable source for allelic variation for breeding efforts. Also, the AFLP technique can be further refined and may allow genotype identification for proprietary purposes.