Transfer of Downy Mildew Resistance from Wild Basil (Ocimum americanum) to Sweet Basil (O. basilicum)

Abstract

Sweet basil (Ocimum basilicum) is susceptible to downy mildew caused by the oomycete foliar pathogen Peronospora belbahrii. No resistant varieties of sweet basil are commercially available. Here, we report on the transfer of resistance gene Pb1 from the highly resistant tetraploid wild basil O. americanum var. americanum (PI 500945, 2n = 4x = 48) to the tetraploid susceptible O. basilicum 'Sweet basil' (2n = 4x = 48). F1 progeny plants derived from the interspecific hybridization PI 500945 × Sweet basil were resistant, indicating that the gene controlling resistance (Pb1) is dominant, but sterile due to the genetic distance between the parents. Despite their sterility, F1 plants were pollinated with the susceptible parent and 115 first backcross generation to the susceptible parent (BCs1) embryos were rescued in vitro. The emerging BCs1 plants segregated, upon inoculation, 5:1 resistant/susceptible, suggesting that resistance in F1 was controlled by a pair of dominant genes (Pb1A and Pb1A'). Thirty-one partially fertile BCs1 plants were self-pollinated to obtain BCs1-F2 or were backcrossed to Sweet basil to obtain the second backcross generation to the susceptible parent (BCs2). In total, 1 BCs1-F2 and 22 BCs2 progenies were obtained. The BCs1-F2 progeny segregated 35:1 resistant/susceptible, as expected from a tetraploid parent with two dominant resistant genes. The 22 BCs2 progenies segregated 1:1 resistant/susceptible (for a BCs1 parent that carried one dominant gene for resistance) or 5:1 (for a BCs1 parent that carried two dominant genes for resistance) at a ratio of 4:1. The data suggest that a pair of dominant genes (Pb1A and Pb1A') residing on a two homeologous chromosomes is responsible for resistance of PI 500945 against P. belbahrii.

CRISPR/Cas9-mediated mutagenesis of sweet basil candidate susceptibility gene ObDMR6 enhances downy mildew resistance

“…Resistance has been found in an exotic sweet basil variety Mrihani and other Ocimum species that differ vastly from popular sweet basil cultivars in ploidy, appearance, aroma and taste [ 12 – 14 ]. Transferring disease resistance to popular sweet basil varieties through traditional breeding is very time-consuming and faced with significant challenges, such as sexual incompatibility, hybrid F1 sterility, and linkage drag [ 7 , 13 , 15 ]. After eight years of breeding work, several BDM-resistant varieties were recently commercialized for sweet basil production [ 14 ].…”

Characterization of the infection process by Peronospora belbahrii on basil by scanning electron microscopy

“…Pathogen resistance to fungicides has also been reported (Cohen et al., 2013a). Breeding for varieties of sweet basil resistant to downy mildew also is promising and has yielded highly resistant germplasm (Wyenandt et al., 2010; Djalali Farahani-Kofoet et al., 2014; Pyne et al., 2014; McGrath et al., 2014; Ben-Naim et al., 2018).…”

Detection of the Downy Mildew Pathogen on Seed of Basil Following Field Infection in Southern New Jersey

“…Importantly, the lack of genetic resistance to the pathogen in widely grown sweet basil cultivars continues to make control difficult (Homa et al, 2014;Pyne et al, 2015Pyne et al, , 2017Wyenandt et al, 2015). Efforts have been made in recent years at breeding downy mildew resistance into commercially acceptable sweet basil lines (Ben-Naim et al, 2018;Pyne et al, 2015Pyne et al, , 2017Pyne et al, , 2018, but this has focused on using a few strains of the pathogen and, as with all disease-resistant plants, growers still need to use the improved germplasm within an integrated pest management program that still requires fungicide applications. Should additional strains (e.g., races) of basil downy mildew develop, such as for other downy mildews on other hosts, continued breeding efforts will be required.…”