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Robert M. Pyne, Adolfina R. Koroch, Christian A. Wyenandt, and James E. Simon

Sweet basil (Ocimum basilicum) is one of the most economically important culinary herbs in the world, yet global production has become increasingly challenging due to the destructive disease downy mildew (Peronospora belbahrii). Although multiple sources of resistance have been identified, there are no resistant sweet basil cultivars with a commercially acceptable chemotype and phenotype available. The commercial basil cultivar Mrihani (MRI) was identified as resistant and crossed with a Rutgers University susceptible sweet basil inbred line (SB22) to generate a full-sibling family. To determine the mode of inheritance for resistance to downy mildew in basil, six related generations of the MRI × SB22 family were evaluated using a disease severity index (DSI) at northern and southern New Jersey locations over 2 years. All siblings in the F1 and BC1P2 generations were resistant (0.33 > DSI) providing strong evidence that inheritance of resistance from MRI was conferred by dominant alleles. Segregation ratios in the F2 and backcross to the susceptible parent (BCP1) generations demonstrated chi-square goodness of fit to the two-gene complementary (F2: P = 0.11, BC1P1: P = 0.04) and recessive epistatic (F2: P = 0.03, BC1P1: P = 0.63) models. Further analyses of gene effects using a weighted six-parameter scaling test provided evidence that nonallelic additive × additive and additive × dominant gene effects were highly significant (P < 0.001) and resistance reducing. This is the first report of heritable genetic resistance that can be introduced to sweet basil without the issue of sterility barriers. Plant breeding strategies using the MRI × SB22 family should exploit dominant gene action and remove recessive, resistance-reducing alleles from the population.

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Robert M. Pyne, Adolfina R. Koroch, Christian A. Wyenandt, and James E. Simon

Sweet basil (Ocimum basilicum L.) is among the most widely popular and economically important culinary herbs. Worldwide production of sweet basil has been threatened by a newly emerging disease, downy mildew (Peronospora belbahrii). Although tolerance and resistance have been identified in other Ocimum species, the traditional sweet basils all have been reported to be highly susceptible. There is an urgent need for evaluation of basil germplasm to identify sources of host resistance to P. belbahrii within Ocimum spp. and especially among O. basilicum species. In searching for genetic resistance, we developed a rapid approach to screen and evaluate downy mildew response at the cotyledon and true leaf growth stages under controlled environmental conditions. To confirm the reliability and reproducibility of this screening method, an experiment was conducted in which three basil species (Ocimum basilicum, sensitive; O. xcitriodorum, tolerant; and O. americanum, resistant to basil downy mildew) were evaluated for response to downy mildew inoculations at three growth stages. Disease incidence (DI) at the cotyledon growth stage was equal to or greater than true leaf growth stages for all species indicating that cotyledon response to downy mildew inoculations is a viable marker for predicting true leaf stage resistance. This approach was then used to screen 36 USDA-NPGS O. basilicum accessions at cotyledon and first true leaf growth stages to identify promising downy mildew-resistant breeding lines. Thirty accessions were susceptible at both growth stages (DI = 1.0). Four accessions exhibited little or no sporulation at either growth stage (DI less than 0.06), three of which showed other symptoms including chlorosis and necrosis. One accession, PI 652053, demonstrated no signs or symptoms but differed greatly from other accessions in regard to leaf morphology and habit. Results show that a resistant, mature plant can be identified at the cotyledon growth stage, providing a robust, low-input approach to identify promising downy mildew-resistant breeding material. Field evaluations of basils under high downy mildew pressure confirmed the applicability of this new screening approach to identify resistance to basil downy mildew.