Identification of quantitative trait loci associated with bacterial leaf spot resistance in baby leaf lettuce

Authors: Sthapit Kandel, Jinita; SANDOYA, GERMAN - University Of Florida; Zhou, Wei; Read, Quentin; Mou, Beiquan; Simko, Ivan

Submitted to: Plant Disease
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/10/2022
Publication Date: 9/26/2022
Citation: Sthapit Kandel, J., Sandoya, G.V., Zhou, W., Read, Q.D., Mou, B., Simko, I. 2022. Identification of quantitative trait loci associated with bacterial leaf spot resistance in baby leaf lettuce. Plant Disease. 106(10):2583-2590. https://doi.org/10.1094/PDIS-09-21-2087-RE.
DOI: https://doi.org/10.1094/PDIS-09-21-2087-RE

Interpretive Summary: Spring mix is a popular type of packaged salad and baby leaf lettuce (Lactuca sativa L.) is one of its main ingredients. Baby leaf lettuce is usually sown at very high densities, which enhances occurrence of bacterial leaf spot (BLS) disease, caused by Xanthomonas hortorum pv vitians (Xhv). BLS symptoms typically start as small water-soaked leaf spots which later turn to brown or black necrotic lesions. In severe cases, in humid conditions, the smaller spots coalesce to larger lesions and can make the crop unmarketable. The market demands disease-free, high-quality baby leaf lettuce all year round. Growing highly BLS-resistant cultivars will reduce yield/quality losses and minimize economic harm to lettuce and spring mix growers. Our objectives were to identify lettuce varieties resistant to BLS and to determine genetic regions (known as quantitative trait loci, QTL) associated with the BLS resistance. Nearly 500 lettuce accessions were screened with six Xhv isolates (BS0347, BS2861, BS3127, L7, L44, and Sc8B). Accessions showing high level resistance to three or more isolates include Bunte Forellen, PI 226514, ARM09-161-10-1-4, La Brillante, PI 358004, Bella, Tango, Romana Verde del Mercado, W6 29841 0-COS- Ferro, Grenadier, PI 491056 0-COS- WP 070, Delight, Little Gem, and PI 491221. Four thousand six hundred and fifteen high-quality, polymorphic molecular markers were used to determine marker-trait association of BLS resistance. Significant QTL were identified in six out of nine lettuce chromosomes (2, 3, 4, 6, 7, and 8). Lettuce accessions with high level of resistance detected in this study are valuable resources for lettuce germplasm improvement. Molecular markers closely linked with QTL can be applied for marker-assisted selection of BLS-resistant lettuce germplasm and development of new lettuce cultivars with multiple disease resistance.

Technical Abstract: Spring mix is a popular packaged salad that contains leaf lettuce (Lactuca sativa L.) as one of its main ingredients. Plants for baby leaf lettuce (BLL) production are grown at very high densities, which enhances occurrence of bacterial leaf spot (BLS) caused by Xanthomonas hortorum pv. vitians (Xhv), a disease that can make the crop unmarketable. The market demands disease-free, high-quality BLL all year round. Growing highly BLS-resistant cultivars will reduce loss of yield and quality thus minimizing economic detriment to lettuce and spring mix growers. The research objectives were to identify lettuce accessions resistant to BLS and associated quantitative trait loci (QTL). A total of 495 lettuce accessions were screened with six isolates (BS0347, BS2861, BS3127, L7, L44, and Sc8B) of Xhv. Accessions showing high level resistance to three or more BLS isolates in the order of resistance phenotype are Bunte Forellen, PI 226514, ARM09-161-10-1-4, La Brillante, PI 358004, Bella, Tango, Romana Verde del Mercado, W6 29841 0-COS- Ferro, Grenadier, PI 491056 0-COS- WP 070, Delight, Little Gem, and PI 491221. Genome-wide association studies of BLS resistance by mixed linear model analyses identified significant QTLs in six out of nine lettuce chromosomes (2, 3, 4, 6, 7, and 8). The most significant QTL was in chromosome 8 (P = 1.42 x 10-7) that explained 6.7% of total phenotypic variation for the trait. Accessions with high level of resistance detected in this study are valuable resources for lettuce germplasm improvement. Molecular markers closely linked to QTLs can be considered for marker-assisted selection to develop of new lettuce cultivars with resistance to multiple races of Xhv.