Location: Crop Improvement and Protection Research
2017 Annual Report
Objectives
The focus of this research program is on quality traits, resistance to diseases, insects and abiotic stresses of lettuce, spinach and melon considered by the respective industries and the scientific community to be the most critical to production. The overall goal is the development of elite germplasm and cultivars with improved quality and productivity, and new knowledge of the genetics and breeding of lettuce, spinach and melon.
Objective 1: Identify and select for novel sources of high-level host plant resistance in lettuce to priority diseases, insects, physiological defects and improved phytonutrient content. Improved lettuce germplasm and/or finished varieties will be released and incorporated into the ARS National Plant Germplasm System.
Objective 2: Identify novel sources of host plant resistance in spinach to the new and emerging priority diseases, continue selection and improvement of host plant resistance to leaf miner, and elucidate genetic variation for low oxalic content to improve bioavailability of iron in fresh product. Improved spinach germplasm and/or finished varieties will be released and incorporated into the ARS National Plant Germplasm System.
Objective 3: Identify novel sources of host plant resistance in melon to Cucurbit yellow stunting disorder virus and other priority diseases of melon, determine their genetic bases and evaluate potential for higher levels of resistance through pyramiding of non-allelic resistance genes. Improved cantaloupe and honeydew germplasm and/or finished varieties will be released and incorporated into the ARS National Plant Germplasm System.
Objective 4: Develop high yielding germplasm and cultivars of leafy green vegetables with optimal levels of nutrients, examine the effects of environment on microbiota found on vegetables, and determine how consumption of food from these cultivars may impact human metabolism, including changes in gut microbiota [NP 301 C1 PS1A, 1B; NP 107 C1 PS1D].
Approach
Objective 1: Collect, identify, characterize, and evaluate wild and unadapted germplasm of lettuce. Evaluate germplasm for new sources of resistance to viruses (lettuce dieback), fungi (downy mildew, lettuce drop, powdery mildew, Verticillium wilt), bacteria (bacterial leaf spot, corky root), and insects (lettuce aphid, leafminer). Evaluate lettuce for differential pre- and postharvest survival of E. coli O157 on lettuce germplasm. Improve phytonutrient content of lettuce. Enhance germplasm, develop improved and elite populations via selection, hybridization and backcrossing. Determine heritability of host plant resistance traits. Determine inheritance and linkage relationships of phenotypic and molecular markers.
Objective 2: Collect, identify, characterize, and evaluate unadapted germplasm of spinach. Evaluate germplasm for new sources of resistance to downy mildew and leafminer insect. Reduce oxalate content of spinach. Enhance germplasm, develop improved and elite populations via selection, hybridization and backcrossing. Determine heritability of host plant resistance traits. Determine inheritance and linkage relationships of phenotypic and molecular markers.
Objective 3: Collect, identify, characterize, and evaluate wild and unadapted germplasm of melon. Evaluate germplasm for new sources of resistance to Cucurbit yellow stunting disorder virus and powdery mildew. Enhance germplasm, develop improved and elite populations via selection, hybridization and backcrossing. Determine heritability of host plant resistance traits. Determine inheritance and linkage relationships of phenotypic and molecular markers.
Objective 4: Collect, identify, characterize, and evaluate wild and unadapted germplasm of lettuce, spinach and other leafy vegetables. Evaluate germplasm for unique variants with respect to nutrient content, including assessment of bioavailability. Characterize host plant genotype variation, environmental and GxE effects on microbiota of lettuce, spinach and other leafy vegetables, including effects of plant pathogens and micro-parasites, and enteric organisms. Determine effects of ingested nutrient-enhanced leafy vegetables on human metabolism, including gut microbiota. Enhance germplasm, develop improved and elite populations via selection, hybridization and backcrossing. Determine inheritance and linkage relationships of related phenotypic, biochemical, physiological, and molecular markers.
Progress Report
Substantial progress was made related to Objective 1, which focuses on lettuce.
Lettuce dieback is caused by two closely related viruses from the family Tombusviridae. The disease is widespread in all types of commercially grown lettuce with the exception of modern iceberg-type cultivars. We tested 225 accessions for resistance to the disease and distributed information to seed/breeding companies. These accessions were genotyped with single-nucleotide polymorphism (SNP)-based molecular markers to identify marker-trait association. Five highly resistant accessions were used for mating and seed production. A single dominant gene confers complete resistance to the disease. Two resistance alleles were identified in cultivated lettuce, while three additional alleles of the same resistance gene were identified in three Lactuca species sexually compatible with cultivated lettuce.
Many cultivars of lettuce (Lactuca sativa L.) are susceptible to downy mildew, a nearly globally ubiquitous disease caused by Bremia lactucae. Testing of two mapping populations developed from a cross between resistant and susceptible accessions in multiple field experiments revealed loci conferring polygenic resistance in cultivars. Grand Rapids and Iceberg. Additional tests were performed on the population derived from a cross between resistant cv. La Brillante and susceptible cv. Salinas 88. These analyses revealed a novel dominant gene (Dm50) on linkage group 2 and quantitative trait loci (QTL) located on another four linkage groups.
Downy mildew is endemic in many lettuce-growing regions of the world. Invasion by plant pathogens may create new portals and opportunities for microbial colonization of plants. The occurrence of outbreaks of Escherichia coli O157:H7 (EcO157) and Salmonella enterica infections linked to lettuce prompted us to investigate the role of downy mildew in the colonization of romaine lettuce by these human pathogens under controlled laboratory conditions. We tested interaction of downy mildew and Escherichia coli on cultivars of lettuce differing in their level of resistance to downy mildew. These results indicate that the plant basal defense gene(s) may play a role in survival of human enteric pathogens on lettuce.
Bacterial leaf spot (BLS) of lettuce, caused by Xanthomonas campestris pv. vitians, (Xcv) is an important disease of lettuce that is dependent on wet conditions in spring and fall production in California. We performed nine greenhouse experiments to evaluate resistance on the mapping population developed from a cross between resistant cultivar 'Batavia Reine des Glaces' and susceptible cultivar 'Eruption'. Results indicate that resistance in ‘Batavia Reine des Glaces’ is polygenic. Mapping of the resistance loci revealed two quantitative trait loci (QTL) located on linkage groups 2, and 9. The QTL on linkage group 9 was detected in all environments and explained up to 20% of the total variation for the resistance to bacterial leaf spot.
Lettuce Drop is caused by the fungal species Sclerotinia minor and S. sclerotiorum. The latter species is globally distributed and predominates in the coastal valleys of California. Sclerotinia sclerotiorum is economically damaging to lettuce production in the Central and Imperial valleys of California, and Yuma Valley, Arizona, and causes economically important diseases on numerous other crops as well. Symptoms include wilting and collapse of lettuce heads. We are developing breeding lines with partial resistance to the disease derived from cultivar 'Eruption'. We have performed field experiments to map the QTL involved in resistance to lettuce drop and identified a major gene in linkage group 5 that explained up to 25% of the total variation.
Fusarium wilt of lettuce continues to be a serious problem in Yuma, Arizona and each year new fields are infested with the soilborne fungal pathogen. Six lines were selected to continue combining resistance with iceberg lettuce type and adaptation to the early fall planting season, which is affected by the disease.
The soil borne fungus Verticillium dahliae causes the highly destructive disease Verticillium wilt on many crop species including lettuce. The disease first appeared on lettuce in 1995 and has spread to ranches throughout Monterey and Santa Cruz Counties. Symptoms on lettuce include lower leaf wilting and yellowing, dark green discoloration along the vascular bundles in the taproot, formation of microsclerotia and shrinking and collapse of the mature head. The disease is most destructive on iceberg lettuce, where losses are typically complete. Fumigation with methyl bromide is effective, but is not economically feasible for lettuce. We tested 241 breeding lines developed from crosses between cultivars with resistance to Verticillium wilt race 1 and four other diseases and selected genotypes with resistance to Verticillium wilt race 1, corky root, bacterial leaf spot, and acceptable resistance to downy mildew. A release notice was submitted for three breeding lines of iceberg lettuce.
While resistance to Verticillium wilt race is currently effective in many commercial fields, diverse isolates of the pathogen are being introduced on spinach seed, a common rotation crop with lettuce. A significant portion of these spinach isolates are race 2, which overcomes resistance to race 1. Screening and development of lettuce germplasm for resistance to race 2 isolates is imperative. We have tested 1,231 genotypes for their resistance to the disease. These genotypes include wild species of lettuce, cultivars originating from diverse locations, and breeding lines developed from crosses between the closest wild relative of lettuce, Lactuca serriola, and cultivated lettuce. QTL for resistance to the pathogen were located on several linkage groups, with the most significant QTL located on linkage group 6.
We continued our crossing, selection, and seed increase of lettuce to breed for resistances to leafminer, corky root, yellow spot, herbicide tolerance, nutritional improvement, appearance, and horticultural traits. Lettuce breeding lines in advanced generations were tested in field trials with control varieties and commercial cultivars, and some lines are ready to be released. The corky root and leafminer resistance of the breeding lines were similar to or better than resistant controls, while their plant weight, height, core length, tipburn, and downy mildew resistances were comparable or better than control cultivars.
Progress was also made in support of Objective 2, which focuses on spinach. We made crosses and selections to breed for resistances to downy mildew, leafminer, and herbicide tolerance, as well as horticultural traits using recurrent selection. Selected plants were transplanted into isolators to produce seeds for the subsequent round of selection. Testing of spinach varieties for cadmium content was continued. Spinach genomic studies continued with collaborators in three universities to find molecular markers for disease resistances, insect resistances, nutritional content, and horticultural traits.
In support of Objective 3 related to melon, breeding of melon for resistance to Cucurbit yellow stunting disorder virus was continued in naturally-infected field tests in Imperial Valley, California. Resistant selections were taken from populations segregating for resistance from crosses with three sources of resistance. The selections were backcrossed in a greenhouse in 2017 and will be inbred for the next round of selection in fall 2018.
Sweet potato whitefly is a major insect pest of melon through direct feeding action, reproduction, and transfer of several viruses. It has eliminated fall season melon production in Imperial Valley, California and drastically curtailed production in Yuma, Arizona. Research on resistance to the whitefly per se was continued using 12 susceptible and putative sources of resistance in field tests and in controlled infestation greenhouse tests.
Accomplishments
1. Heat tolerant lettuce to compensate for climate change and extend the growing season. Climate change poses serious threats and challenges to the production of leafy vegetables, as they generally are cool-season crops. Lettuce is especially vulnerable to heat stress. ARS researchers at Salinas, California evaluated the performance of leaf lettuce varieties for heat tolerance by growing them in different locations in California that differ in temperatures during the growing season. Fifteen green leaf and 21 red leaf lettuce varieties that were selected from more than 1,000 varieties that were screened in three different commercial lettuce production environments, and planted in March and May in the San Joaquin and Imperial Valleys, and in June in the Salinas Valley. Results suggest that lettuce planting can be extended from January to March beyond the normal growing seasons in San Joaquin and Imperial Valleys, where yields may be higher than in the Salinas Valley. Results of this research should be useful for the development of heat-tolerant lettuce cultivars and for extending the growing season in warmer areas with lower production costs, which will help maintain or increase the profitability and sustainability of this important crop in a changing climate.
2. Cucurbit powdery mildew database. Cucurbit powdery mildew is a fungal disease of melon that is ubiquitous worldwide; early disease can kill plants, and late disease can reduce yield and quality. Many pathogenic races of powdery mildew have been identified on melon since PMR 45 (resistant to race 1) and PMR 5 (resistant to race 2) were released in the 1930s. Many of the new races are able to cause disease on PMR 45 and PMR 5, rendering these varieties susceptible to the disease. The pathogenic race situation in California has varied every year since 2003, making it difficult for plant breeders to develop resistant varieties and for farmers to know what to plant. An ARS researcher in Salinas, California and a researcher at University of California, Riverside collected cucurbit powdery mildew from fields in three production areas of California in 2015, 2016 and 2017, tested them for their ability to cause disease on a set of 11 melon varieties, and developed procedures for reproducing them and their long-term storage at -80 degrees Celsius. These researchers developed the first cucurbit powdery mildew database website (powderymildew.ucr.edu) to provide the information from the tests to the public, and melon powdery mildew scientists.
Review Publications
Hayes, R.J., Simko, I. 2016. Breeding lettuce for improved fresh-cut processing. Acta Horticulturae. 1141:65-76. doi: 10.17660/ActaHortic.2016.1141.7.
Lafta, A., Turini, T., Sandoya, G.V., Mou, B. 2017. Field evaluation of green and red leaf lettuce genotypes in the Imperial, San Joaquin, and Salinas Valleys of California for heat tolerance and extension of the growing seasons. HortScience. 52:40-48.
Xu, C., Mou, B. 2016. Short-term effects of composted cattle manure or cotton burr on growth, physiology and phytochemical of spinach. HortScience. 51:1517-1523. doi: 10.21273/HORTSCI11099-16.
Sandoya, G.V., Gurung, S., Short, D.P., Subbarao, K.V., Michelmore, R.M., Hayes, R.J. 2016. Genetics of resistance in lettuce to races 1 and 2 of Verticillium dahliae from different host species. Euphytica. 213:20. doi: 10.1007/s10681-016-1813-0.
Qin, J., Shi, A., Mou, B., Bhattarai, G., Yang, W., Weng, Y., Motes, D. 2017. Association mapping of aphid resistance in USDA cowpea (Vigna unguiculata L. Walp.) core collection using SNPs. Euphytica. 213:36. doi: 10.1007/s10681-016-1830-z.
Xu, C., Jiao, C., Sun, H., Cai, X., Wang, X., Ge, C., Zheng, Y., Liu, W., Sun, X., Xu, Y., Deng, J., Zhang, Z., Huang, S., Dai, S., Mou, B., Wang, Q., Fei, Z., Wang, Q. 2017. Draft genome of spinach and transcriptome diversity of 120 Spinacia accessions. Nature Communications. 8:15275. doi: 10.1038/ncomms15275.
McCreight, J.D. 2017. Botany and culture. In: Keinath, A.P., Wintermantel, W.M., Zitter, T.A., editors. Compendium of Cucurbit Diseases and Pests. 2nd edition. St. Paul, MN: APS Press. p. 1-9.
McCreight, J.D., Keinath, A.P. 2017. Crown blight of melons and crown decline of watermelon. In: Keinath, A.P., Wintermantel, W.M., Zitter, T.A., editors. Compendium of Cucurbit Diseases and Pests. 2nd edition. St. Paul, MN: APS Press.p. 185–186.
Dhillon, N.P., Sanguansil, S., Singh, S.P., Masud, M.A., Kumar, P., Bharathi, L.K., Yetisir, H., Huang, R., Cahn, D.X., McCreight, J.D. 2017. Gourds: Bitter, Bottle, Wax, Snake, Sponge and Ridge. In: Grumet, R., Katzir, N., Garcia-Mas, J., editors. Genetics and Genomics of the Cucurbitaceae. New York, NY: Springer. p. 1–18.
McCreight, J.D. 2016. Cultivation and uses of cucurbits. In: Grumet, R., Katzir, N., Garcia-Mas, J., editors. Genetics and Genomics of the Cucurbitaceae. New York, NY: Springer. p. 1–12.
Dhillon, N.P., Sanguansil, S., Schafleitner, R., Wang, Y., McCreight, J.D. 2016. Diversity among a wide Asian Collection of bitter gourd landraces and their genetic relationships with commercial hybrid cultivars. Journal of the American Society for Horticultural Science. 141:475–484. doi: 10.21273/JASHS03748-16.
Nimmakayala, P., Tomasson, Y., Abburi, V.K., Saminathan, T., Rodrigguez, A.G., Vajja, V.G., Salazar, G., Girish, P., Levi, A., Wechter, W.P., McCreight, J.D., Korol, A., Ronin, Y., Garcia-Mas, J., Reddy, U. 2016. Genome-wide differentiation of various melon horticultural groups for use in genome wide association study for fruit firmness and construction of a high resolution genetic map. Frontiers in Plant Science. 7:1437. doi:org/10.3389/fpls.2016.01437.
