Melon Team members: Jim McCreight (USDA, ARS), Shaker Kousik (USDA, ARS), Michael Mazourek (Cornell Univ.), Pat Wechter (USDA, ARS), Bill Wintermantel (USDA, ARS)
Develop common genomic approaches and tools for Melon
1.2. Perform GBS analysis of PI collections, establish core populations, provide community resource for genome wide association studies (GWAS)
1.2.1. GBS of cucurbit species, establish molecular-informed core populations
Genotyped the available NPGS melon accessions and heirloom (n= 2,084) by GBS methods. Population structure, pattern of LD, and redundant accessions were analyzed using the genotype data.
Selected 384-member molecular-informed core population, i.e., functional panel or diversity panel, from the 2,084 based on molecular data analysis and historic importance.
- Validated the utility of the diversity panel for identification of loci that determine quantitative and qualitative traits based on GWAS of 100-seed weight, fruit characteristics, and flower sex expression.
- The panel was phenotyped in a 2018 field test in Imperial Valley, with particular respect to those traits that define melon horticultural groups.
- A manuscript is in preparation to report results of population structure analyses using a suite of tools, LD decay, Core collection selections, and GWAS using historical and project-generated data.
The melon core population was planted in a greenhouse at Salinas in December 2108 for selfing and subsequent resequencing; one plant per member. Seeds of each member will be increased for deposit in USDA, NPGS. Fruits from 57 members have been harvested to date (3/28/19).
GBS and endorna virus analysis of 42 C. melo ssp. agrestis var. texanus, and bona fide C. melo ssp. melo var. chito and var. dudaim accessions confirmed the unique identity of var. texanus among the 19 melon horticultural groups in the recently revised melon classification scheme.
Powdery Mildew (Podosphaera xanthii) resistance in MR1xAY RIL
Awaiting for growth chamber space availability for Race 2 test.
Powdery Mildew (Podosphaera xanthii) resistance in Top Mark x PI 313970 F2:3
Growth chamber test for resistance to race S is underway.
Growth chamber test for resistance to race 1 scheduled to follow the race S test.
Powdery Mildew (Podosphaera xanthii) California Field Tests
Two replicated field tests of cucurbit powdery mildew race differentials subjected to natural infection were planted in
• Imperial Valley. University of California, Desert Research and Extension Center, Holtville; watered 8 March 2018; insufficient infection to evaluate.
• Central Valley. University of California, Westside (Westside Research and Extension Center, Five Points; Planted 25 June 2018; insufficient infection to evaluate.
Downy mildew
• Initiated phenotyping resistance in the MR1 x AY RIL.
CYSDV
- PI 313970 x Top Mark–Produced 200 F2:F3 progenies. GBS analysis completed. Evaluated for CYSDV reaction replicated, naturally-infected field tests in Imperial Valley at the University of California, Desert Research and Extension Center, Holtville; watered 16 August 2018.
- PI 313970 x TGR 1551 (PI 482420)–Produced 184 F2:F3 progenies. GBS analysis completed. Evaluated for CYSDV reaction replicated, naturally-infected field tests in Imperial Valley at the University of California, Desert Research and Extension Center, Holtville; watered 16 August 2018.
CMV
- Evaluated 25 advanced Cornell University CMV-resistant melon lines developed by M. Kyle-Jahn and H.M. Munger. Limited quantities of seed were produced in 2017 and transferred via MTA to USDA-ARS, Salinas.•
- Greenhouse evaluation–Controlled-inoculation tests at Salinas, Spring 2018: nine lines exhibited resistance: six lines were asymptomatic, with limited or no virus accumulation (Figure 1, Left), three lines exhibited only local lesions against a subgroup 1 CMV isolate from melon; the other 16 lines were susceptible with 15 lines exhibiting mosaic reactions.
Figure 1. Susceptible and resistant reactions to CMV inoculation: Left, virus-free, asymptomatic plants of resistant line 17-4065-1; Right, Susceptible line 17-4028-1 showing mosaic symptoms.
View figure 1 in the PDF version
• Field evaluation–Central Valley, University of California, Westside (Westside Research and Extension Center, Five Points; Planted 25 June 2018 for disease reactions to natural CMV-infection, adaptation, and fruit quality. Field was infested with melon aphid that was controlled with insecticide application. CMV was not present in the field; sampled plants were negative for the virus. The lines appeared to be poorly adapted to the Central Valley, CA, as indicated by plant size and condition. None of the lines exhibiting resistant reactions in the greenhouse were U.S. western shipper (USWS) type melons (Figure 2). Additional backcrossing is needed to combine CMV resistance with USWS type melon.
Figure 2. Fruits of two CMV-resistant Cornell breeding lines did not exhibit western U.S. shipping type, orange flesh melon characteristics.
View figure 2 in the PDF version
2.2 Marker development and verification
Powdery Mildew
Identified QTL on Chromosome 5 and 12 for resistance to Powdery mildew race 1.
KASP markers for both Powdery mildew QTL have been developed and are in the process of being validated.
KASP markers for sulfur resistance have been developed and are in the process of being validated.
Fusarium wilt
KASP markers have been developed for resistance to Fusarium wilt race 1 and Fusarium wilt race 2, and are being validated
CYSDV
No progress to date.*
CMV
No progress to date.
Cucurbit chlorotic yellows virus.
During the summer of 2018, melon plants from a germplasm diversity study in the Imperial Valley, CA were found infected with Cucurbit chlorotic yellows virus (CCYV; genus Crinivirus, family Closteroviridae). Two melon plants were found exhibiting interveinal yellowing and chlorotic spot symptoms similar to those caused by a crinivirus, but varying from symptoms normally observed during infection by Cucurbit yellow stunting disorder virus (CYSDV; genus Crinivirus). Total nucleic acid was extracted from leaves of both plants and tested negative for CYSDV, but positive for CCYV by RT-PCR using primers specific to portions of RNA2 of each virus encoding the virus coat protein genes. The CCYV amplicon was sequenced and shared 99% sequence identity with most of the CCYV isolates from around the world sequenced to date. A second set of CCYV-specific primers were designed to a region within RNA1 encoding the RNA-dependent RNA polymerase (RdRp) gene and amplification of a 370 nt amplicon was confirmed. This 370 nt RdRp amplicon sequenced was a 100% match to 20 CCYV isolates from around the world. Due to the similarity in symptoms between CCYV and CYSDV, several archived and frozen total nucleic acid and RNA extracts from Imperial Valley melon, collected over the course of 9 years (2010-2018), were re-analyzed for CCYV to determine whether the virus was newly emerged or if it had evaded detection due to similarity in symptoms to CYSDV. Nineteen of 23 samples collected between 2014 and 2018 were positive for CCYV, and many samples contained mixed infections of CCYV with CYSDV and/or the ipomovirus, Squash vein yellowing virus (SqVYV). All eighteen archived samples collected from 2010 to 2013 tested negative for CCYV, but extracts were confirmed as viable because parallel amplification of CYSDV from these samples was successful. Therefore, CCYV most likely emerged in the Imperial Valley during in 2014 but remained undetected due to similarity with CYSDV in symptoms on cucurbit host plants and vector transmission. CCYV is prevalent in East Asia, the Middle East, and North Africa, and is transmitted efficiently by the whitefly, Bemisia tabaci. Both CCYV and CYSDV have long retention times in their whitefly vector, facilitating transmission throughout the region. Further studies will be necessary to evaluate epidemiology of CCYV in the southwestern US desert production region, and to determine its impact on melon production and development of crinivirus-resistant cultivars. (See Wintermantel et al. 2019)
2.3 Introgress resistance into advanced breeding lines
Melon USVL206 (derived from an MR-1 x AY cross) with resistance to Fusarium wilt race 1 and 2, sulfur resistance, powdery mildew race 1 resistance, and has orange sweet flesh has been crossed into Top Mark, Charentais and backcrossed into AY.
