Melon (Cucumis melo L.) is a fresh vegetable and dessert fruit that may also be cooked or dried, processed for juice and flavoring, and the seeds of which are a source of high-quality cooking oil and high protein seed meal. Melon production throughout many parts of the world is now threatened by the crinivirus Cucurbit yellow stunting disorder virus (CYSDV) in tropical and subtropical areas favorable to its whitefly vector. CYSDV is transmitted by the sweetpotato whitefly, Bemisia tabaci Gennadius, biotypes A, B, and Q. CYSDV first appeared on melon in the 1980s in the United Arab Emirates and emerged on melon in the Yuma, AZ, and Imperial Valley, CA, regions and western Mexico during the Fall season of 2006 followed by Florida in 2007. PI 313970, C. melo var. acidulus Naudin, a salad-type melon from India, expressed high-level resistance to CYSDV in Yuma and Imperial Valley in Fall 2006, but it was not immune; the virus was detected in asymptomatic plants. Inheritance of resistance to CYSDV in PI 313970 was studied in three naturally infected, replicated field tests in Imperial Valley during the Fall seasons of 2007 and 2008 and the Spring season of 2009. Resistance in PI 313970 was recessive: all F1 PI 313970 (PI) × susceptible ‘Top Mark’ (TM) and BCTM individuals were susceptible, and the F2 and BCPI segregated 3:1 and 1:1 susceptible to resistance, respectively. Frequency distributions of CYSDV symptom severity ratings suggested a single recessive gene in PI 313970 for resistance to CYSDV. PI 313970 was, however, observed to be variable for resistance; a few plants in each test expressed distinct symptoms of CYSDV infection and its frequency distributions overlapped those of ‘Top Mark’. This variation may represent genetic variation selectable for uniform reaction to infection by CYSDV or phenotypic variation in the resistant reaction. The genetic relationship between the genes for resistance to CYSDV in PI 313970 (recessive) and TGR-1551 (dominant) is not known.
James D. McCreight and William M. Wintermantel
Prabin Tamang, Kaori Ando, William M. Wintermantel, and James D. McCreight
Cucurbit yellow stunting disorder virus (CYSDV) is a devastating viral disease of melon that can cause significant yield and quality losses. This disease has recently emerged as a major concern in the southwest United States and major melon-growing regions across the world. Coinfection of melon by Cucurbit chlorotic yellows virus (CCYV) was recognized in Imperial Valley and neighboring production areas of California and Arizona in 2018, but its importance remains largely unknown. Identifying and deploying CYSDV resistance from elite germplasm is an economical and effective way to manage the disease. A F2:3 population was developed from a cross of susceptible ‘Top Mark’ with CYSDV-resistant PI 313970, which was shown to possess a single recessive gene for resistance to CYSDV. The F2:3 population was phenotyped in the field in response to natural, mixed infections by the two viruses, CYSDV and CCYV in the Fall melon seasons of 2018 and 2019. Phenotypic data (foliar yellowing) from both years were not useful for mapping CYSDV resistance quantitative trait loci (QTL), as PI 313970 and CYSDV-resistant F2:3 plants exhibited yellowing symptoms from CCYV coinfection. QTL analysis of the relative titer of CYSDV calculated from reverse transcriptase quantitative polymerase chain reaction (RT-qPCR) data identified one locus on chromosome 3 at the physical location of S5-28,571,859 bp that explained 20% of virus titer variation in 2018 but was undetected in 2019. A locus on chromosome 5 between S5-20,880,639 to S5-22,217,535 bp explained 16% and 35% of the variation in CYSDV titer in 2018 and 2019, respectively. One or both of the markers were present in six of 10 putative melon CYSDV resistance sources. Markers flanking the 2019 QTL were developed and can be used in marker-assisted breeding of CYSDV-resistant melons.