Identifying new sources of resistance to tomato leaf curl New Delhi virus from Indian melon germplasm by designing an improved method of field screening

Tomato leaf curl New Delhi virus (ToLCNDV) is an emerging constraint in muskmelon production in India and other parts of the world. This study aims to identify the new sources of resistance against ToLCNDV from Indian melon germplasm, which has not been evaluated globally. Sixty melon germplasm comprising of both cultivated commercial types (vars. reticulatus and inodorus) from the subspecies melo and wild germplasm (vars. momordica, conomon, and callosus) from subspecies agrestis were screened in the field for two consecutive years under natural epiphytotic condition. The infected plants showed varying degrees of phenotypic symptoms, such as yellow mosaic, stunting of plant growth, and restricted fruiting. The disease response of ToLCNDV in melon genotypes were measured by a robust rating scale, which was developed by providing differential weightage to morphogenic symptoms on foliage, reduction of vine length and fruiting of the plant. The genotype DSM 132 (C.melo var. callosus) could be identified as highly resistant to ToLCNDV, which recorded the minimum disease severity index (DSI) of 0.00, 0.00, followed by DSM 19 (3.50, 4.50) and DSM-11-7 (7.00, 6.11) from C. melo var. momordica for two consecutive years. The resistance in these genotypes was further confirmed through challenge inoculation with viruliferous whitefly (Bemisia tabaci) carrying ToLCNDV in the greenhouse conditions, which showed a minimum vulnerability index in genotype DSM 132 (VI = 2.0) followed by DSM 19 (VI = 6.67) and DSM-11-7 (VI = 11.34). The molecular technique of virus detection through polymerase chain reaction (PCR) specific to ToLCNDV failed to detect the presence of tomato leaf curl New Delhi virus in resistant genotypes DSM 132, DSM 19, and DSM-11-7. Quantitative PCR (qPCR) showed very low viral titer in resistant genotypes DSM 132, DSM 19, and DSM-11-7 compared to susceptible genotypes. This study could identify three Indian melon genotypes with high levels of resistance to ToLCNDV, which will be useful for resistance breeding across the globe.