We can take advantages of NGS approaches in identification and mapping of desirable genes. However, it is useful in crops where genome has been sequenced. How can we use NGS in the crops where genome has not been sequenced yet?
With crops you mean some plant species, so if not the same species some related species or taxa must have been sequenced. That can be used as a reference
Still if no suitable reference is available, de-novo assemblies can be performed. This was/and still used for the cases similar to yours. On model plants which you think is most closely related to your plant can be used.
There are many good software with good algorithms are available now a days. So it is much easier to do such analysis as it used to be before few years.
True, NGS can be applied to a variety of uses along with the discovery of novel useful variations. The genome sequence of a closely related taxa/species can be used as a reference genome sequence for the crops where genome has not been sequenced. We use the genome sequence of Sorghum as a reference for sugarcane.