Is there a way to tell if an Esrog is murcav?

The was an old method of opening it up and looking at the seeds. In classical etrog, the seeds are aligned in rows along the axis of the fruit, and the murkav from a lemon often has the radially spaced seed of typical lemons and other citrus. Also, the etrog is supposed to be all pith with little or no fruit. This has all gone into disuse, and now we rely only on the pedigree of the orchard and the source of the tree stock. The reasons for this is the contamination of many stocks with the Corfu strain which was a graft to a lemon. Also, actual grafted trees can produce etrogim for a long time, and the features of the rootstock remain in latency; for example, my neibor’s 30 year old lemon tree, started putting our sour oranges, the type of the rootstock it was grafted to long ago. There is also an issue of cross pollenation which can change the fruit immediately, or lay dormant in the seeds of the next generation. I have cut open etrogim in large batches many times, and about 5% were full lemons inside. In Yalut Yosef, he famously said that MOST Israeli etrogim are murkav. The best bet is with fruit that came from the stocks of isolated places like the Moroccan from the Atlas mountains, and meyuhas Teimani.

DNA testing would indicate.

AronChaim, if my pollen contamination theory is correct, the Chazon Ish strain would not be immune the ‘murkave bidei shamayim. I don’t know how the C.I. got its yichus and what the Gadols means of telling was. A Moroccan importer I knew three shmitas ago claimed a 600 year mesorah.

Sam2 for starters read Rav Kook’s Pri Ezt Hadar, and there is massive 19th century debate on the Corfu saga.

DNA testing would indicate.

First of all you’d need to know what pure Esrog DNA is and then youd have to determine which variations are naturally occuring and which come from other species.