Further of Etrog

Purity
In order for a citron to be kosher it must be pure, not grafted nor bred with any other species, therefore only a few traditional varieties are used. In addition, the plantations must be under strict rabbinical supervision.

Genetic research
The Citron varieties traditionally used as Etrog, are the Diamante Citron from Italy, the Greek Citron, the Balady Citron from Israel, the Moroccan and Yemenite Citrons.
A general DNA study was arranged by the world-renowned researcher of the etrog, Prof. Goldschmidt and colleagues, who positively testified 12 famous accessions of citron for purity and being genetically related. As they clarify in their joint publication, this is only referring to the genotypic information which could be changed by breeding for e.g. out cross pollination etc., not about grafting which is not suspected to change anything in the genes.
The Fingered and Florentine Citrons although they are also Citron varieties or maybe hybrids, are not used for the ritual. The Corsican Citron is no longer in use, though it was once used and sacred.

Selection and cultivation
In addition to the above, there are many rabbinical indicators to identify pure etrogs out of possible hybrids. Those traditional specifications were preserved by continues selections accomplished by professional farmers.
The most accepted indicators are as following: 1) a pure etrog has a thick rind, in contrast to its narrow pulp segments which are also almost dry, 2) the outer surface of an etrog fruit is ribbed and warted, and 3) the etrog peduncle is somewhat buried inward; a lemon or different citron hybrid is opposing one or all of the specifications.
A later and not so widely accepted indicator is the orientation of the seed, which should be pointing vertically by an etrog, except if it was strained by its neighbors; by a lemon and hybrids they are positioned horizontally even when there is enough space.
The etrog is typically grown from cuttings that are two to four years old, the tree begins to bear fruit when it is around four years old. If the tree germinates from seeds, it will not fruit for about seven years, and there may be some genetic change to the tree or fruit in the event of seed propagation.

Source: http://en.wikipedia.org/wiki/Etrog
See Also: bunga, toko bunga, bunga papan