Resistant Varieties

Varieties resistant to PVY have been cultivated in Europe for many years. They already existed before the arrival of necrotic strains of potato virus Y (PVY-N), in the late forties. Among these traditional varieties are: Geudertheimer, Paraguay, Mont-Calme Yellow. At that time, given the severity of necrotic strains, especially on Virginia type tobacco, a search for PVY-N resistance was undertaken. Virgin A mutant genotype (VAM), obtained by X-ray irradiation of Virgin A, showed good resistance to PVY-N strains. Subsequently, other resistant genotypes were created, such as SCR (Aurea), Perevi, Polalta and Havana II C which served as sources of resistance used in many countries.

Breeding programs around the world have introduced PVY resistance into tobacco, using either resistant cultivars of European background (Paraguay, Alta, SCR, Perevi) or VAM. All these sources of resistance seem to be governed by a recessive gene located in the same locus (the va gene), which has different forms of expression as determined from several tests of allelism.

Strategies for selecting resistance to PVY must take into consideration the existence of the American VAM-B strain, and the European or the South American strains, which show comparable virulence to the va gene. Another PVY resistance source has been developed in the USA by androgenesis in vitro from the susceptible cultivar McNair 944. The created variant (NC TG52) possesses a semi-dominant gene named VR, resistant to VAM-B necrosis. It is only effective against a few strains of PVY, particularly the VAM-B strain. Unfortunately, many strains in Europe induce severe necrosis in this genotype limiting the value of this source of resistance in the "old continent".

Studies attempting to use the PVY resistance in Nicotiana africana, which is governed by a dominant gene are underway. This resistance should allow control of all the strains of PVY. For the moment, the introduction of this resistance into Nicotiana tabacum presents major difficulties because of the low pairing frequencies between chromosomes of the two genomes. Other species of Nicotiana are also resistant to PVY and therefore can potentially be used as sources of resistance.