Virus Resistance in Potato Clones

Symptoms caused by potato viruses include spraing, potato tuber necrotic ringspot disease, net necrosis, and deformed tubers.

PLRV is probably the most damaging and widespread virus of potato.

PVY is next in importance after PLRV.

PVA occurs worldwide and causes symptoms ranging from very mild to fairly severe. PVV is relatively restricted in its distribution and tends to cause few symptoms.

PVX occurs worldwide and tends to have mild symptoms.

Comprehensive ER has obvious advantages over specific HR for breeding purposes because it is effective against several strains of a virus or even several viruses, providing broader protection compared to specific HR.

Relatively few cultivars are known to have ER to PVX and PVY because ER genes are generally much more recent introductions to the S. tuberosum gene pool and were introduced from relatively few sources, and S. stoloniferum, the source of some ER genes, does not intercross freely with S. tuberosum.

Ry genes are possibly associated with wild characteristics, and relatively few U.K.-bred cultivars have an Ry gene because a higher priority is given to cultivars resistant to PVY in the countries where Ry cultivars have been produced, and possibly an association of Ry genes with wild characteristics.

It is difficult and time-consuming to select ER when HR is also present in the population because expensive and laborious field trials are necessary to assess the resistance to infection, and ER genes may dissipate in outcrossing and are difficult to accurately assess for selection.

Resistance to virus accumulation is the ability of plants to limit the spread and multiplication of viruses, and it is useful for reducing virus spread within the crop because virus is less likely to be acquired and transmitted to other plants by aphids from plants with this type of resistance.

Pathogen-derived resistance involves genetic engineering of plants to resist viruses by introducing viral genes or proteins. An example is the transformation with viral coat protein genes, which can work against virus accumulation. Another example is the coat protein-mediated resistance (CPMR) to potato viruses.

The different forms of resistance to PLRV accumulation include single dominant major gene control in S. chacoense, resistance associated with limited virus spread in certain diploid potato clones, and resistance resulting from impaired movement of the virus from sieve elements to companion cells in the external phloem bundles.

Tolerant cultivars may be preferred for reasons like yield, quality, and other disease resistance when resistant cultivars are limited. However, there are risks of virus spread from infected symptomless stocks, introducing soil-borne viruses to uncontaminated sites, and infecting new virus-free sites if the vector nematodes are present.

Resistance to virus movement refers to mechanisms that hinder the spread of viruses within plants. An example is the resistance to PLRV in S. tuberosum clones, which may result from impaired virus movement from sieve elements to companion cells in the external phloem bundles.

The available host resistance includes HR and ER to various potato viruses in breeding lines, wild species, and cultivars. Some resistances have been used effectively in cultivars for many years. Additionally, there is resistance to PLRV infection, accumulation, or movement in a few cultivars and, to a greater extent, in breeding lines and wild species.

The major viruses that affect potato are Potato leafroll virus (PLRV), Potato virus Y (PVY), Potato virus X (PVX), Potato virus S (PVS), Potato virus M (PVM), Tobacco rattle virus (TRV), Potato mop-top virus (PMTV), and Potato spindle-tuber viroid (PSTVd). PLRV is transmitted by aphids in a persistent manner, PVY by aphids in a nonpersistent manner, and PVX is transmitted by contact. PSTVd can be transmitted by aphids if encapsidated by PLRV.

Host resistance to viruses includes extreme resistance (ER) and hypersensitive resistance (HR), which can prevent virus multiplication or cause cell death and necrosis respectively. Infection resistance can be manifested as resistance or unattractiveness to the vector, and resistance to virus accumulation allows the virus to reach only a relatively low concentration in the plant.

The need for virus resistance is great due to the economic losses and costly control measures associated with viral infections. Resistant cultivars are the most economic and environmentally acceptable solution to this need.

Different types of host responses to virus infection include extreme resistance (ER), hypersensitive resistance (HR), infection resistance, and resistance to virus accumulation.

Resistance gene nomenclature indicates the type and origin of resistance and the virus or strain resisted. It follows the conventions described in Solomon-Blackburn & Barker (2001) for resistance gene nomenclature.

Tolerance is the ability of a plant to exhibit little or no symptoms in the presence of disease, but is not synonymous with resistance.

Host resistance is the ability of a plant to limit the spread of a pathogen.

Sources of host resistance in S. tuberosum include existing cultivars, improved breeders' clones, and resistance genes in wild species.

The early resistance breeding programmes in the 1930s focused on the characterization of numerous resistance genes and phenotypes in Solanum species.

HR and ER genes are effective, quite durable, and simply inherited, making them a focus for breeding.