Viral diseases of potato
In fact, the joint occurrence of PVX and PVY, or either or both of them with PLRV or Potato virus A (PVA), produces much more severe symptoms than separate infection of each.
Since potato is an asexually propagated species, the viruses present in a given plant are passed on to the next clonal generation through the tuber pieces used for multiplication.
However, these symptoms do not always manifest themselves due to interactions between the virus(es) involved, the potato variety and the environment (soil fertility, climate or the age at which the plant is infected, among many other variables).
For this reason, in recent years, serological and molecular detection techniques have been used to diagnose and characterize the viruses affecting the crop to take the most appropriate control measures.
This virus needs 20 to 30 minutes to be acquired and 24 to 48 hours to be transmitted by aphids, since it needs to move inside the insect's digestive tract and come out again through the salivary glands.
Symptoms of PLRV in potato include a characteristic erect growth habit and the classic "inward" rolling of the leaf blades that gives the virus its name.
In addition, chlorosis (i.e., yellowing) or reddening of leaves and necrotic spots along the leaf veins due to phloem cell death may be visualized.
Because the acquisition, transmission and dissemination of PLRV requires a few days, insecticide applications to eliminate insect vectors can be effective as a control measure.
On the other hand, some potato cultivars are hypersensitive to certain variants of the virus and react to infection with apical necrosis, i.e. death of the plant apex.
Cultivars more susceptible to PVS react with severe browning (leaves become redder in color), chlorotic spots that later become necrotic, and even leaf drop (a symptom called defoliation).
PVX, PVS, and PVM are controlled by clonal selection during the multiplication of seed tubers, i.e. by observation, uprooting and discarding of those plants that show symptoms of virosis during their cultivation.
Cultivars more sensitive to this virus may react to infection with apex necrosis, leaf deformations, dwarfing and/or delayed emergence.
The relative impact of both viruses on potato yields is unknown, although it is speculated that the effect of APMV may be more severe on susceptible cultivars.
The most effective control of both viruses is by clonal selection during seed tuber propagation and by uprooting and discarding diseased plants.
The importance of these diseases, which generally occur under cool temperature conditions, depends on the causal virus and the potato cultivar affected.
Insecticide applications to control the vector are not an effective method because they may even lead to an increase in disease incidence as populations of whitefly predatory insects are also affected.
Serological methods such as ELISA and nucleic acid hybridization or PCR (polymerase chain reaction) techniques offer all these characteristics and are currently widely used during clonal selection for seed tuber production.
With current diagnostic and micropropagation technology it is possible to start a program with completely healthy material obtained by tissue culture and its subsequent multiplication through successive generations avoiding or reducing reinfection.
Thus began the commercial production of clean seed tubers in specific regions, an activity highly perfected and technified in many countries of the world.
As a perennial, it is exposed to attack by numerous pathogenic organisms such as fungi, bacteria and Viruses over a long period of time.
In addition, and unlike other major crops, potatoes are reproduced vegetatively, as clones, which guarantees a stable, unalterable multiplication of the original genotype.
Examples of such virus resistance mechanisms and their genetic basis are given below: A cultivar is said to be immune to a given pathogen when, despite predisposing environmental conditions, infection does not occur.
The International Potato Center also has several clones of the andigene subspecies with immunity to PVX, which is supposed to be governed by the same Rxadg gene.
Several genes have been reported that govern non-specific hypersensitivity, i.e., encompassing the entire spectrum of variants of a given virus, following mechanical inoculation.
An example of a hypersensitivity mechanism has been reported for clone "USDA 41956" and its derivatives, which have a dominant gene (called Rx) that can produce local necrotic lesions after infection with PVX.
Antixenosis, on the other hand, is the non-preference or rejection of vectors by certain hosts due to the presence of volatile toxins or repellents produced by those plants.
In contact with atmospheric oxygen, the light-colored, water-soluble exudate contained in the glands changes to an insoluble black material that impedes the aphid's movement until it is completely immobilized.
[16] Using both approaches, several groups of researchers around the world have achieved transgenic potato plants that exhibit resistance to viruses.
It is also possible that it inhibits cell-to-cell movement of PVX, as the capsid protein is an essential cofactor of systemic virus translocation.
It was shown that expression of this protein in transgenic potato plants protected them against a variety of viruses, whether they were inoculated mechanically or by aphid vectors.
