Viral diseases of potato are a group of diseases caused by different types of Viruses that affect potato crops worldwide and, although they do not affect human or animal health since they are viruses that only infect vegetables, they are a source of great economic losses annually. About 28 viruses have been reported infecting potato crops. However, potato virus X (PVX), potato virus Y (PVY), and potato leafroll virus (PLRV) are the most important viruses worldwide. Some others are of economic importance only in some regions. Such is the case of potato virus M (PVM) in some Asian and European countries.
An additional problem is the co-infection of two or more viruses on the same plants. 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. The magnitude of economic losses associated with this synergistic effect depends on the types of viruses that are interacting, their respective races, their interaction with the host potato cultivar, the viral vectors involved, and the environment.
Diseases caused by viruses are one of the main limiting factors of potato cultivation worldwide, not only because of the immediate damage they can cause, but also because their effect is cumulative over time. 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. In the new generation, new viruses of the same or other species may be added to the existing ones, increasing the virus load of these plants. This accumulation of viruses is mainly responsible for the so-called gradual degeneration of potato varieties, the consequences of which are a gradual decrease in the vigor and yield of the crop as well as an increase in quality losses due to a reduction in the possible shelf life of the tubers after harvest.
Most potato viruses can be diagnosed by the presence of characteristic symptoms, such as mosaic patterns on leaves, stunting of plants, and deformations of leaves and tubers. 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.
The following is a description of several of the viral diseases of potato crops, their symptoms and management possibilities.
Main viruses
Potato leafroll virus (PLRV)
Potato leafroll virus (PLRV), also known as "leafroll" or "yellow dwarfism", is a phloem-limited Luteovirus, which is transmitted by aphids, the green peach aphid (Myzus persicae) being one of the most important vectors. 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. The consistency or texture of the leaves also changes, as they become more leathery. Reduced plant height (a symptom called "dwarfism") and net necrosis in tubers complete the repertoire of symptoms shown by potato plants affected by PLRV.
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. The use of virus-free seed tubers is of fundamental importance to achieve healthy crops. During cultivation, removal of plants showing symptoms helps prevent the spread of PLRV. Also, early harvesting of tubers can help prevent infection later in the season. The virus is not spread or transmitted mechanically.
Mosaics
In plant pathology, "mosaic" is a symptom, characteristic of many viruses, in which diseased leaves show undefined dark green sectors alternating with lighter colored sectors, ranging from light green to yellow. Potato leaf mosaic symptoms can be caused by different viruses individually or in combination. Some of them are potato virus X (PVX), potato virus S (PVS) and potato virus M (PVM), as well as PVY and PVA.
Depending on the virus variant and the potato cultivar considered, PVX can decrease tuber yield by more than 10%, making it one of the most important potato viruses after PLRV. It is transmitted by infected tubers and by contact; it is not transmitted by aphids. The characteristic symptom of PVX is mild leaf mosaic (i.e., leaf coloration variations are not accentuated) and it is often latent (i.e., the plant is infected but does not show symptoms). The more virulent variants of potato virus X|PVX virus can cause more severe symptoms, such as leaf curling and necrosis of certain areas of the leaves, as well as mosaic. 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.
PVS is a fairly widespread virus and generally causes mild symptoms. Plants affected by PVS do not show significant yield reduction. It is transmitted by infected tubers, by contact and, in certain variants, also by aphids. PVS infection is usually latent, although some cultivars show a soft mosaic or fainter color bands on the veins when infected with this virus. 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).
PVM is a less widely distributed virus than those mentioned above and little is known about its effects on yield. It is perpetuated by infected tubers and is transmitted by contact and aphids. The virus remains dormant in some cultivars, but in the most susceptible cultivars it causes mild to severe mosaic and leaf deformations. Under certain environmental conditions, symptoms may be more severe, as plants may develop necrosis on petioles and leaf veins.
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. This discarding is effective only when symptoms are evident; it is almost useless in those cases in which the infection is latent. There are cultivars with genetic resistance to PVX.
Andean Potato Mottle Virus and Andean Potato Latent Virus
Andean potato mottle virus (APMV) and Andean potato latent virus (APLV) are two common viral diseases in the potato-growing areas of the Andean plateau of Peru, Bolivia and Argentina. Both are transmitted by contact and by insect vectors. Unlike the previous viruses, the vectors in these two cases are not aphids but beetles. APMV usually produces a mild to severe mottling (irregular arrangement of dark and light areas on leaves). Cultivars more sensitive to this virus may react to infection with apex necrosis, leaf deformations, dwarfing and/or delayed emergence. APLV is usually dormant but often causes chlorosis, or soft mosaics and leaf roughness. APLV is transmitted by the "potato pulp beetle" (Epitrix sp.), while APMV is transmitted by the "green leaf beetle" (Diabrotica sp.). Both viruses are also transmitted by mechanical contact between plants. 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.
Potato mop-top virus
Potato mop-top virus (PMTV), also known as potato stem dwarfism virus, is prevalent in growing regions with cold and humid climates, where it favors the spread of the fungal vector, Spongospora subterranea. Economic losses due to this virus can be very high; cases of up to 25% yield reduction have been reported. Moreover, such losses can be accompanied by reductions in tuber quality, making the tubers unmarketable. Tubers can become infected after planting, directly from the surrounding soil where the fungus vector lives.
The primary symptoms consist of the formation of brown or necrotic, ring-shaped spots on the tuber surface. These spots extend into the tuber as necrotic arcs. Towards the center of these necrotic rings there is usually a rot caused by the fungal vector. Symptoms on the aerial part of the plants are secondary, subsequent to the subterranean infection and can be of three types. Stem dwarfing ("mop-top", origin of the virus name), bright yellow spots (called "aucuba" spots) especially on the lower leaves and pale V-shaped spots on the upper leaves. Occasionally, only some of the stems of a plant are infected while the remaining stems remain healthy and symptomless.
After infection and appearance of symptoms in the aerial part of the plant, secondary symptoms may appear in the tubers, which consist of deformations, deep cracks in the flesh, fine cracks on the surface, and brown ring-shaped spots at the apex of the stolons. The virus survives in the soil within the Spongospora fungus and spreads to other areas within the vector spores.
Regarding the control of this disease, treatment of infected soil with fungicides can prevent infection of a healthy crop by eradicating the virus vector. Discarding diseased plants is an effective control measure in highly susceptible cultivars.
Aucuba leaf
Different viruses cause the same symptom on the leaves of affected potato plants, which is called "aucuba leaf". This symptom consists in the presence of bright yellow spots of different sizes, which may appear with yellowing around the nerves. In some cases the leaflets turn completely yellow. This symptom is called "aucuba leaf" or simply "aucuba" because of its similarity to the normal leaves of the species Aucuba japonica. The consequences of these viruses are a decrease in yield and, in some cases, the quality of the tubers can be drastically affected by the presence of spots on the tubers that can become necrotic.
The viruses that usually produce these symptoms in potato are "alfalfa mosaic virus" (AMV), potato aucuba mosaic virus (PAMV), tobacco ringspot (TRSV), potato black ringspot (PBRSV) and tomato ringspot (TBRV). The importance of these diseases, which generally occur under cool temperature conditions, depends on the causal virus and the potato cultivar affected.
Control consists of discarding plants with symptoms during seed tuber production and by applying chemicals to kill the virus vector, which differs from case to case. Thus, the vectors are aphids in the case of AMV and PAMV, and nematodes in the case of TFISV and TBRV.
Yellowing of potato veins This virosis is quite frequent in some South American countries. Apparently, the causal agent is a virus, not yet characterized, transmitted by the whitefly Trialeurodes vaporariorum. Symptoms become evident immediately after infection, and consist of a bright yellowing of the tertiary leaf veins. As the disease progresses, the secondary veins and leaf lamina turn yellow, sometimes without affecting the primary veins, which remain green. Affected plants have not been reported to show other symptoms such as dwarfing or weakness. However, the yield of affected plants may be reduced by up to 50%.