Causal agent(s) and transmission
Potato virus Y (PVY, type-member of the genus Potyvirus) is one of the most economically serious plant viruses. It is currently recognised as the most damaging potato virus in the world for seed and processed potatoes. PVY is transmitted by at least 70 aphid species in a non-persistent manner. Acquisition and inoculation periods are short (around one minute) and retention of the virus in the aphid vector usually lasts no more than 1 to 2 hours. PVY can infect a very wide range of cultivated and weed plant species, belonging mainly to the family Solanaceae (tomato, tobacco, pepper, etc.).
The sources of PVY inoculum resulting in the infection of potato crops are usually infected seed tubers or volunteers within the field but virus reservoirs may also be present in neighbouring fields, gardens or the adjacent environment, either on infected potatoes or on other host plants (tomato, tobacco, weeds such as nightshades).
PVY can cause yield reduction up to 50%, and even 80% on susceptible cultivars or in the case of co-infection with other viruses.
Symptoms on foliage
Symptoms caused by PVY vary widely with the combination of the virus strain, the potato cultivar, the environmental conditions and the type of infection (either primary i.e. current- season infection, or secondary i.e tuber-borne infection).
Field infection during the current-season with PVYO isolates (primary infection) results in symptoms of vein necrosis and necrotic spots on the underside of the leaflets (photo 1). The leaves become brittle and dry, but remain attached to the plant (so-called leaf-drop symptom). Primary infection can also produce a distorting mosaic symptom and crinkling, often confined to one stem or one part of the plant.
Secondary infection, i.e. infection resulting from tubers contaminated from the previous year, produces more severe symptoms, which vary widely with the infected cultivar. One of the three following types can usually be observed:
- crinkling : leaf deformation (crinkling) and size reduction with shiny appearance and mottling associated with pendulous growth habit and stunting of the plants (photos 2 to 6);
- rugose mosaic : severe mosaic pattern and leaflets reduced in size, with large necrotic spots on the veins, dwarfing and severe plant deformation (photos 7 and 8);
- mosaic pattern or mottling : alternating patches of light green and dark green areas without any leaf distortion, more or less visible depending on the potato cultivar and the climate (usually more visible in overcast weather) (photos 9 to 11).
PVYN isolates cause faint mosaic patterns in plants infected during the current season, while infection in the previous year (infected mother-tubers) gives more or less marked or mild mosaic symptoms.
PVYN-W isolates induce faint mild mosaic symptoms (mainly in the case of primary infection) with some vein deepening (photo 12).
The development of PVY infection results from the combination of different sources of infection, either internal (volunteers, seed tubers, weeds) or external (nearby potato crops, gardens, weeds such as Solanum dulcamara or Datura spp.), the significance of flights of aphids during the growing season (related to climatic conditions and nearby crops), the level of control measures taken in seed potato production (roguing, crop management such as weed control, crop protection, haulm destruction or field and lot inspection) and finally the level of the susceptibility of the grown potato cultivar.
It is essential to use certified seed potatoes to limit the infection of potato crops by viral diseases, particularly PVY. Production of certified seed potato is formally based on a range of strict measures designed to limit the infection by viral diseases. They include:
- the use of certified seed tubers resulting from the multiplication of virus-free material in a certification programme (including multiplication systems such as in vitro micropropagation, intensive field inspections and large-scale post-harvest testing);
- the production in a favourable environment in isolated areas with low virus/vector pressure and in fields away from household gardens or away from fields used for the production of ware/processed potatoes;
- the early roguing of diseased plants and eradication of all sources of inoculum, e.g. weeds or volunteer potatoes to inhibit the spread of viruses within the field;
- the treatment with mineral oils to reduce the transmission of non-persistent viruses such as PVY. Insecticides are only effective against persistent viruses such as the Potato leaf roll virus;
- the use of potato cultivars with a good level of resistance to PVY may be useful in areas with favourable conditions for vectors and virus development.
Additional control measures in seed potato production taking into account risks of aphid infestations that are usually higher during warm periods, include:
- early planting to avoid the flights of aphids in summer;
- early haulm destruction (by chemicals and/or mechanical means) prior to maturity to limit the infection of daughter tubers.
PVY strains are extremely variable and can be classified according to their biological, serological and molecular properties. Three main strains of PVY are usually distinguished:
- PVYO isolates, ordinary strains (old), causing chlorosis symptoms in tobacco and in potatoes with obvious mosaic and leaf curl symptoms and losses in yield
of around 50%;
- PVYN isolates, more recent strains (new), which induce necrosis in the veins of tobacco leaves and, in the potato, mosaic symptoms with usually slightly lower yield (25-30%);
- PVYC isolates (with serological properties similar to the PVYO isolates serological and also causing chlorosis symptoms in tobacco which overcome certain resistance genes in the potato).
More recently, new variants of PVY virus have been identified in the PVYN strain group:
- PVYN-W variants, characterised as isolates of the PVYO strain group with serological tools but with biological properties normally associated with strains from the PVYN strain group;
- PVYNTN variant capable of inducing potato tuber necrotic ringspot disease (or PTNRD).
The emergence of PVY variants (PVYNTN, PVYN-W) has resulted from genetic recombination events between isolates of the PVYN and PVYO strain groups.
This capacity for genetic recombination allows for the extensive variability of the PVY virus and is associated with an increased risk of new emerging isolates with increased pathogenicity (aggressivity and virulence).