Causal agent(s) and transmission
Potato tuber necrotic ringspot disease or PTNRD is mainly caused by particular isolates of PVY, named PVYNTN, belonging to the PVYN strain group.
PVYNTN isolates can be transmitted in the same way as the other PVY strains, by the seed tubers and by numerous aphid species. PTNRD is present in most European countries and is very widespread in areas with warm climates such as Central Europe and the Mediterranean Basin.
The economic importance of PTNRD is mainly related to the presence of necrotic symptoms on tubers which may lead to the downgrading or the rejection of the affected lots.
Extensive work on the characterization of PVY isolates (PVYNTN or others) able to cause potato tuber necrosis is being carried out at an international level (PVY wide network ), chiefly for the prupose of identifying the molecular determinants of the necrosis.
Symptoms on foliage
PVYNTN isolates cause mosaic patterns and crinkling of varying intensity in the foliage, similar to the symptoms caused by PVYO isolates.
With primary infections (i.e. occurring during the current year), other symptoms may be found:
- necrotic spots on the leaves (photo 1);
- vein necrosis mainly seen on the underside of the leaves;
- necrotic streaks on the stems (photo 2) and the petioles or oak-leaf-shaped leaf necrosis (photo 3);
- yellowing with the presence of green rings or particular patterns (“Maritta-type” symptoms: photo 4).
Symptoms on tubers
- brown blisters in the form of arcs, rings or lines, caused by PVYNTN (photo 5);
- necrotic arcs and rings: a symptom often caused by a primary PVYNTN Infection (photos 6 and 7).
The development of PVY infection results from the combination of sources of infection, either internal (volunteers or groundkeepers, seed tubers, weeds) and/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 the susceptibility of the potato cultivar.
Expression of PTNRD after harvesting is also enhanced by an early infection and by warm temperatures during the growing season and during storage.
The use of resistant (or tolerant) cultivars and the use of certified seed potatoes are measures essential to the control of infections due to viral diseases, in particular the development of necrosis caused by necrotic viral isolates in potato crops intended for consumption and processing.
Production of certified seed potato is based on a range of strict measures limiting the infection by viral diseases and involving:
- the use of certified seed tubers resulting from the multiplication of virus-free material in a certification programme (including multiplication systems such as micropropagation in vitro, intensive field inspections and large-scale post-harvest testing);
- 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;
- early roguing of diseased plants and eradication of all sources of inoculum, e.g. weeds, 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 Potato leaf roll virus.
Additional control measures in seed potato production taking into account risks of aphid infestations which are usually higher during warm periods, include:
- early dates of planting;
- early haulm destruction (by chemicals and/or by mechanical means) prior to maturity to limit the infection of daughter tubers.