Potato Diseases |
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Potato Virus Y Strains
Background
Potato Virus Y (PVY) is a monopartite, single stranded RNA virus that infects mainly Solanaceous plants including, potato, tomato, pepper, tobacco and eggplant. There are multiple strains of the virus including the common strain, PVYO, which causes mosaic symptoms in most hosts. The necrotic strains PVYN and PVYNTN both cause a veinal necrosis on tobacco; PVYNTN also causes a tuber necrosis in certain potato varieties. This necrosis is referred to as potato tubers necrotic ringspot disease (PTNRD). In general, the necrotic strains tend to cause a more mild mosaic symptom than PVYO in potato. The distinction among strains is imprecise because recombination among PVY strains is common, i.e. the recombinant isolates contain genome information from both common and necrotic strains. Recent surveys indicate that recombinant isolates are not uncommon in many potato production areas.
PVYN is common in Europe and has been reported from other countries around the world including Canada. PVYN was first reported in the Pacific Northwest of the U.S. in 2002. Subsequently it has been identified in several other regions of the northern U.S. and several Canadian provinces. In response to the first report of PVYN in Canada in the early 1990's, Canada and the U.S. implemented a PVYN Management Plan with the objective of controlling the spread of PVYN. In light of reports of PVYN in recent years and the finding of potato mop top virus (PMTV) in 2002, Canada and the U.S. revised and expand the PVYN management Plan to include tobacco rattle virus (TRV) and PMTV. All three viruses are capable of causing tuber necrosis in potato. PVYN is classified as a non-quarantine regulated pest under USDA-APHIS guidelines. The Canada/US-Management Plan for Potato Viruses that Cause Tuber Necrosis includes management methods for PVY, TRV, and PMTV.
Disease Spread
PVY is spread naturally in a non-persistent manner by many aphid species, and can be mechanically transmitted. Virus is transmitted when an aphid initially probes the plants. These brief "taste testing" probes that last from several seconds to several minutes are long enough to both acquire virus from infected tissue and to inoculate healthy plants if the aphid has previously fed on an infected plant. Due to the short durations required for virus acquisition and inoculation, this type of disease spread is difficult to control. Insecticides may help to reduce aphid populations within the infected crop, but aphids entering the crop still have enough time to contribute to the spread of virus, because the insecticide action is not rapid enough to kill a virus-carrying aphid alighting on the crop prior to virus transmission.
Diagnosis
Detection and diagnosis of PVY strains is difficult because there is not currently one method that will separate all strains into their respective types. Serology can be used to distinguish between PVYO and PVYN, but does not separate N from NTN strains. A tobacco bioassay remains the best method to identify necrotic isolates, but a tuber grow-out on sensitive potato cultivars under standard conditions is needed to confirm NTN, because development of tuber necrotic symptoms has been shown to be affected by environment and cultivar. Finally, many primer sets have been developed for use in RT-PCR tests, but they may not detect all possible strains.
Symptoms
PVYO causes a mosaic pattern in the leaves of infected plants. This mosaic is seen as a green and yellow variation, but can also be expressed as a roughness in the leaves. Yield reduction is the main effect of PVYO is as it does not affect tuber quality. PVY-necrotic strains can also cause a mosaic in the leaves, but experience has shown that most of the time it is a milder symptom than with PVYO. However, the tuber symptoms produced by necrotic strains introduce a quality defect to the tubers that is not easily remedied. Some varieties seldom produce distinct PVYO symptoms in the foliage. These reduced-symptom varieties can be difficult for seed growers to rogue (remove diseased plants) in order to reduce the level of virus inoculum in a field. As a result, PVY levels can increase, affecting whole fields or areas. The symptoms shown (select link below) are primary symptoms (healthy plants infected with PVY) and are characterized by 'yellow flecking' in the leaves. As evident in the figures, symptoms in one variety infected with different strains range from none to the characteristic yellow flecking.
Control
Use of certified seed potatoes that are low in virus levels provides the best method currently available for control of PVY strains in the potato industry. Knowledge of susceptibility and symptom expression in the varieties grown is necessary for effective use of cultural practices to reduce virus levels.