Potato-infecting viruses cause significant damage worldwide and represent a significant threat to seed potato industries. The incidence of virus in seed potatoes can have a significant impact on crop quality (both seed and ware). Virus infection can result in seed crops not meeting the official standard for a class (downgrading) or even for certification as seed potatoes (failure).  As the cultivation of potatoes involves vegetative (asexual) reproduction, virus infection in the parent tuber is generally passed on to the next generation.  Virus free crops are at risk from transmission of virus from sources of infection outwith the planted crop.  The greatest risk is presented by virus infections in neighbouring potato crops, in groundkeepers originating from previous potato crops that have continued to grow in the soil, and from virus reservoirs in the environment, e.g. weeds.
Work carried out at SASA using data from the Scottish Seed Potato Classification Scheme has shown that crops grown from a parent crop in which symptoms of infection had been seen at the previous year’s classification inspections have a far greater likelihood of exhibiting virus symptoms than crops grown from a parent crop in which no virus had been seen during inspection.  For example, over the period 2009-2011, whilst virus symptoms were observed in 16% of the crops grown, the virus incidence was 52% for stocks grown from parental material, and 13% for crops grown from parent stock in which no symptoms had been visible.  These data indicate a four-fold difference in the likelihood of mosaic being seen in a daughter crop depending upon whether virus had been observed in the parent crop.
The location where crops are grown has an effect on the likelihood of a crop acquiring infection, with the probability increasing when crops are grown in areas where more commercial stocks are in cultivation.  The area of ware potato crops is likely to have a more significant effect than the area of seed crops.
Potato viruses are transmitted by a number of vectors.  Whilst over 75% of the virus infections seen in Scottish seed potato crops are transmitted by aphids (e.g. Potato Leaf Roll Virus – PLRV, potyviruses such as Potato Viruses Y, A and V), other viruses may be transmitted by nematodes (e.g. Tobacco Rattle Virus - TRV), fungi (Potato Mop Top Virus - PMTV) or be transmitted by physical contact (e.g. Potato Virus X).  Under Scottish field conditions the symptoms of virus infection are not usually seen during the growing season in which the transmission takes place.
Aphid transmitted potato viruses may be transmitted in a persistent (e.g. PLRV) or a non-persistent manner (e.g. Potato Viruses Y, A and V).  Persistently transmitted potato viruses infect the vector aphid for its lifetime and any plants on which such an aphid then feeds will be at risk of acquiring the virus.  Non-persistently transmitted potato viruses can only be transmitted immediately after aphids have fed on an infected plant.  Non-colonising aphid species, such as cereal aphids, that do not use potato as a host but alight on potato plants and probe the leaves, can transmit these viruses.
Details of the virus tolerances for classified seed potatoes in Scotland are set out in the SPCS in Scotland leaflet.
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