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Date: Mon 29 June 2009
Source: Proplanta [in German, trans. & summ. Mod.DHA, edited] 

New highly contagious strains of potato virus Y

The potato virus Y (PVY) is transmitted by several species of aphids and causes significant losses in yield and quality. Numerous new isolates of this virus have emerged in recent years in Swiss potato crops. Experiments at Agroscope Changins-Waedenswil (ACW) show that the epidemiology of the PVY strains and the sensitivity of potato varieties have changed completely.

The seed potatoes imported into Switzerland are from different regions of Europe. This exchange promotes the dissemination of various genotypes of PVY. The 1st symptoms of tuber necrosis were observed at the beginning of the 1990s and were caused by so-called NTN strains of the virus. One of the new strains was imported from Germany in planting material. The spread of the new isolates in Switzerland made the cultivation of certain very susceptible varieties difficult. At the beginning of the 2000s virologists at ACW discovered Wilga-types of the virus in Swiss crops. These strains can also cause significant damage.

ACW investigated the spread of different PVY strains under field conditions. The experiment showed that the variety Lady Christl has surprisingly good resistance to different isolates of the virus. The new strains are spreading faster and more effectively than the older strains of PVY. This new situation calls for more consistency and attention from seed potato producers. In the future, the sensitivity to these new PVY strains should be taken into account when choosing varieties, and the most sensitive varieties should be avoided.

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[_Potato Virus Y_ (PVY; type member of genus _Potyvirus_) is one of the most damaging potato viruses affecting crop yields and tuber quality. PVY can also affect tomato, capsicum, and some other related species. It is transmitted by aphid vectors in a non-persistent manner, with _Myzus persicae_ being the most efficient vector. PVY can also be spread by mechanical means, plant-to-plant contact, or with infected planting material such as seed tubers. Disease management of PVY and other potato viruses relies largely on clean planting material and use of resistant crop varieties, but reduction of available virus reservoirs and vector numbers may also be useful.

The PVY isolates reported so far have been classified in 3 main strains, PVY-N, PVY-O, and PVY-C, according to leaf symptoms induced on the experimental host _Nicotiana tabacum_. PVY-N isolates have been divided into 2 groups, one causing mild mosaic in most potato cultivars, while the other induces "potato tuber necrotic ring disease" and severe chlorotic mosaic of leaves. It is referred to as PVY-NTN (necrotic group [N] and inducing tuber necrosis [TN]) and is the most virulent strain of PVY. It has been suggested that PVY-NTN isolates may have resulted from natural combination of local strains of PVY-N with PVY-O or PVY-C.

PVY-O isolates induce severe symptoms on potato leaves, such as crinkling, leaf drop, or severe necrotic mosaic. PVY-C isolates causes stipple streak on potato cultivars carrying the Nc resistance gene. Some isolates determined serologically as PVY-O and inducing less severe symptoms in potato than the PVY-N isolates have been called PVY-N-Wilga isolates.

A build up of PVY due to farm-saved seed potatoes has also been reported from the UK earlier this year (2009; see ProMED-mail post 20090212.0628).
Around 40 viruses have been reported to affect potato, and since the crop is vegetatively propagated, many of them may be disseminated in tubers. If virus-infected seed tubers are used, virus populations and numbers of co-infecting species and strains will build up with every crop cycle. This invariably leads to severely reduced plant vigour and a dramatic drop in yield. Even more severe problems may result if viruses in infected seed tubers are moved to a different area where they may combine with local pathogen populations and/or encounter lack of host resistance, as suggested above for Switzerland. The important role that tubers play in virus and viroid spread is recognised by the strict requirements for certified seed potato production in many countries worldwide.

Maps of Switzerland:
<http://www.idsia.ch/wea2003/map_switzerland.gif> and <http://healthmap.org/r/00v2>
 Pictures PVY symptoms on potato:
<http://www.inra.fr/hyp3/images/6034404.jpg>  (leaves), <http://www.umext.maine.edu/images/PVY.jpg>  (leaves), <http://www.inra.fr/hyp3/images/6034401.jpg>  (plant), and <http://www.ascenion.de/fileadmin/ascenion/Technology_Offers/Agro_Technology/Plant_Virus_Resistance.jpg>
(tubers)
PVY symptoms on tomato leaf:
<http://www.avrdc.org/photos/tomato_diseases/PVY_01.jpg>
PVY particles, electron micrograph:
<http://www.ppi-bg.org/i/sn5.jpg>
Links
Information on _Potato virus Y_:
<http://www.inra.fr/hyp3/pathogene/6potviy.htm>,
<http://www.oregon.gov/ODA/CID/PLANT_HEALTH/profilepvy.shtml>  and Information on PVY and other potato viruses:
<http://www.umext.maine.edu/onlinepubs/htmpubs/2492.htm>  and <http://vegetablemdonline.ppath.cornell.edu/factsheets/Virus_Potato.htm>
Review and diagnosis of PVY strains:
<http://www.virologyj.com/content/6/1/48>
PVY taxonomy and description:
<http://www.ncbi.nlm.nih.gov/ICTVdb/ICTVdB/00.057.0.01.001.htm>
Agroscope potato research (in German):
<http://www.acw.admin.ch/themen/00568/02136/02173/index.html?lang=de> . - Mod.DHA]