A ProMED-mail post
<http://www.promedmail.org>
ProMED-mail is a program of the
International Society for Infectious Diseases <http://www.isid.org>

Date: January 25, 2008
Source: Farmers Guardian [edited]
<http://www.farmersguardian.com/story.asp?sectioncode=19&storycode=15895>

Seed potato growers will need to start their insecticide applications earlier and put together more comprehensive programmes to tackle an increasing threat of virus transmission by aphids, according to leading Scottish advisor, Stuart Wale of SAC [Scottish Agricultural College]. He said that some of the non-persistent viruses -- such as _Potato virus A_ -- are appearing early in the season. "We have witnessed an unexpected increase in virus levels, even before aphids conventionally associated with virus spread have been caught in monitoring traps."

Dr Wale advised growers that they will have to revise the practice of waiting for aphid warnings to be issued, or treating when there are signs of them building up in the crop. "The suggestion is that it is non-colonising aphids, such as cereal aphids, migrating through the potato crop that are feeding on plants and transmitting the viruses," he said.
"The changing physiology of virus transmission suggests that growers will need to start programmes as soon as the crop has emerged this season."

He acknowledged this may mean growers have to spray more often over the season, but that high grade seed producers have to err on the side of caution -- especially on varieties susceptible to virus A. "Tolerance of virus infection in Scottish seed is among the lowest in the world. The incidences we have seen are extremely low, but do give cause for concern.
Growers must act now, to prevent any escalation."

He believes new insecticides available will allow the more comprehensive approach without risking resistance development. Dr Wale advocates that fast aphid knockdown is a key element in preventing virus transmission, along with the potential of residual activity and repellency to stop aphids landing and feeding on the crop.

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[_Potato virus A_ (PVA, also called potato mild mosaic virus, potato virus P; genus _Potyvirus_) infects only potatoes. It is one of the most widespread potato viruses and is found in most potato growing areas. Leaf  symptoms include mild mosaic, slight crinkling, and a rough surface ("rugose") with sunken veins and raised interveinal areas. Affected plants tend to open up because the stems bend outward, and tuber size may be decreased. Severity of symptom expression depends on weather conditions, the potato cultivar, and the strain of PVA. In plants infected with certain strains of _Potato virus X_ or _Potato virus Y_, co-infection with PVA may lead to a synergistic (symbiotically enhanced) increase in symptom severity.

PVA is spread in infected plant material (including tubers) and by vector aphids in a non-persistent manner. While the virus survives only a few minutes in the aphid, if a virus-carrying aphid probes a potato plant, infection is instantaneous. Potato tubers are probably the primary overwintering reservoir of PVA. Disease management of PVA (and other aphid-transmitted potato viruses) includes the use of disease-free seed tubers, insecticides for vector control, and resistant varieties. Additional PVA susceptible varieties exhibiting a hypersensitive response
(necrosis of virus-infected tissue resulting in pathogen elimination) under field conditions can also be used.

Around 40 viruses have been reported to affect potato, and a number of them can be spread by aphids. In addition, since the crop is vegetatively propagated, many pathogens including viruses such as _Potato leafroll virus_, _Potato virus X_ and _Potato virus Y_, as well as _Potato spindle tuber viroid_ are disseminated in tubers. Some viruses, like PVA, may only have a minor effect on yield if they are the only infecting virus species and if a crop is newly infected in the field. However, if virus-infected seed tubers are used, the virus population will gradually build up during every crop cycle, both in number of co-infecting (possibly synergistic) virus species and in amounts of infectious units per species. This invariably leads to severely reduced plant vigour and a dramatic drop in yield. 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 the UK:
<http://www.lib.utexas.edu/maps/europe/united_kingdom.gif>  and
<http://healthmap.org/promed?v=54.5,-2,5>
Pictures
PVA leaf symptoms:
<http://www.inra.fr/hyp3/images/6034372.jpg>  and
<http://www.inra.fr/hyp3/images/6034373.jpg>
Potato virus symptoms, photo gallery:
<http://vegetablemdonline.ppath.cornell.edu/PhotoPages/Potatoes/Viruses/PotVirusPhotoList.htm> 

_Potato spindle tuber viroid_ tuber symptoms:
<http://vegetablemdonline.ppath.cornell.edu/Images/Potatoes/PotatoViruses/PotatoVirusfs7.jpg> 

Links
Story also at:
<http://www.freshplaza.com/news_detail.asp?id=15127>
Information on potato viruses, including PVA:
<http://www.hort.uconn.edu/Ipm/veg/htms/viruspot.htm>,
<http://www.ipm.ucdavis.edu/PMG/r607101411.html>  and
<http://vegetablemdonline.ppath.cornell.edu/factsheets/Virus_Potato.htm>
PVA taxonomy:
<http://www.ncbi.nlm.nih.gov/ICTVdb/ICTVdB/57010056.htm>
List of potato diseases and pathogens:
<http://www.apsnet.org/online/common/names/potato.asp>
SAC:
<http://www.sac.ac.uk/>.  - Mod.DHA]