Have you got to grips with Blue 13 in potato blight programmes yet? Well a fresh wave of virus vectors and a newly-dominant strain of Myzus persicae demand an aphicide strategy rethink, too.
A number of previously ignored aphid species are piling on the pressure from potato viruses A and Y. These new villains have caused up to a quarter of certified seed crops in England to be downgraded and home-saved seed with over 10% infection has suffered serious yield penalties.
The so-called non-persistent viruses hitch a ride between plants on the mouthparts (stylets) of a wide range of aphid species, spreading infection during the briefest probing of leaf tissues as aphids move from plant to plant seeking suitable food.
Jon Pickup at SASA in Edinburgh has shown that the rose-grain aphid and cereal aphid are significant sources of PVY.
Infection can be carried on stylets for 18-21 hours, so infected aphids moving on the prevailing wind could transfer viruses significant distances, if there is no feeding in between to purge their mouthparts.
"The world has changed," says Eric Anderson, senior agronomist with Scottish Agronomy. "Previously, we focused on Myzus persicae and PLRV. Now we need to think about cereal aphids as significant potato virus vectors, too."
Brian Fenton and Gaynor Malloch at SCRI in Scotland have also logged the rapid rise of new o-genotype MACE-resistant Myzus persicae. Unknown before 2006 and from just 43% of the population in 2007, it hit 92% in Scottish Agronomy's Fife trial site last year. That mirrors national data and the o-genotype is now considered endemic in England and Scotland.
The good news is that it is fully susceptible to pyrethroids - the link between MACE and kdr (knock-down resistance) has been broken.
More good news is that this spring's aphid attack should be up to a fortnight later than recent seasons. "The cold start means we should expect less of an impact, because aphids should fly in later," says Stephen Foster of BBSRC's Rothamsted Research. "But it isn't all good news. The winter will have hit predators and parasitoids, too, so when aphid numbers start building, they could do so very quickly."
Allowing for label restrictions, Mr Anderson urges growers to use a neonicotinoid in the first and third aphicide spray to combat PVY. In susceptible varieties, including Desiree, King Edward, Lady Rosetta, Maris Peer, Maris Piper, Pentland Dell, Saturna and Shepody, start control with the first blight spray as plants emerge. In less susceptible varieties start when colonising aphids or cereal aphids first appear in suction traps or in yellow water traps within the crop.
The dominance of pyrethroid-susceptible, MACE-resistant o-genotype Myzus persicae, plus the need to combat other aphid species, means it is essential to include a pyrethroid, he adds. "We have seen a definite additive effect," says Mr Anderson. "In trials we have seen a mix of Hallmark Zeon plus Biscaya, Teppeki, Plenum or Actara, can give almost 100% virus control."
Lab tests monitoring aphid honeydew production have confirmed Biscaya's speedy action, says Bayer's Eileen Bardsley. "Most feeding stopped in 30 minutes and within 60 minutes it had all but finished. That compares with 60-90 minutes for competitor neonicotinoid products actemiprid and thiamethoxam. And it translated into less virus in field trials, where Biscaya at treatment one and three gave nearly 0% PVA and PVY, as compared with 6% untreated."
Syngenta's Michael Tait advocates Actara use early to protect new growth, where persistence of up to three weeks was seen in trials, thanks to the aphicicde's high water solubility. "The speed of kill is on a par with pyrethroids," he comments. Lab studies also show Hallmark Zeon physically repels aphids, which seemed to be confirmed in field trials last year when it helped mixes outperform straight thiacloprid or flonicamid, he adds.
Aphid control in potatoes
- Beware PVY and PVA from non-colonising aphids
- Neonicotinoid aphicides work well, but should be used in tank mixes with pyrethroids
- Pyrethroids kill dominant o-genotype MACE-resistant aphids
- Cold winter means more normal first aphid attack, numbers likely to be lower