Covering a small patch of potatoes with insect mesh has highlighted the potential damage leafhopper species might be doing to the UK crop.

Grown under the mesh, the processing variety Markies was almost completely free of the brown necrotic lesions found in the uncovered crop, which independent agronomist and researcher Martyn Cox believes is, at least partly, due to feeding by swarms of leafhoppers.

Leafhoppers are from the same order of insects as aphids and white flies, and, like all Hemiptera, are defined by long, needle-like, piercing-sucking mouthparts that form a beak or stylet.

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Damage in potato crops in other parts of the world, including the US and Europe, is caused by either direct feeding damage or through transmission of viral or phytoplasma infections.

The latter has the potential to be most serious for UK growers, Martyn says.

Found in part of south-eastern Europe and Mediterranean countries, potato stolbur is caused by a bacteria-like phytoplasma – Candidatus Phytoplasma solani, transmitted by leafhoppers.

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Symptoms of stolbur include aerial tubers, reddening and upward rolling of the top leaves, and reduced size of leaves, he says.

More importantly, stolbur-affected plants produce spongy tubers with elevated sucrose levels that will destroy fry colours.

“It’s not unlike zebra chip – you’re looking at very dark fries, rubbery soft potatoes in badly affected crops. It would be a major problem for the processing industry.”

Fortunately, stolbur hasn’t yet been found in the UK. “But if you see very dark fry colours, uncharacteristically severe, then it should be checked,” Martyn stresses.

First concerns

His concern about leafhoppers was originally prompted by an investigation into a stunted patch of Markies, when he discovered a swarm of the insects when parting the crop.

Having already been aware of the damage leafhoppers can do in herbs, he discovered that potatoes in the US can suffer from a condition called hopperburn, which is caused by feeding damage.

Symptoms of hopperburn start with leaves turning yellow around the margins – often resembling a disease or nutritional deficiency, before turning brown at the edges and leaf tips – like the leaves have been burned, and curling slightly.

“There are a couple of schools of thought about how this is caused,” says Martyn. “One is when the leafhoppers feed, they inject a toxic saliva that kills the plant tissue, while the other is that it is just mechanical damage.”

The leaf damage can often let in secondary fungal pathogens, such as botrytis and alternaria, he says.

Feeding damage also shows as a white flecking on upper leaf surfaces, appearing early and continuing all season.

It is characteristic of adult leafhopper feeding on herbs such as mint, lemon balm, basil and rosemary, as well as broad-leaf weeds including nettles, he adds. “It’s easy to find on broad-leaf plants in field margins.”

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Varietal differences

Research has shown there is a strong varietal influence on levels of damage in potatoes. “In the US, researchers have seen up to 62% yield loss in some varieties, while in the same trials other varieties have shown no damage.”

That led to observations in variety trials last season at two sites in Norfolk comparing eight processing varieties. The trials, which will be repeated this season, also showed significant visual varietal differences. “There was a broad correlation in the varieties that were better at both sites,” Martyn says.

While the mechanism for varietal tolerance is not yet known, evidence suggests it could be to do with leaf hairs and glandular trichomes.

“Once we have established differences between varieties, we will look at leaf hair structure to see if it there are any trends.

“That could allow us to understand whether a variety is susceptible by looking at these features rather than needing to test every variety.”

A set of trials carried out by VCS Potatoes, Provenance Potatoes and Blackthorn Arable involving the insect mesh, funded by Cambridge University Potato Growers Research Association (Cupgra) has started to answer some key questions around leafhopper damage, including which species are involved.

Three species

That’s no small task, with more than 20,000 species of leafhoppers globally, and about 300 identified in the UK.

Three were identified by Fera Science in the Cupgra study – a green leafhopper, Empoasca decipiens, and two cream species which have dark markings, Eupteryx aurata and Eupteryx atropunctata.

Typically about 3-4mm long and very mobile, Eupteryx species overwinter locally as eggs, usually in nettles.

“We know they host swap as the year progresses,” Martyn says. “The eggs are laid into the leaf, generally in leaf veins, and hatch out as small nymphs. Egg predation by a parasite can reach high levels.”

Nymphs go through about five moults in about two weeks before developing wings.

In the trial, adult feeding damage first appeared in potato crops soon after crop emergence, with adult populations increasing to massive numbers by mid-July.

“Nymphs appeared in potatoes in the latter part of June in both the trials and in commercial crops,” says Martyn.

“We’ve established they will lay eggs in potatoes. But evidence suggests they get the same level of predation in that crop.”

Monitoring

Adult and nymph populations were monitored in the Fenland trials on two varieties, Taurus and Markies, with a very different picture emerging between the two.

“Adult numbers were broadly the same on both varieties, while nymph numbers were very different,” Martyn says.

“There was around 1,000 adults for every nymph on Taurus, but 175 adults for every nymph on Markies. That would indicate some form of varietal resistance.”

Meshing over an area of Markies showed that feeding damage could be minimised by direct protection, with up to 30% improvement in green leaf area just prior to burn off.

“We didn’t see the brown spotting, which is associated with alternaria under the mesh.”

Unfortunately, the mesh also affected the growth of the potatoes, Martyn notes.

“I think it reduced the wind flow and, because the Markies wanted to grow to 1.3m tall, the mesh ended up pressing the foliage down, bent stems and sclerotinia and blackleg infection took hold.”

That meant yields were lower under the mesh despite the improvements in haulm health, he says.

“Without yield data we cannot say for sure if the visual symptoms are as severe as they appear. Last year was a cooler season, and we have seen severe damage earlier in other years.”

Further work

Improving the technique for covering with mesh will help answer some questions. “That will help us produce more data and answer questions about leaf spotting, necrosis and disease.

“If we can pin down whether there is any yield loss from leafhoppers, even better.”

Control of leafhoppers using insecticides looks difficult. Monitoring with sticky traps one week after spraying the whole field bar a tramline showed no difference in adult catches between the treated and untreated, although feeding damage was reduced for a while, Martyn says.

“At the moment, it is early days – enabling research that is giving the industry information about something that could be a problem.”