Potatoes and TPP - An Update

The use of translucent mesh crop covers has proved very effective in controlling the Tomato/Potato Psyllid (TPP) and an unexpected bonus was that it reduced potato blight dramatically. Further work has shown that it appears the UV light blocking effect of mesh is what is controlling the blight and also suppresses TPP.

In 2014 Rural Delivery ran a story about the use of mesh covers to control TPP on potatoes. In that, Dr Charles (Merf) Merfield, head of the BHU Future Farming Centre based near Lincoln, described how the arrival of the psyllid in NZ in 2006 posed a serious problem for growers. The TPP will attack everything in the Solanaceae family –  tomatoes, potatoes, capsicums, aubergines, chillies, peppers and tamarillos and it will also host on most of the Solanaceae weeds. 

Consequently it will survive in an environment even when there are no valuable crops present. In addition it carries a bacterium Candidatus Liberibacter solanacearum (CLso or ‘Liberibacter’), that causes “zebra chip” discolouration when potatoes are processed into crisps and chips, and can cause significant crop damage, yield losses and downgrading of fruit.  TPP can also kill tamarillo trees.

In 2006 potato growers were thrown into disarray. Up until then their IPM strategy had provided good control of pests without the need for much in the way of sprays. However, the IPM methods did not work on the TPP and the only option was to spray thoroughly and frequently.

In 2012 Merf began looking at a non-chemical control method – the use of a mesh cover to prevent the psyllid from reaching potato plants. In small-scale trials it proved very effective and gave an unexpected bonus – it seemed to reduce potato blight. Rural Delivery reported on those results in 2014.

Merf says that the difference between the covered and non-covered plants was like chalk and cheese. “The plants outside the mesh were dead with blight but the plants underneath the mesh were green, so we did a second trial focusing in on potential causes – the obvious suspects of temperature, humidity and possibly of the mesh being a barrier to spore transmission,” he says. “However, it wasn’t any of these things. Both sets of plants had had the same number of spores and the same number of Smith periods* so we came to the conclusion that it must be a spectral filter effect – that's where in the likes of a polytunnel the spectrum of light reaching the plants is modified, and this can have some really quite profound effects.”

 [* A Smith period is the combination of relative humidity and temperature required for late blight to become a problem.]

In a follow-on trial he looked at a number of covers comprising both meshes and polytunnel film to see what effect changing the amount of UV light hitting the crop would have on potato blight. There was a strong correlation between UV and blight (r=0.7), and while correlation does not mean causation it’s a fair old smoking gun, according to Merf. “The other thing we noticed during this trial was that we seemed to be getting a large difference in the psyllid yellows, which is the kind of visual indication that a crop is being infected by psyllids,” he says. “We got an almost equally strong correlation between reduced UV and suppressed psyllid yellows.”

Potato blight is caused by two different species – “early blight” is Alternaria solani and “late blight” is caused by Phytophthora infestans. In this trial it wasn’t clear whether one or both organisms were present and so which was being suppressed but there was a remarkable difference between the high and low UV treatments – low UV treatment looking green and healthy with a little psyllid damage a few blight spots, and the high UV looking very sick and damaged.

Those trials used mesh, and so some sunlight will be filtered by the plastic of the mesh and some would go directly through the holes. “So we also included a pair of polytunnel plastic sheets that were purpose designed to have either a high level of UV transmission or a low level, virtually zero,” says Merf. “We also had two more meshes that were also designed as kind of plus and minus UV, and these gave us the wide spread of UV light levels from practically zero and to high UV, and the results showed that the less UV the less disease, so clearly there was something going on.”

Obviously you can't put plastic sheets on a crop of potatoes like you can with mesh because you will kill them or cook them. This was a scientific investigation whereas the first two trials were simulation of real-world potato production, but the indication was that the mesh was reducing UV light, which reduced both the amount of blight and the amount of psyllid damage on the plants. Even if you got the odd psyllid under the mesh it wouldn’t spread.”

“It appears that psyllids require UV light for some aspect of their lifecycle, and certainly plant pest insects have two main light frequencies they respond to. There's the yellow-green band, and UVA, the lowest level of UV that's close to violet light. By shutting off that UV it appears that they can’t behave normally.”

In a more recent experiment that is still ongoing Merf is using UV fly traps and the psyllids are greatly attracted to them. Conversely reducing UV light appears to inhibit some aspect of their behaviour. So, what are the implications for the potato industry? Can they get good crops without having to use lashings of agrichemicals?

Last year Merf and his team did a large field trial that compared the use of mesh with the industry’s standard chemical use. The results were jaw-dropping, says Merf. “Overall there was a 12% increase in potato yield, but that was everything from marble-sized up. In terms of marketable tubers over 60g (golf ball sized) there was a 24% increase in yield. In terms of supermarket potatoes of 125g (tennis ball sized) or more there was a 60% increase in yield. “In this trial the mesh was dug in and so there were essentially no psyllids in the crop whereas in the industry standard chemical treatment plot using insecticide and fungicide every week there were thousands of psyllids.”

“The total yield I got from the best mesh was 94 tonnes per hectare, and the Canterbury average is about 60 tonnes. Plant & Food research calculate the theoretical maximum is 90 tonnes per hectare so it beat that! However, being realistic it was on a fresh site where no potatoes had been grown before and soil conditions were perfect, so it would be hard for the average grower to match those conditions.”

Merf points out that the mesh acts as a sort of tunnel house and protects the growing plants from wind as well as insects and fungi. Growers can simply plant and cover immediately. A pre-emergence herbicide spray can be applied through the mesh, as can irrigation. Potato plants are strong enough to lift the mesh as they grow but it is important to leave enough slack to allow for this at planting time. The mesh is reusable and in Europe has as 10 year guarantee. For New Zealand conditions it may need to have greater UV resistance built in, and this will give added protection to the crops as well.

Merf is quick to point out that he has shown correlation but not causation. He needs to find out whether it is indeed UV light that is controlling both species of blight or whether there is any other cause, such as temperature. Unfortunately he is having some difficulty in raising the $50,000 needed, which is strange because the potential advantages to potato growers, not only in New Zealand, but globally are huge.

“I got some numbers from key potato industry consulting and supply firms and found that the cost of using mesh is about $1,000 less per hectare than using agrichemicals. The mesh has to be laid and later removed but using it avoids the time and cost of regular spraying,” says Merf.

“Taking into account cost savings and better yields there is was profit increase in the field trial of between 27% and 75%, depending on whether you are getting low, medium or high yields. So it’s cheaper, more effective, gives a higher yield and greater profit, and you can stop spraying agrichemicals. Spray-free potatoes should command a premium.”

Is there potential for using mesh technology on other crops? Almost certainly yes, says Merf. “We’ve demonstrated the potential here and we need to be trying this out on a whole bunch of crops to see what the effects on yield, chemical reduction and profits are,” he says. “To me, field tomatoes really are a no-brainer and there’s every reason to expect good results with brassicas and other crops where protection from insects and blights would be of benefit.”