Keeping on top of virus, pest and disease – British Potato Review
This month Andrew Goodinson looks at some of the viruses and pathogens affecting potato crops, offering key insights into how to achieve the best control possible ...
Based in Herefordshire, Agronomist and Potato Specialist Andrew Goodinson has been working for Hutchinsons for 17 years and looks after nearly 8000 ha of farmland, including the Welsh borders, south Shropshire and Worcester. Most of the potato crops he looks after are destined for the crisping or processing markets. This month he looks at ways to help this year’s crop get the best start possible.
Pragmatic strategies are needed for keeping on top of early blight this season, with the future loss of the only multi-site active blight control active looming large.
But while the loss of mancozeb is a cause for concern, there is no need for blind panic, according to Andrew.
The dates for withdrawal and use-up are yet to be officially announced. It can still be used for this season at least, but as soon as the death sentence has been given, processors may decide to remove it from their preferred product list for 2025, he said.
“Mancozeb has been on the market for 60 years, and as there are now concerns over its effects on humans as well as its environmental profile, we should anticipate it loss, so we need to move on.”
The impact of Alternaria on a field relies on a number of criteria such as varietal susceptibility, plant stress and weather conditions, and isolates may be more virulent owing to different weather and/or climate conditions, he suggests, cautioning that this needs scientific confirmation.
“Extended weather conditions can create more plant stress, and therefore higher stress levels, so we need to think about how we can best manage that stress. We know that the pathogen grows better at higher temperatures, thus the link with climate change has been suggested.
“More research is being done on the performance of potential replacement actives as well as integrated crop management solutions, but because last year’s weather was surprisingly not conducive to A. Alternata, results were often inconclusive.”
At the time of writing, soils were very wet, and 2024 may have a difficult planting season subjecting potato crops to stress, owing to soil conditions restricting root growth and nutrient utilisation. This will make them more susceptible to pathogens such as Alternaria.
If the weather later in the year becomes hot and dry (often in early July) creating heat stress for the crop, later-planted crops which have not achieved full canopy, appear to suffer first.
Of the two species affecting potato crops, A. solani and A. alternata, A. solani comes in earlier, is more aggressive and tends to affect the newer leaves which then become covered by the canopy as the plant grows.
“A.alternata needs more help from factors such as plant stress for it to infect the plant, and the six to eight-day latent period between infection and symptoms appear. However, once it has become established, it is more difficult to control.”
Andrew recommends targeted applications of Alternaria-active fungicides before flowering to high-risk crops, to protect the crop before the disease can get established.
The lesions caused by A. solani are larger, more random, and angular, and the pathogen is more aggressive, and affects the new leaves, it is easier to control.
He warns that there is increasing concern about Alternaria spp isolates showing reduced sensitivity to fungicides belonging to the Quinone outside Inhibitor (QoI) group of fungicides, and both species are also less sensitive to strobilurins and older SDHIs.
“This implies we need to incorporate IPM methods and treatments which help reduce stress and infection, as they are likely to play a key role in preventing disease development. Ensuring plant nutrient levels and soil moisture are adequate for the crop is key, and if it is irrigated, ensure the strategy is suited to the environmental conditions during the first six weeks of crop growth.”
He often recommends strategic use of biostimulants to help reduce crop stress(it’s a bit like ensuring lambs and calves have colostrum soon after birth) and this year is going to look at potential benefits of Scyon as a means to reduce Alternaria susceptibility. It is currently used in cereals at T0 as a biostimulant.
“These strategies are particularly relevant if you are growing susceptible varieties such as Markies, Melody or King Edward” said Andrew.
He also points out that Alternaria can be confused with magnesium deficiency, which can easily be clarified by nutrition tests. Botrytis can also have symptoms at flowering time.
But the news is not all bad, he emphasises, noting that products such as Caligula (fluopyram and prothioconazole) have raised standards above those of difenoconazole.
“When used together as part of a well thought-out IPM strategy, growers should be able to keep on top of this pathogen.”
There has been a lot of discussion about aphids and the devastating effects they can have when they transmit viruses or damage plants by direct feeding damage.
Both seed and ware sectors are affected. Aphid-transmitted viruses are a major reason for crops failing seed certification and in ware crops they cause damage to haulm and tubers affecting yield and quality.
Key factors to of virus management remain location choice for seed, the correct timing of chemistry applications combined with cultural controls, Andrew said. This includes removing volunteers from elsewhere in the rotation and in neighbouring crops as well as other virus host plants such as black nightshade
Aphid control has become much more challenging, particularly in ware crops, because of a lack of effective chemistry.
Andrew said this is partly because historical over-reliance on pyrethroids led to resistance developing in the peach potato aphid (Myzus Persicae) and the grain aphid (Sitobion avenae) and also because the available actives, flonicamid (Teppeki) and acetamiprid (Insyst)have label restrictions for ware.
“Insyst can be used once on ware, but the window for use is very narrow, between tuber initiation and July 31” he said. “The majority suppliers and/or processors do not accept ware crops treated with Teppeki. As a result, many growers still apply pyrethroids, despite warnings to avoid them because of resistance issues.
“Monitoring and managing aphid populations is really important because they vector persistent and non-persistent viruses to potato plants as they feed on them.”
He draws attention to complementary ways of reducing the number of aphids landing in the crop, as a means of reducing the virus load. This includes research evaluating regenerative methods of growing potatoes using straw mulch. These methods are being used in seed growing areas.
“Aphids are attracted by the colour difference between the brown soil and the green crop, so by creating a lower visual contrast of plants we may be able to reduce the attraction of the crop to in-coming aphids.
“This method could prove to be effective in the growth period from emergence until canopy closure in seed crops. In addition, mineral oils can be applied in seed crops in the early stages of crop growth because they can block up the aphid’s stylet and reduce virus transmission”
Andrew goes on to point out that not all aphids landing on the crop are colonising and non-colonising aphids can also create challenges for growers.
“This is because these aphids briefly settle on crops and may introduce virus into the plant before moving on to other crops.”
He explains that persistent viruses are only acquired from infected plants by aphids.
“The aphid picks a persistent virus only after feeding on an infected plant for several hours and it takes several hours more to go through its digestive system before it can then transmit the virus.
“However, after this stage the aphid is infected for life and has the capability of infecting potato plants it subsequently feeds on.”
Persistent viruses include Potato Leaf Roll Virus (PLRV) and Potato VirusX.
“PLRV has a latent period of 24 hours, and one of the challenges in seed potatoes is that it can be difficult to see symptoms. There is some upward leaf rolling and some purple edges of leaves.”
Last year, seed-borne PLRV was mis-diagnosed in a few ware crops as being aphid feeding damage, reports Andrew.
“Infection was random, sometimes affecting a few plants in a field to, occasionally, whole areas. This was down to variation in the seed lot because the virus can be passed into daughter tubers, causing stunted plants, and, owing to high starch accumulation, older leaves become rolled and tough, reducing their ability to photosynthesise.”
As a result, yield losses can be around 5%, depending on the varietal susceptibility. Daughter tubers can develop tuber necrosis, which affects fry quality and tuber shape.
Andrew points out that aphids that do cause problems by transmitting non-persistent also known as mosaic viruses.
“The aphid picks up a non-persistent virus holds the virus in the mouthpart, passing the on, said Andrew.
“Potato VirusY, which is non-persistent, is acquired in seconds when vectored and has no latent period, is spread mainly by the peach potato aphid, with other aphids also able to transmit virus.
“Symptoms of these mosaic viruses can be similar, showing lack of plant vigour, smaller leaves, leaf distortion and mottling, and cracking on the tuber surface.
“As a company, Hutchinsons nationally monitor aphids, using the forecasting tools and water traps to help predict numbers and timing for their entry into crops. This is a real challenge for growers to deal with, as transmission to a potato crop is really fast and often the aphid has transmitted the virus before the insecticide kills it.”
Planting certified seed will help reduce the risk of virus inoculum being present in the daughter tubers, emphasises Andrew.
“When presented for certification, tolerances to virus have been reduced over the past ten years, so the risk of seed-borne virus is higher, resulting in a greater risk of infection and spread.”
Carried by the powdery scab fungus Spongospora subterranea, PMTV can remain in the soil for many years, infecting potato crops when conditions become favourable, such as wet soils.
“It is often spread from seed tubers with powdery scab and infects roots and tubers, and part of the challenge is that if the mother tuber has symptoms, PMTV does not show until the second generation,” said Andrew.
The virus causes spraining and is often associated with internal blemishing leading to rejections.
“It is a virus we rarely see in the west but ensuring that volunteers and potato relatives such as nightshade are promptly removed will help reduce risk of infection, as will ensuring that seed potatoes are clean.”
The pathogen appears to be soil-borne, so when it is already present in the soil, anything causing crop stress is likely to make the plant more susceptible, says Andrew.
“It is a fungal disease of the vascular tissue of the potato and is often seen in small patches.
It can affect one side of the stem and can be identified by cutting the stem at an angle and seeing some vascular browning.
“Once Verticillium has infected the plant, it damages the stem, affecting nutrient uptake, and therefore results in stunting and causes premature plant senescence, creating a yield hit.”
He notes that there are many different sub-species, but in general the fungus occurs in wet, low-lying areas.
“Control is best done by choosing the right location to grow the crop, ensuring all seed is clean and avoiding areas with high nematode populations that can damage the roots.”
Sclerotinia is another soil-borne pathogen with a wide host range, which allows it to survive from season to season, warns Andrew.
Tending to occur where oilseed rape is in the rotation, it can infect potato crops post-flowering.
“It is often confused with black-leg because the stems go soggy, but as the crop dies off you can see the sclerotia” he says.” Effective control can be achieved with the use of fluazinam, which can be applied twice at full canopy as part of the blight control strategy making sure you add in another blight controlling active.”
Planting crops too early before conditions are ideal, can increase the risks of damage from both PCN and free-living nematodes (FLN), warns Andrew.
This is because in poorer planting conditions root systems are reduced leaving less ability for the plant to compensate for any soil pest attacks.
He recommends testing soils for FLN at the same time as PCN, based on previous history.
“If you suspect you have FLN, it is important to test to see if they are carrying tobacco rattle virus, which can cause internal defects, gappy, patchy, stunted crops. Such symptoms are often incorrectly attributed to soil-borne rhizoctonia as the pathogenic FLN species damage roots, opening them to infection.”
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