‘Know your enemies’ – British Potato Review

Andrew Goodinson, Agronomist and Potato specialist at Hutchinsons, offers advice and insights to help growers ensure the best results from their potato crops. Based in Herefordshire, Andrew has been working for Hutchinsons for 18 years and looks after 8000 ha of farmland, including farms in the Welsh borders, south Shropshire and Worcester. Most of the potato crops he looks after are destined for the crisping or processing markets.

This year growers need to strengthen their strategies for aphid control as the incidence of aphid-transmitted viruses such as potato leaf roll virus (PLRV) and PVY are on the increase, especially in seed crops, according to Andrew.

Once a plant is infected, the virus alters its defence mechanisms, affecting the movement of carbohydrates and sugars.

The virus challenge forms part of a general trend in yellows viruses in potato crops.

Potato leaf roll virus can affect tuber numbers, size and quality characteristics, and therefore impact on marketability and profitability. Virus in ware crops causes between 30-80 % yield loss, as well as affecting quality – and fry colours in processing varieties, he warns.

“PLRV vectors settle on potato plants to feed from the phloem and acquire the virus which can take up to four hours to be absorbed. The virus eventually enters the salivary gland and is transmitted through the probe into the leaf, so it takes a few days for aphids to become infective.”

The main aphids involved in transmission of PLRV in potato are the colonising aphids, such as the peach potato aphid (Myzus persicae), the potato aphid (Macrosiphum euphorbiae), and the glasshouse potato aphid (Aulacorthum solani).

Non-persistent viruses, such as mosaic viruses, are vectored by a wide range of non-colonising aphids.

“Transmission is very rapid – between 30 minutes and two hours – and as the virus is held in the mouth parts of the aphid, so just a short probe by the aphid can result in the plant becoming infected.”

“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. This is a real challenge for growers to deal with, as transmission to a potato crop is really fast and there are times when the aphid has already transmitted the virus before the insecticide kills it.

“The main factor for Virus Y infection is location, and the only way to stop this virus is to grow in an area where there are no other potatoes.”

Reasons behind increasing virus incidence are complex and nuanced, but may be partly down to virus evolution, climate change, and a depleted conventional plant protection toolbox which has also been subjected to label changes.

“This implies we need to combine chemicals and cultural controls better, and ensure we are using them correctly to ensure efficacy,” said Andrew, going on to add that a lot of damage is done in the early stages of crop development, but infection can happen later in the season than expected, including at desiccation. It is therefore crucial to keep on top of aphid populations until the haulm is dead.

“Virus in the leaves does not always mean virus in the tubers, so if you are growing a seed crop it is always worthwhile sending leaves for analysis during the growing season and tubers after harvest to accurately diagnose potential virus infection to manage the crop accordingly.”

Monitoring aphid and ladybird populations is key for knowing when, or if, to spray. It is also important to know which aphids are in the crop, because not all of them vector virus. Use of forecasting tools and water traps help predict numbers and timing for their entry into crops.

Moreover, monitoring crops and identifying those that are infected is also key, as roguing infected plants reduces virus inoculum and therefore virus transmission.

Mineral oils, which are only approved for use in seed crops, work by coating the stylet so it becomes more difficult for the aphid to transmit the virus.

“If you opt for applying mineral oil to deter aphids from landing, it is important to stop applying it before tuber initiation. Also, care needs to be taken if the young plants are under stress because if sunlight levels are high, it can scorch the leaves and affect yield,” said Andrew. “If you use it, because it is non-selective, you are also preventing aphid predators and other beneficials from landing on the crop.

“IPM programmes are complicated because of the predator/aphid relationships. You have to have sufficient numbers of aphids for their predators to feed on.”

Flower and grass margins provide habitat for predators, as well as reducing field run-off, but they can also provide a place for aphids to settle and thrive. He warns against treating ware crops with aphicides before the threshold of five aphids per compound leaf is reached.

While there has been widespread resistance to pyrethroids for a number of years, there is hope on the horizon with products offering two new mode of action becoming available. Flupyardifurone, (approved called Sivanto Prime) which works through both contact and ingestion, with knockdown taking seven days has also been approved.

“We also need to look harder at genetics to find new varieties with tolerance to these viruses,” said Andrew.

He goes on to note the importance of undertaking effective testing for home-saved seed for virus.

“You often just take a hundred tubers from a batch of thousands, rather than a properly representative sample, yet the results from testing are used to base a decision on whether or not to plant these potatoes. This is a huge investment on a minute amount of information.

“Planting certified seed will help reduce the risk of virus inoculum being present in the daughter tubers.”

Leafhoppers are piercing and sucking insects and a relatively recent pest, says Andrew.

“At the moment they are more frequent in the east of the country, but they may be more common than we think, as they may not be correctly identified in other areas.”

They insert their piercing-sucking mouthparts into the underside of potato leaves and feed on phloem and xylem, sucking out plant juices, which causes the leaves to appear mottled with ‘hopper burn’ scorch on top of the leaf.

Leafhopper injury reduces production and translocation of photosynthate, and may increase sugar content in the tubers.

The amount of land infested with PCN is increasing, so varietal tolerance and resistance to this pathogen is becoming critical, notes Andrew. “PCN has recently become one of the major limitations to seed potato growing in Scotland as you are not allowed to grow seed on infected soils.”

Tolerant varieties still produce good yields despite PCN infestation, with the downside that the PCN populations continue to thrive. Resistant varieties stop multiplication of the pest, but yields can take a hit if the variety is not tolerant as well.

“This implies that ideally varieties should have both tolerance and resistance to the two species of PCN present in GB, Globodera rostochiensis and G. pallida,” said Andrew.

As there are a number of popular varieties resistant to G. rostochiensis, this has selected for burgeoning populations of G. pallida.

“Breeders are well aware of the problem, and newer varieties with better PCN resistance and tolerance, are coming through the system, and trials held by Hutchinsons have identified some as looking particularly promising.

“These new varieties will widen the processors’ and growers’ choices, whether they are destined for processing or fresh markets.”

The problem is that PCN feed and reproduce on potato roots, causing poor growth, wilting. and can result in early senescence.

“It is always advisable to wait for a couple of days to allow soil conditions to improve if the weather is unsuitable and always ensure seedbed conditions are optimal to give the crop best start and provide good conditions for root development.

“Tolerance is particularly important in years where crops are planted in less than ideal soil conditions leading to crops not having the root system or sufficient biomass to reduce the effect of the pests.”

This is partly because bigger roots are better able to scavenge for nutrients and water, to help the plant reach its maximum potential. Andrew always recommends testing the fields for PCN levels prior to making decisions on which crop to grow, but observes that soil testing needs to become more effective, sub- dividing the field into small plots for testing. “This reduces the chance of missing hotspots.”

More incidence of Alternaria is being recorded in GB, which may be because the pathogen thrives in conditions that stress the potato plants, such as a warm dry period, followed by hot, humid weather.

“Critical times for infection are pre- and post-flowering, and as we are getting such weather patterns more frequently, there is more opportunity for timing to coincide,” he notes.

Under certain weather conditions can the disease progression be mild. “This implies that ensuring sufficient plant nutrient levels and soil moisture is key at this important timing to keep stress to a minimum.” This year, Hutchinsons plans to do potato trials using the biostimulant Scyon, which has given good results in cereals, peas and beans.

The impact of Alternaria on a field relies on a number of criteria such as varietal susceptibility. plant stress and weather conditions.

“We are also getting better at identifying it; previously it was sometimes confused with ozone damage and/or magnesium deficiency.” While no varieties are either resistant or tolerant to the pathogen, there are some particularly susceptible varieties, including some old favourites such as Markies, Melody, King Edward and Vivaldi.

“We know that the pathogen grows better at higher temperatures, thus the link with climate change has been suggested.”

He goes on to explain that the name ‘early blight’ for Alternaria is misleading because Alternaria is a soil-borne fungal pathogen, while Phytophthora infestans (also known as late blight) is an oomycete, which, despite appearing similar, is a completely different species.

“Until now we have been able to control Alternaria with mancozeb, but most blight sprays do not control it.”

Currently available control options include Belanty (Revysol), which is a new generation of triazole; Caligula (fluopyram and prothioconazole); Carial Star/Amphore Plus (difenoconazole and mandipropamid); Signum (boscalid+ pyraclostrobin); Amistar (azoxystrobin); and Narita (difenoconazole).

He notes that while Azoxystrobin (which is a Qol fungicide) and Boscalid (which is a SDH), will offer some control, growers will still need to consider adopting anti-resistance strategies in the absence of mancozeb.

“We need to look at specific Alternaria sprays pre-flowering, ensuring adequate nutrition by tissue and sap testing, managing irrigation. At Hutchinsons we are doing more research on the performance of potential replacement actives as well as integrated crop management solutions.”