Crop Advice Directory: Potatoes

Name

Potatoes

Subtitle

Solanum tuberosum

Description

Potatoes are an important crop for Scotland, and it has a reputation for producing good seed potatoes. The total crop area in 2011 was 13,305 hecatres of seed potatoes and 17,768 of ware potatoes. Scotland is proud of its reputation as a producer of high quality seed potatoes. As such, many resources are spent in maintaining this reputation. Scotland's climate is ideal for the production of potatoes. Cool and wet seasons mean that yields can be high, whilst keeping aphid populations down. Aphids are an important pest which can transmit potato viruses.

Weather conditions are important for all crops, but more so for potatoes. Dry conditions are required at planting time (usually April for main crops, earlier in February for early varieties). Rainfall is important to bulk up tubers, but warm and humid weather also leads to problems with the disease late blight.

Dry hot weather can also cause problems at bulking up, but irrigation can be used to alleviate this. However, dry warm weather in the summer may lead to increases in aphids. At harvest, wet weather can severely hamper lifting crops and early frosts can also cause damage.

Once lifted and in store, sophisticated store climate management can ensure good quality seed and ware is maintained over the winter. Problems may occur if potatoes are transported via road or sea in frosty conditions, whilst the need for washed high quality ware by supermarkets leads to some challenges in keeping blemish diseases at bay, since washing, bagging and placing tubers in warm supermarket shelves provides ideal conditions for some blemish and rot diseases to develop.

Crop Diary

April: Forgotten potato dumps
Check old potato dumps and prevent any growth on them. Most outbreaks of potato blight can be traced back to someone's long forgotten potato dump. For low risk crops, the first blight fungicide should be applied at the high-risk warning or when the leaves begin to meet along the drills, whichever is sooner. A key factor in successfully controlling blight with fungicides is to start programmes early enough. In some situations, fungicide needs to be applied before the crop meets along the rows, e.g. very susceptible varieties, locations that experience suggests are prone to early infection, crops grown from seed stocks that contained blighted tubers.

May: Aphids, blight protection
Virus levels in Scottish seed potatoes continue to remain low as a consequence of effective virus management over the last few seasons. To maintain this low level of virus in seed potatoes a balanced management programme for the prevention of virus transmission by aphids is required, that takes into account several factors which are outlined below. Aphids pick up and transmit viruses through their feeding. Potato dumps can be a source of virus so it is essential that dump management is undertaken to prevent a local source of virus and blight being present.

Targeted use of aphicides to control aphids on seed potato crops is recommended to minimise the transmission of virus. This does not mean blanket spraying of crops from crop emergence onwards, growers should be going out into their seed crops and actively looking for aphids and only applying treatments if aphids are present. Aphid control alerts are now no longer issued by SASA and the SPCS, so it is down to the grower to determine when the aphid control programme should begin.
Potato aphid (Macrosiphum euphorbiae) has been caught in the aphid suction traps at Edinburgh and Dundee – no doubt making the most of the warm, settled weather.

Blanket treatments with aphicides is also discouraged because of the threat of aphicide resistance in the peach-potato aphid (Myzus persicae). In the recent past, aphicide resistant aphids have been found in Scottish potato and vegetable crops, so choice of aphicide is key in minimising the risk of resistant aphids arising on crops and consequently transmitting virus.

Blight protection

Fungicide protection can be severely tested during rapid growth of the haulm because of the considerable new growth produced between fungicide applications. Protecting new growth is obviously more difficult when the haulm is growing particularly quickly. Rapid growth will be the case when temperatures are high and soil moisture is not limiting during the period of rapid haulm growth. Also, haulm growth will be more rapid for some varieties compared with others. The rate of production of new leaves and growth of existing leaves will be greater for any crops planted later.

The main choice as far as fungicides are concerned is between using systemic fungicides and non-systemic fungicides. Trials continue to demonstrate the benefit of applications of systemic fungicides. The use of systemic products should ideally start when the leaves are meeting along the drills, i.e. when there is plenty of haulm to intercept the spray. Keep intervals appropriate to the risk.
Trials have demonstrated that a robust programme applied during rapid haulm growth can make a substantial difference not only when the crop is severely challenged by blight during rapid haulm growth but also when blight risk is high during the canopy stable phase of growth.

The threat from blight each year is mainly determined by the prevalence of weather conditions suitable for its spread and development. Assuming weather conditions favour the development of the pathogen the current dominance of the more aggressive genotypes of the blight pathogen means that blight threats are higher compared with a few years ago. The recent mild weather has triggered High risk weather for late blight in some regions, and in these regions, you may still be looking at little haulm growth, but the risk of infection is still there.

1. Blight outbreaks are likely to occur earlier. This is not because outbreaks are being started by soil-borne oospores but because more aggressive genotypes of the pathogen produce spores earlier and in greater numbers. Current evidence suggests that the primary sources of inoculum remain the same, i.e. outgrade piles, groundkeepers and seed tubers.

2. There is likely to be less scope to extend spray intervals. If there’s any gap in fungicide protection then this is more likely to be exposed than previously. Fungicide programmes must be started early enough and spray intervals must be short enough for the risk conditions. The shorter latent period and ability to produce spores more quickly means that when blight develops it will do so more quickly. Growers should adhere to a maximum 7-day interval unless there are periods of extended low risk. 3. For some varieties recent trials have confirmed that resistance ratings obtained when tested using the new, more aggressive strain of blight 13_A2 were lower than the published ratings. There is no doubt that 13_A2 is at least partly responsible for changes in varietal resistance ratings. If growers are relying on cultivar resistance as part of the blight control programme then they should refer to the most recent ratings, derived from tests in the previous three years using 13_A2.

For optimum blight control it is not sufficient simply to use the most effective products because the decision over which fungicide to apply when can be critical. In recent trials blight was best controlled where the crop received the most effective fungicide, or combination, during periods of greatest risk of infection. At times curative activity and/or tuber protection are required in addition to good protection against foliar blight. For optimum control the blight fungicide programme should be based on a pro-active approach, using the most appropriate fungicide product for the growth stage and the challenge posed by blight.

June: Aphids
Check crops for aphids which may have begun to take to the wing and fly into crops during the warm weather. Peach-potato aphids (Myzus persicae) have been flying into crops, but cool, unsettled weather will slow them down. Growers should be checking seed potato crops regularly for the presence of aphids, and consider beginning their aphid control programme if aphids are present.

July: Aphids, blight
Check crops for aphids and consider treatment with an aphicide if aphids are found. For seed crops aphid control is essential to minimise the spread of aphid-borne viruses such as leaf roll and severe mosaic. If aphids are being found on seed crops then the aphid management programme should begin immediately. On ware crops, aphid numbers need to reach 5 aphids per leaf to justify an aphicide treatment. Hallmark Zeon or Aphox can be applied as single applications, but check the efficacy of the product and if aphids are still alive after application of either of these products, there may well be aphicide resistant aphids present, in which case apply Plenum.

Check for blight
If blight is found, it is important to deal with the outbreak as quickly as possible. Where only a few plants are infected then remove these and destroy them. Where it is not practical to do this then desiccate the infected patches using a fast acting desiccant, e.g. diquat. For crops entered in the seed classification scheme it is important to contact the Scottish Agricultural Science Agency (SASA) prior to desiccating patches in a seed crop. The healthy plants next to infected patches are likely to have been infected but symptoms have not appeared yet. It is sensible to desiccate an area slightly larger than the patches with visible symptoms. Blighted crops, or parts of crops, should not be irrigated when blight is active. This increases the risk of tuber infection.

August: Late blight
Continuing mild and wet weather increases the risk of late blight attacking potato tubers. Ensure late blight fungicides are maintained at short intervals to help maintain protection of tubers.

September: Rots at lifting
Wet weather at lifting can lead to extensive potato rots, so take care with storage conditions. Tubers lifted wet need to be positively ventilated in store and store temperatures reduced to reduce the risk of wet bacterial rots developing. Tubers lifted in dry conditions may be dry cured to help heal damage to the skins caused at lifting. This minimise diseases which may infect tubers via damage and develop during storage.

November: Sample next season's fields for pests
Sample fields destined for potatoes net year for Potato Cyst Nematodes (PCN) and Free-living nemaatodes.

The nematodes will be near the soil surface and the sooner fields are sampled, the more time you have to decide whether they need to take action for potato cyst nematode (PCN) or spring next year. SAC can assess population size and species of potato cyst nematode (PCN) which can allow you to take decisions on whether a nematicide may be needed or whether growing a resistant potato variety may be the best management option.

Free-living nematodes have become more problematic over the last decade, with populations of Trichodorid nematodes (which transmit tobacco rattle virus that leads to spraing) increasing threefold. Trichodorid (and Longidorid) nematodes can also cause direct feeding damage to potato crops at emergence, as well as vectoring the virus that causes spraing. Consequently, an assessment of nematode populations as well as determining if they are carrying tobacco rattle virus (TRV), is an important factor in effective nematode management.

SAC can carry out an assessment of nematode populations and test them for the presence of TRV, so that an accurate assessment of the risk of direct feeding damage and spraing risk can be provided for individual fields.

Seed

Seed potatoes produced and marketed in Scotland must be classified under the Seed Potato Classification Scheme. More details are available from the SASA web site.

Diseases

Potato Late Blight
Late blight in potatoes is the key foliar disease to affect potato crops in Scotland. Potato blight is caused by a fungus (Phytophthora infestans). The main source of the disease are potato dumps (areas where diseased potatoes from previous years have been left). These dumps tend to have a high concentration of diseased tubers and growth develops early in the season, resulting ina a localised source of inoculum at the time when field crops are emerging. Other sources included infected seed and potato volunteers. To date, soil borne infection is not known to occur in the UK.

Weather conditions which suit potato blight are mild and humid. High risk periods can be forecast based on minimum temperatures & humidity. Varieties vary in their resistance to blight on the haulm and the tuber. No current variety is totally resistant.

Late blight is caused by the fungus Phytophtora infestans and it first became a major problem in Europe in the 18th Century, where it led to extensive destruction of potato crops in Ireland.

Although seed and potato volunteers are potential sources of the disease, the key source are potato dumps. A potato dump is a pile of potato tubers which have been discarded in previous years. These dumps are a concentration of diseased tubers which, if ignored, sprout early in the year and allow the disease to develop.

Typical disease symptoms comprise pale brown or black watersoaked spots on leaves. The white fungal growth of the fungus is generally seen in the zone between the lesion and healthy plant tissue. This is because late blight is a biotroph and lives on healthy green tissue. Lesions can also develop on the stems and appear as pale brown watersoaked marks. The spores which develop from the fungus fom these lesions are dispersed in the wind. These spores may infect more potato plants directly, or produce swimming spores known as zoospores which infect more plants. Heavy rain can wash these spores from the leaves and stems onto the ground where they can infect the tubers. Symptoms on the tubers comprise brown mottled lesions can be seen inside the tubers close to the surface. In severe cases, the marks can be seen without cutting open tubers.

Recent changes to the GB blight population
The most recent information from the Potato Council-funded project on the UK blight population is that the population is now dominated by two genotypes of P. infestans, i.e. genotypes 13_A2 and 6_A1, which in 2008 comprised 79% and 12% of the population respectively. Genotype 6_A1 is of similar aggressiveness to 13_A2 therefore most of the comments in 1 and 2 below regarding 13_A2 also apply to 6_A1.

The threat from blight each year is mainly determined by the prevalence of weather conditions suitable for its spread and development. Assuming weather conditions favour the development of the pathogen the presence of 13_A2 means that:

1. Blight outbreaks are likely to occur earlier because 13_A2 will be more active at lower temperatures than most other genotypes. In cooler conditions 13_A2 produces larger lesions that sporulate sooner than other genotypes. The 13_A2 genotype also spreads quickly.

2. There is likely to be less scope to extend spray intervals because 13_A2 has a shorter latent period between leaf infection and the production of new spores. This means that the pathogen will complete its life cycle in less time than previously. If there’s any gap in fungicide protection then this is more likely to be exposed than previously. Fungicide programmes must be started early enough and spray intervals must be short enough for the risk conditions. The shorter latent period and ability to produce spores more quickly means that when blight develops it will do so more quickly. Growers should adhere to 7-day intervals unless there are periods of extended low risk.

3. In the last few years comments were made about some varieties appearing to be no longer as resistant to foliar blight as their published resistance ratings. Recent trials, some funded by the Potato Council, have confirmed that for some varieties resistance ratings when tested against the new, more aggressive strain of blight 13_A2 were lower than the published ratings. There is no doubt that 13_A2 is at least partly responsible for changes in varietal resistance ratings. However, further testing is required before sound conclusions can be reached. It is important for growers to refer to the most recent ratings, derived from tests in 2007 and 2008 using 13_A2.

Risk of oospore-derived outbreaks
The situation for the last few years has been that some blight outbreaks in crops, on volunteers and even in outgrade piles, have contained both the A1 and A2 mating types of P. infestans. Consequently it is possible that oospores are present in fields that are going back into ware potatoes this year and the risk of blight outbreaks initiated by oospores is higher than before. The use of short rotations increases the risk considerably where the previous potato crop or volunteers were blighted and both the A1 and A2 mating types necessary for the production of oospores were detected. The production of oospores can be minimised by dealing very quickly and effectively with blight outbreaks and the risk of oospores initiating outbreaks can be reduced by minimising volunteers as much as possible and avoiding short rotations.

Ensure there is no growth on discarded dumps of potatoes. Covering the heaps with black polythene is one method to prevent growth.

All crops should be protected with fungicide when a High Risk period is triggered ,or as soon as the haulm meets in a drill. High Risk Smith Periods are defined as a period of two consecutive days where the minimum temerature remains at is 10C over the whole 48 hour period and where the relative humidity remains above 90% for at least 11 hours on each day.

Pests

Potato Cyst Nematode
Potato Cyst Nematodes (PCN) are an important pest of potatoes. There are two species namely Globodera rostochiensis (golden cysts) and Globodera pallida (white cysts).

The cysts present in the soil, hatch in the spring. Hatching can be triggered by exudates from potato roots, but some may also hatch even where there is no potato host in the field. Nematodes feed on the roots and high numbers can lead to plants becoming stunted with pale leaves. The damage usually occurs in patches in the field. Where growing conditions are stressful (i.e. wet soils, or drought conditions), symptoms will show up more readily.

On close inspection of the rooots, you will find cysts present on the roots. Crop losses can be high if the plants die-back early. Cysts in the soil will hatch during the spring. The nematodes then feed on potato roots. This can lead to symptoms in the plant. Feeding damage to the roots may also lead to secondary fungi attacking the plants. Females feeding on the roots form cysts which can be seen by eye on affected plants.

The mature cysts can be harvested with the cop and transmitted to other fields on affected seed. Others will remain in the soil ready to hatch in future potato crops. A proportion of the cysts will hatch every year even where no potato crops are grown. Exudates from potato roots will lead to a higher proportion hatching when a potato crop is grown again in the field.

Fields can be tested for the presence of potato cyst nematodes. SAC can test for cysts, species and viability of cysts. On the basis of the results, advice can be given on treatments and varieties.

Slugs
Field slugs can cause damage to a wide range of plants, particularly seedlings, which can be killed out, but also leaves, stems, grains and tubers. Slugs are most active in mild wet weather. During dry weather, they are most likely o be found at night, but when weather conditions are ideal they can be found on plants, including the heads of cereals.

Subscriber Info

For 2009, as in previous years, it is important to deal with any sources of primary inoculum. This will reduce the risk of crop infection and allow fungicides to be more effective. One of the mainstay methods of preventing haulm growth on outgrade piles is to use dichlobenil granules, e.g. Casoron G. Note that approval for dichlobenil has been revoked and consequently approval for the storage and use of dichlobenil granules already on farm is only until the 18th of March 2010.

An alternative method of preventing haulm growth is to use black polythene to cover the outgrade pile. Glyphosate can be used to kill off any haulm that has already grown but such treatment may be too late to prevent the spread of blight from the outgrade pile. In addition, repeat applications will most probably be required to deal with successive emergences of haulm because tubers are at different depths.

Potato blight control early in the growing season
There are many fungicides suitable for use early in the season, i.e. prior to rapid haulm growth, for example, Curzate M, Electis, Invader, Ranman TP, Revus, Shirlan, Valbon or Volley. Where curative activity is required cymoxanil (C50, Cymbal or Option) should be added to those products above without curative action, i.e. Electis, Ranman TP and Shirlan. Where the Shirlan soil treatment for the control of powdery scab (SOLA) has been used then foliar sprays of fluazinam cannot be applied to the crop for the control of blight. If there is a risk that blight has been introduced on seed then it is appropriate to use a fungicide with curative activity. Zoospore activity is also important, e.g. fluazinam.

Protecting potato crops against blight during rapid haulm growth
It is anticipated that 13_A2 will be prevalent from the beginning of the 2009 growing season, as it was in 2007 and 2008. It is prudent to start fungicide programmes early and maintain short spray intervals unless weather conditions are very clearly not conducive to the spread of P. infestans and infection. Robust protection from the start is required. This should apply to almost all varieties until there is more robust information on changes in varietal resistance. Strain 13_A2 combines phenylamide insensitivity with aggressiveness and fitness therefore the use of blight fungicides containing phenylamide active ingredients to protect the crop during rapid haulm growth is not recommended. Phenylamide fungicides are mefenoxam (also known as metalaxyl-M) in Fubol Gold and Epok, and benalaxyl in products such as Tairel and Galben M.

An alternative to the use of phenylamide-containing fungicides during rapid haulm growth is to use some of the newer generation contact fungicides, Ranman TP and Revus, that in trials have been shown to offer very good protection of new growth. The ability to spray at short intervals, no greater than 7 days, is essential to the success of this approach. It is strongly recommended that Ranman TP and Revus are alternated and cymoxanil, e.g. C50, Cymbal or Option, is tank-mixed where appropriate. In recent years Belchim has investigated the protection of new growth by Ranman TP compared with systemic fungicides. In a large number of trials, including Scottish ones at SAC, Ranman TP has given very good protection of the new foliage. This effect has been shown to be consistent across years and many countries (and therefore varieties) in Europe. There are fewer results for Revus but there is good evidence that Revus also protects new haulm growth.

An alternative systemic fungicide to the phenylamides is propamocarb, e.g. Consento, Merlin and Tattoo. These products control phenylamide-resistant strains. However, in many trials the protection of new growth by Ranman and Revus at 7-day intervals was superior to that achieved using propamocarb products.

Protecting new growth is obviously more difficult when the haulm is growing particularly quickly. More rapid growth than normal will occur when temperatures are high and soil moisture is not limiting. Also, haulm growth will be more rapid for some varieties compared with others and for any crops planted late.

General comment
For optimum blight control it is not sufficient simply to use the most effective products, the decision regarding which fungicide to apply when can be critical. SAC trials in 2006 and 2007 demonstrated that blight was best controlled where the crop received the most effective fungicide, or combination, during periods of greatest risk of infection. At times curative activity and/or tuber protection are required in addition to good protection against foliar blight. The results of the SAC trials also argue against having too rigid a programme planned in advance of the season and in favour of a pro-active approach to fungicide selection.