Technical Guidelines for Seed Potato Production

Denis Griffin – Potato Breeder Teagasc

Introduction Unless good quality seed is planted, attempting to grow a high yielding crop of potatoes that meets market requirements is futile. Tubers are clonally propagated which can lead to accumulation and spread of seed and soil borne diseases.

Seed quality is governed by seed certification legislation and for many export markets additional conditions may apply. Seed production for export, therefore requires specialist husbandry with a focus on producing seed that is disease free and of high varietal purity to comply with relevant certification legislation. However, over the last 20 years in Ireland most seed crops have been grown as dual crops for both seed and ware.

The purpose of this paper is to provide general guidance and highlight some of the key issues that must be taken into consideration if you are intending to grow a high quality crop for seed.

Rotation and field selection  Certification requires 6 years between potato crops for pre-basic grades and at least three years for basic seed grades to minimise the build up of soil borne diseases, longer rotations are always desirable.  Heavy land with low pH will limit common scab development. On higher pH soils and particularly on light soils irrigation will limit common scab development.  Seed crops should be isolated from lower grade ware crops which may act as a reservoir of virus disease.  Fields chosen for seed production must be free from potato cyst nematode.  Fields with a history of powdery scab should be avoided.

Virus prevention The major reason for producing seed potatoes in northern Europe is to limit the spread of virus diseases by aphids which are more common in warmer climates. In this respect the northern regions of Ireland have an advantage over southern regions due to lower temperatures.  PVY, PVA and PLRV (leaf roll) are the most important viruses spread by aphids.  PLRV is a persistent virus which is acquired by the aphid vector feeding on an infected plant and can be carried by the aphid for a long time. However, transmission to the plant requires a long feeding duration; insecticides are therefore a useful tool to prevent the build up of infected aphid populations and reduce PLRV incidence.  PVA and Y are non persistent viruses, aphids become infected by probing an infected plant to feed and can transmit the virus within 30 seconds of starting to feed on a healthy plant. Insecticides do not kill aphids quickly enough to prevent infection but do prevent build up of aphid populations.  Potato Virus Y is the most important virus disease and several strains exist. PVYO causes typical severe mosaic and leaf drop streak symptoms. PVYN is considerably more difficult to detect as symptoms are mild and many varieties can be symptomless leading to considerable difficulty rouging crops.  Paraffinic oils are used in other seed producing countries which reduce incidence of PVY infection by removing the virus particles from the aphid mouthparts during feeding. Oils must be applied weekly to protect new growth.  Early burn off dates should be considered to avoid high aphid populations.  Insecticide use must be carefully planned to avoid the development of resistance.  PVX and PVS are spread by contact such as machinery passing through the crop and humans or animals walking through crops. Incidence of these diseases has declined greatly but rouging of seed crops for all virus diseases is essential and remains the only effective control strategy for contact spread viruses.

Size management  Specialist seed crops must be managed to maximise tuber numbers in the 35-55 mm seed fraction.  To maximise tuber numbers seed crops are planted at higher seed densities than ware crops usually 4 to 5 tonnes of seed per hectare depending on variety.  Physiological management of seed for planting is an important tool, seed which has been cold stored for a period will not exhibit apical dominance and will produce more stems and tubers per plant.  Sprouting of seed for production of seed crops is not recommended due to the risk of virus transmission to sprouts by aphids.  Generally seed crops will need to be burned down earlier for virus avoidance but also for early harvest for export markets. To maximise yield, early planting should be considered.

Bacterial diseases  Blackleg caused by Pectobacterium spp is a major problem in seed production in North Western Europe, It is exacerbated by wet weather, in this respect Southern Ireland has an advantage over northern regions due to dryer conditions.  Currently no control measures for blackleg exist in the growing crop.  Harvesting in dry conditions, crop drying and correct storage are important blackleg reduction methods.  Rouging of blackleg plants is essential to meet certification standards.  Care is necessary to prevent import and establishment of Dickeyea spp, avoiding seed from areas where it is known to be endemic is the best control measure. There is a nil tolerance of Dickeyea spp in certified seed in Ireland.

Foliar Diseases  Late blight remains the most important foliar disease of seed or ware potatoes.  Control programmes must focus on prevention of blight and particularly protection of tubers at the end of the season.  No differences in fungicide programmes exist between ware and seed crops.

Rogues and variants  Seed crops must be rouged to remove stray plants of any other potato varieties  Variants of the variety being grown such as bolters and wildings must also be removed.

Storage and handling  Seed crops grown for early export may negate the need for refrigerated storage.  Seed must be protected from frosts during storage.  Hygiene of stores, grading lines and boxes is of utmost importance. Maintaining a clean grading line and disinfecting between lots limits the spread of blackleg and soft rots.  Dust and soil should be minimised in store to prevent spread of gangrene, dry rot and silver scurf. Use of misting or fogging equipment to reduce airborne dust and spores helps reduce disease spread.  Grading should only be undertaken when stocks are at the correct temperature to avoid damage and potential disease infection. Seed and ware lots must be clearly labelled and segregated to comply with certification requirements.