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Potato Seed Production
Pests and Diseases:
Introduction
Second only to maize, "Irish" potatoes play a very big role in the Kenya food security situation. Potatoes can be grown within a growing period of 3 months, produce good yields of highly nutritious food and be stored for some months under the right conditions.
However growing potatoes in mixed cropping systems as currently practised by many small scale farmers where volunteers from previous crops are not weeded out has by now managed to spread serious seed borne plant diseases to most potato growing areas. This has resulted in declining yields and again undermined the possibility of good food security in These areas.
In order to clean up the disease situation and to increase clean side production it is necessary to understand exactly what needs to be done (for more articles on clean seed potato production please also see TOF June 2010
Farmers can also grow their own seed and share with neighbours, if good field hygiene is observed as described in this article.

On-farm production of potato seed tubers by smallholder farmers in small seed-plots
Principle of seed plot system
This involves maximising tuber production per unit area of limited, disease-free land through high-density planting in a seed plot and also separation of seed and ware (larger tubers meant for consumption) potato production. The system uses 50% less land than the ware production to meet on-farm seed tuber requirements


Land requirement and site selection

Commercial Opportunity:
In Kenya there is a very large commercial opportunity in the production of certified seed potatoes. The demand for and commercial price of clean potato seed tubers is far higher than potatoes for the food market. Mid 2010 it was established that Kenya only produces about 1% of the potato seeds needed every year by potato Farmers countrywide. Clean seed is the best start for a good potato crop yield as most potato farmers know or have discovered.
So if you have an extra piece of land free of bacterial wilt and other potato plant diseases, it might be a very good ideal to pay a visit to KEPHIS (Kenya Plant Health Inspectorate Service ) and find out from them how to go about such a venture. The production of certified seeds can easly be carried out in farmers fields, but will need inspektion and certification by KEPHIS officers to be able to attract the nice high price tag that currently is offered for such clean seed.



Choose your plot for potato seed production with care:
  • It should not have had any Irish potatoes, capsicums, eggplants (brinjals) and tomatoes grown in it for the last 5 years.
  • Preferably it should have had a dense mat of grass growing on it for at least one year during the last 5 years.
  • Other probable sites for seed plots include virgin land and land that has been under fallow for 2-3 seasons.
  • It should not have been given agricultural lime for the last 5 years
  • The plot should be located in full sunshine in highland areas, and in half shade (for example shaded half the day) in hotter areas.
  • The seed plot should not be situated in low-lying or water-logged areas where run-off water flows into it.
  • There should be a good supply of well composted farm yard manure applied just before planting the certified seed potato tubers. If the amount of manure is not very much, it should be applied in the trenches dug for planting tubers.
  • If Lantana camara bushes grow in your neighbourhood it is very beneficial to also mix in leaves of lantana in the potato planting rows to repel insects.


Sources of seed tubers
Clean seed tubers can be obtained from KARI centres at Tigoni, Embu, Kitale and Oljoro Orok, identified seed growers who work with Ministry of Agriculture, and from authorised farmers groups who produce quality seed with advisory inspection support from Kenya Plant Health Inspectorate Service (KEPHIS). However, commercial clean seed is expensive and mostly not available. Therefore, for most farmers, positive seed selection and seed plot system for seed production offer an immediate low-cost alternative, provided it is done correctly


Positive seed selection technique
This is a procedure where any diseased plant particularly infected with bacterial wilt or viruses is removed from the field immediately it is noticed and the remaining healthy-looking plants at flowering are marked with sticks or pegs. The marked or selected healthy-looking plants should be checked weekly and those which have developed disease symptoms should be unpegged and removed from the field. Pegged plants should not be near (at least 1 metre away) where diseased plants were removed.
  • These pegged plants are considered to be positively selected seed potatoes. The removed diseased plants with bacterial wilt including their tubers should be put in a 1-metre deep pit and covered with clean soil or be burnt. Also wilt infected plants and tubers should not be put in compost heap.

  • Harvest the pegged plants separately and first before the rest of the field to get seed tubers for the next planting. This procedure is in contrast with the usual farmer practice where all plants in the field are harvested together and seed is picked from the harvest for planting in the following season

Guide to knowing healthy and diseased potato plants
How a healthy plant looks:
A healthy plant is vigorous, has many stems, dark-green leaves and at harvest, has more seed-sized of 35-55 mm than smaller ones below 25 mm in diameter.
Healthy Mavuno potato
© A.A. Seif
Potato variety 'Asante'
© A.A. Seif
Potato variety 'Tigoni'
© A.A. Seif
Potato crop flowering
© A.A. Seif


How a bacterial wilt infected plant looks:
Sudden wilting of plants even when it is wet. Entire plants can wilt quickly without yellowing. There is a dark brown colour in the inner section of the stem. A white creamy liquid oozes out when the stem is cut near the base. Tubers are black or have black eye buds. Heavily infected tubers have soil stuck to the tuber eyes and on cutting of the tubers rotting from the vascular ring is observed
Bacterial wilt (rotting of vascular ring)
© A.A. Seif
Bacterial wilt symptoms on potato plants. The first visible symptom on foliage is a wilting of the leaves at the ends of the branches during the heat of the day with recovery at night. As the disease develops, a streaky brown discolouration of the stem may be observed on stems 2.5 cm or more above the soil line, and the leaves develop a bronze tint.
© Mauritius Sugar Industry Research Institute. Reproduced from the Crop Protection Compendium, 2004 Edition. © CAB International, Wallingford, UK, 2004


How a virus infected plant looks:
Plants are stunted (dwarfed). Leaves may have mosaic pattern (yellow and green). Leaf margins may be curled or rolled up and may be thick. In dry season, green aphids may be found on lower leaf surfaces. Tubers produced are small
Potato Leaf Roll Virus 1
© A. A. Seif


Seed plot system
Land preparation, planting and crop management
Mark out beds of 2.0 m width and a sufficient length depending on your seed needs. Plant in furrows at a spacing of 30 x 30 cm at planting depth of 15 cm. Broadcast DAP fertiliser at the rate of 500 gm for every 10 m2. A 90 m2 seed plot (2 x 45 m) can provide enough material to plant 0.4 ha (1 acre) of ware potatoes (4000 m2). Hand weeding is recommended. Weeding should be done on appearance of weeds until there is sufficient potato growth to suppress weed growth.
Seed plot should be weekly inspected to monitor crop performance and particularly occurrence of pests (aphids, potato tuber moth) and diseases (bacterial wilt, viruses, early and late blight). Aphids can be easily controlled by spot insecticide application (consult area extension agricultural officers or the nearest KARI centre for advice). In case of potato tuber moth ridging is recommended. Any plant with bacterial wilt and/or virus infection should be uprooted and be destroyed and should not be used as animal feed as they may be brought back to the field in form of manure. Put 2 hands-full of ashes or 1 hand-full of lime in the hole after removing a wilted plant. Early and late blight can be effectively controlled by using appropriate fungicides immediately when noticed (consult area extension agricultural officers or the nearest KARI centre for advice)
Potato mother plants widely spaced for production of basic seed
© A.A. Seif
Potato mother plants widely spaced (1.5x0.60m) for production of basic seed
© A.A. Seif


Harvesting, grading and storage
On maturity, when leaves begin turning yellow, remove potato vines (haulms / stems) about 2 weeks before harvesting to harden tuber skin. Harvest when the soil is dry. Avoid injuring the tubers.
  • Seed tubers must be unbruised measuring 2.5-5.5 cm in diameter. Larger tubers should be sold off as ware potatoes.
  • Selected seed tubers should be stored in diffused light and ample aeration. Do not store seed tubers in dark stores used for ware potatoes.
  • Store tubers of each variety separately. Do not store seed tubers in gunny bags.
  • Spread the seed tubers thinly on the floor or in wooden crates.
  • Cover with branches of Lantana camara.
  • Farmers in Naivasha have discovered that storing seid potatoes under saw dust keeps away tuber moths and keep the potatoes seeds longer.


Usage of seed plot tubers
Tubers harvested from seed plots are divided into 2 lots: one lot is used to establish a new seed plot (for not more than 3 seasons) as described above and the other lot is used for production of ware potato in main fields.


Seed plot protection
  • Fence the seed plot area to restrict movement that can cause contamination.
  • Plant maize around the seed plot as a border crop to minimize aphid infestation. Plant at least 4 rows of maize 2-3 weeks before seed potatoes.
  • Clean /and disinfect all tools, feet and shoes before entering the seed plot area.
  • Use a household bleach (sodium hypchlorite) (e.g. JIK®) at the rate of 50 ml in 1 litre of water.
Information Source Links
  • Anon. (2006). Select the best: Field guide for positive seed potato selection. CIP Sub-Saharan Office, Nairobi
  • Gildemacher, P., P. Demo, P. Kinyae, M. Wakahiu, M. Nyongesa and T. Zscocke (2009). Select the best. Positive selection to improve farm saved seed potatoes. Farm field aid. 2nd. Edition. CIP & CTA. ISBN: 978-92-9060-362-7
  • Kabira, J.N, M. Macharia, M.W. Karanja and L.M. Muriithi (2006). Potato Seed: How to grow and market healthy planting material. KARI Technical Note No. 20, April, 2006, Kenya Agricultural Research Institute
  • Kinyua, Z.M and J.J. Smith (2007). Multiplication of disease-free seed potato in seed plots. KARI Advisory Bulletin, March 2007, Kenya Agricultural Research Institute
  • Nyongesa, M., C. Lung'aho, C. Kinyae, P. Wakahiu, M. Karinga and J. Kabira (2008). Production of food (ware) potatoes. KARI information brochure series/35/2008, Kenya Agricultural Research Institute
  • Nyongesa, M., C. Lung'aho, P. Wakahiu, J.N. Kabira, P. Gildemacher and G. Forbes (2008). Control of late blight on varieties Asante and Tigoni. Kenya Agricultural Research Institute/International Potato Centre, SSA, Nairobi
Potato field
Certification
A process verifying the compliance of farm management with i.e. organic standards; based on inspection of the farm and its documentation.
Diameter
Thickness or width.
Border crop
Crop grown at the edge of organic fields bordering conventionally managed fields in order to reduce drift of pesticide sprays.
Disinfect
To reduce, by physical or chemical means, the number of potentially harmful microorganisms in the environment, to a level that does not compromise food safety and suitability.
Contamination
Pollution of organic product or land; or contact with any material that would render the product unsuitable for organic certification.
Symptom
Symptom - the plant's response to the disease causing organism, examples are; changes in plant color, death of infected tissues, and wilting. It is the external or internal physical characteristic of a disease as expressed by the host plant.