RapidMultiplication

of Yams(Dioscorea spp.)

Agdex#175-40

CROPSIRETA Publication No. 3/88

AUTHOR: Jill E. Wilson, Senior Fellow in Tropical CropProduction, Department of Agronomy and SoilScience, University of Hawaii at Manoa.

Assisted by Kim Des Rochers and Carl I. Evensen.

Illustrations by Shirley Tjendana.

Trade names mentioned do not constitute IRETA approval to theexclusion of suitable alternative products.

All or part of this publication may be reproduced for educationalpurposes. When doing so, please credit the USP Institute forResearch, Extension and Training in Agriculture (IRETA).

Published May, 1989, by the Institute forResearch, Extension and Training inAgriculture with financial assistance fromthe UNDP/FAO Root Crops DevelopmentSystems Project RAS/86/034.

IRETA PublicationsUSP Alafua CampusP.O. Private BagApia, WESTERN SAMOA

Printing: Communications Support Centre

87/88 - 1 M

RAPID MULTIPLICATION OF YAMS

In traditional systems of producingyams (Dioscorea spp.), farmersdepend on this season’s harvest tosupply planting material for thefollowing season. To produce yamsfor eating or marketing, mostfarmers in the Pacific plant yam settsthat weigh 100 to 600 g or more. Thismeans that they must set aside 10 to30% of their yam harvest for plantingmaterial.

Farmers often encounter shortagesof yam planting material, especiallyfollowing cyclones, droughts, anddisease epidemics, or in years whenmany feasts and celebrations are held.

Research and extension officersmay also be faced with the challengeof producing large quantities of yamplanting material.

Several methods of rapidmultiplication have been developed tospeed production of yam plantingmaterial. One of these methods isminisett rapid multiplication. Thismethod produces small, wholeplanting setts, each weighing 200 to800 g, from minisetts as small as 25 g.

Usually, the aim of minisett rapidmultiplication is to produce a largenumber of small, whole tubers forplanting, NOT to produce tubers foreating and marketing.

However, in the Caribbean wheresmall tubers are preferred for export,minisetts are being used to producehigh yields of marketable yams.

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Who Can Use Minisett Rapid Multiplication?

• Farmers can growtheir own planting setts.

• Farmers can produce planting settsto sell toother farmers.

• Extension workers can grow largequantities of planting setts tosupply farmersafter cyclones and other naturaldisasters.

• Extension and research workerscan rapidly multiply new cultivarsfor distribution to farmers.

• Researchers can rapidly multiplynew introductions for evaluation.

• Researchers can use it to producehigh-quality, uniform plantingmaterial for trials.

Small Whole Tuber Setts

The small, whole tuber setts produced byminisett rapid multiplication areexcellent planting material for farmersand researchers. Each sett has a head,which means that sprouting is early,strong, and uniform. Also, these small,whole tuber setts have no cut surfacesand therefore rot less easily than cutpieces.

Although small, whole tubersproduced from minisetts are excellentplanting material, this is NOT true for allsmall, whole tubers produced in normalyam production. Why? Because innormal yam production, many of thesmall tubers are produced by weakplants that are infected with nematodes,viruses, and other diseases. Some ofthese nematodes and diseases are carriedin the tuber. So, if you plant small tubersfrom infected plants, you will spread andmultiply these nematodes and diseasesand reduce your yields.

small, whole tuber

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For example, when the 1987 drought destroyed most of the plantingyams of farmers in the East Sepik Province of Papua New Guinea, theSaramandi Research Station used the minisett technique to rapidlymultiply D. alata for distribution to these farmers for planting.

Minisett Rapid Multiplication

There are 8 steps in minisett rapid multiplication:

• Presprout the minisetts.

• Plant and maintain the minisett crop.

• Rogue diseased plants.

• Harvest and store the planting setts.

Select Tubers for Cutting

To avoid spreading tuber-borne diseases,only tubers harvested from HEALTHYplants should be cut into minisetts.Select the tubers you will use forminisett multiplication in the fieldduring harvest. Mark weak and diseasedplants during the growing season, so thatyou can recognize them at harvest andavoid using their tubers for minisetts.The most important diseases to watch forare caused by viruses and nematodes.

Virus DiseasesDo not use tubers harvested from plantswith virus symptoms. Look forsymptoms of virus on the leaves.Symptoms include vein banding, veinclearing, mottling and leaf distortion. Ifyou see leaf distortion or curling,examine the undersides of the leaf forinsect damage, which can sometimes beconfused with virus symptoms. It is notnecessary to avoid tubers from plantswith insects on the leaves.

Leaf vein banding is onesymptom of virus.

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• Select tubers for cutting into minisetts.

• Choose minisett size.

• Cut the tubers into minisetts.

• Dry and treat the cut pieces.

NematodesDo not use tubers infestedwith nematodes. At harvest, lookcarefully for the symptoms of root-knotand lesion nematodes.

The symptoms of root-knotnematode (Meloidogyne incognita)include bumps and excesshairiness that are not typicalof the cultivar.

The external symptoms of thelesion nematode (Pratylenchuscoffeae) include vertical cracks on theskin of the tuber. Theskin of the tuber flakes offeasily to expose black-coloured dry rotunderneath. This dry rot looks likeblack fire ash. Also, tubers infestedwith lesion nematode become soft andwrinkled and feel spongy when youtouch them.

While you are cutting minisetts,discard any tubers that show theinternal symptoms of lesion nematode.These include yellow, brown, or blackareas under the skin that are not typicalof the cultivar. Also, discard tuberswith slow growing sprouts since this isanother symptom of lesion nematode.

Only tubers from healthy plants shouldbe cut into minisetts.

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Choose Minisett Size

When choosing the size of minisetts touse in your location, you must consider:

• The species of yam,

• The spacing at which the minisetts willbe planted in the field, and

• The size planting setts preferred byfarmers.

You can use Table 1 to chooseminisett size and plant spacingto produce a desired size of planting sett.

For example, if farmers prefer toplant 500 g setts of D. alata,cut minisetts that weigh about25 g and plant them at a spacing of 100 x25 cm.

Dioscorea rotundata usuallyrequires larger minisetts thanD. alata. The minisetttechnique has not yet been tested on D.esculenta or D. nummularia.

Large minisetts sprout rapidly andgrow into vigourous plants that yieldlarge planting setts. In comparison,small minisetts are more likely to rot orfail to sprout, and they yield smallerplanting setts. But despite thesedisadvantages, small minisetts usuallygive a more rapid multiplication ratethan large minisetts.

Table 1. For different species of yam (Dioscorea), what size planting setts you can expect toharvest from minisetts of different weights planted at different spacings?

Minisett Planting sett

Yam species Minisett wt (g) spacing (cm) produced (g)

D. alata 10-20 100 x 25 100-600

25-30 100 x 25 200-800

40-60 100 x 40 400-1800

D. rotundata 25-30 100 x 25 100-300

45-60 100 x 50 100-400

70-100 100 x 50 400-500

D. esculenta No data

D. nummularia No data

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Cut Tubers into Minisetts

Some cultivars of yam produce aerialtubers, and these can also be used forminisett rapid multiplication. Use smallones whole and cut large ones to thecorrect size.

There is no evidence that yamviruses can be spread by the cuttingknife, but to avoid spreading rotdiseases, disinfect cutting knives bydipping frequently in a strong (70%)solution of Clorox (or other householdbleach containing SODIUMHYPOCHLORITE).

Very small tubers would be planted whole.

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You can cut selected tubers intominisetts when they have brokendormancy and begun to sprout. Use aclean, sharp knife to cut each tuber intopieces that are the size you have chosen.First cut off the head and tail, and cutthem each into at least 2 pieces. Then cutthe rest of the tuber crosswise into disks,and cut each disk into 2, 3, 4 or morepieces. Each piece should have as muchskin as possible and enough flesh tosupport sprouting.

Tubers that are irregular in shape aremore of a challenge to cut. But withplanning, you can cut them in such away that you get the most minisetts, eachwith as much skin as possible. For roundyams that have a low ratio of skin toflesh, you can carve out some of thecenter flesh and use it for cooking.

Very small tubers should be plantedwhole.

Dry and Treat Cut Piece

To reduce rotting, minisetts should betreated and dried as soon as possibleafter cutting.

Dust the cut surfaces with dry,sifted fire ash, or beach sand (wellwashed or collected from above thehigh tide mark), or treat with thefungicide and/or insecticiderecommended in your country fornormal yam planting setts.

Air dry the treated minisetts for 2days, then plant them. Do NOT drylonger. Minisetts are so small thatdrying for longer than 2 days reducessprouting because too much moistureis lost.

Dry the minisetts in a place that isshady, well-ventilated, and protectedfrom rain and sun. Spread them in athin layer with cut surfaces facing up,so that air can circulate freely aroundthem.

oil palm

Fire AshTo treat 500 minisetts each weighing25 to 30 g, you will need about 1 kg (oralmost 2 litres) of dry, sifted fire ash.

Fire ash made from burning theinflorescences of oil palm isparticularly effective as a treatment forminisetts. This may be related to itshigh potassium content of 4% or more,compared to 1% for ash made fromburning most woods. Oil palm (Elaeisguineensis) grows in Solomon Islandsand Papua New Guinea. For othercountries, alternative high-potassiumashes should be identified.

FungicidesIf rotting of normal yam planting settsis a serious problem in your location,you can also expect it to be a problemin minisett rapid multiplication.Minisetts can be treated with the samefungicide/insecticide slurry or dustrecommended by your ExtensionDivision for treating regular yamplanting setts.

For example in one Pacific countrythe following is recommended: Dustfreshly cut pieces with undilutedpowder of captafol (Difolatan, 80%WP), then dry and plant.

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Presprout Minisett

After drying, minisetts can be planteddirectly into the field. With some yamcultivars, however, better results areobtained if minisetts are presproutedbefore planting. They can bepresprouted in a nursery bed, woodenbox, or pit that is well drained. Theseshould be protected from the rain andshaded from the sun.

Place old, rotten sawdust or verylight, sandy soil in the nursery bed,box, or pit. These media are betterthan topsoil because they allowexcess water to drain away, and theymake it easy for you to remove theminisetts without damaging sproutsand roots.

After burying the minisettsin the sawdust or sandy soil, applywater as necessary to keep damp butNOT wet. This is important, becauseminisetts that are kept too wet will rot.

NOTE: Do not use fresh saw-dustbecause it tends to heat up and that cankill young plants.

Not all the minisetts you plant willsprout. Sprouting usually ranges from60 to 80%, but this varies with speciesand cultivar. Also, the range in time tosprouting varies with species andcultivar and the variability in time tosprouting among minisetts of onecultivar can sometimes be a problem.

Plant and Maintain Minisett Crop

Most operations involved in plantingminisetts and maintaining the crop arethe same as for normal yam.

Time of Planting. The best time ofplanting will vary with location, but isusually during the normal yam plantingseason. Your aim is to harvest setts thatwill break dormancy as close as possibleto the time when they are needed forplanting.

Soil. It is possible to plant minisetts onsoils that are too low in fertility or tooshallow to grow normal yams for eatingand, marketing.

However, remember that your goal isto produce healthy planting setts that arefree from soil-borne nematodes anddiseases. Therefore, minisetts shouldNOT be planted on soils infested withlesion or rootknot nematodes or yamdiseases. Minisetts should not be plantedrepeatedly on the same land, and theyshould not be planted after a crop ofnormal yam. Newly cleared fallow is thebest land for planting minisetts, becausenematode populations are low.

Land should be prepared as iscustomary for growing yam in yourlocation, either by digging holes or bybuilding mounds or ridges.

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Planting. Plant the mini-setts at aspacing of 1 x 0.25 m or 1 x 0.5 m(see Table 1) and cover with about 5cm of soil.

Mechanized land preparation.Where fields are flat and soils are freeof large stones (like some locations inTonga, Cook Islands, and Fiji),minisett production can bemechanized to reduce labour costs.The soil can be prepared byploughing and harrowing. Ridges canthen be made mechanically before theminisetts are planted by hand.

If yams are usually planted on theflat, as in Tonga, shallow furrows canbe made with a chisel plough orsubsoiler. The minisetts are thenplaced in the furrow by hand andburied either by hand or bymechanically ridging over the furrow.

If using fertilizer, phosphorousand potassium can be broadcastbefore ridging or furrowing.

Staking. If yams are staked in yourlocation, then your minisett cropshould also be staked. However, thestakes can be lighter and shorter.Stakes 1 to 2 m high are adequate.Split bamboo, thin sticks, or otherinexpensive local materials can beused. Also, single maize plantsspaced at 1 x 2 m can be planted tosupport the light vines of minisettswithout significantly reducing theyield of the yams.

Where yams are grown onbranches scattered over the soil, as inTonga, minisetts can also be grownthis way.

Where yams are traditionallygrown without stakes, minisetts canalso be grown without stakes. Insome tropical countries whereminisetts are being grown withoutstaking, WHITE plastic mulch hasbeen used successfully (not clear orblack plastic). This mulch is spreadover the ridges, and the minisetts areplanted through holes cut in it.

Light stakes 1 to 2 m highare adequate for minisettproduction.

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Weed control. Sprouting ofminisetts is often delayed, and theyoung sprouts are weak and tender.Good weeding is therefore criticalduring the first 12 weeks afterplanting. If herbicides arerecommended for normal yamproduction in your location, they canalso be used for minisett production.

Fertilizer application. If fertilizer isrecommended for yam production inyour location, it should also be appliedfor minisett production.

Pest control. To protect your minisettproduction from diseases, insects, andnematodes, use the methodsrecommended by your ExtensionDivision for normal yam production.

Rogue Diseased Plants

Virus diseases can be carried in the yamsetts you grow and distribute. To avoidspreading virus diseases, you shouldreduce the incidence of virus in yourminisett production as much as possibleby roguing.

Roguing is the process of inspectingthe crop and eliminating unwantedplants.

Walk your minisettmultiplication plots regularly, andpull out all plants with virussymptoms on their leaves. Burn theseplants, or bury them very deep. Alsoeliminate plants that are weak andgrow slowly. Start roguing early inthe growing season and roguefrequently to prevent insects fromspreading virus from infected plantsto other plants.

Harvest and Store Planting Setts

Harvesting must be done carefully sothat yam tubers are not cut, scraped,bruised, or sunburned. Immediatelyafter digging, tubers should be movedout of the sun, cleaned, sorted, andplaced in storage. Damaged tubers andtubers showing symptoms of nematodesshould be eaten or fed to livestock, notstored.

Storage areas must have adequateshade, good ventilation, and securityagainst pests, especially rodents.Regular inspection and disposal ofrotting tubers is recommended. Sproutsshould be kept pruned.

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Curing

Planting setts will store longer if theyare cured before they are placed instorage. Curing helps to heal woundsand toughen the skins.

Curing should be carried outIMMEDIATELY after harvesting andtransporting yam setts to the storagearea. Curing requires hightemperatures (32 to 40oC) and highhumidity (95%) for 2 to 4 days. It isdone under a cover that traps self-generated heat and moisture.

One way of curing is to heap thetubers in a lightly-shaded area andcover them, first with grass or matsand then with a CANVAS tarpaulin.Do not use plastic sheets for curing.Plastic will make the yams too hot. Ifyou do not have a canvas tarpaulin,you can use several layers of sacks ormats. Leave the yams to cure for 2 to4 days - NO LONGER. Check to besure temperatures do not get higherthan 40oC.

For more information on storingyams see: “Agro-Facts, CarefulStorage of Yams” by Jill E. Wilson,IRETA Publication No. 15/87.

canvas tarpaulin

grass or mats

cut-away view of yams curing in aheap on the ground.

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Distribution of Minisetts

The small, whole tubers produced by theminisett rapid multiplication are easilytransported and distributed to farmers.

Tubers that are smaller than desiredfor planting setts can be used for anothercycle of minisett rapid multiplication.