CROPSIRETA Publication No. 15/87

Careful Storage of YamsSome Basic Principles to Reduce Losses

AUTHOR: Jill W. Wilson, Senior Fellow, USP Institute forResearch, Extension, and Training in Agriculture,assisted by Linda S. Hamilton, Project Manager,South Pacific Region Agricultural DevelopmentProject.

Adapted from: Wilson, J. E. 1980. Careful Storage ofYams. Commonwealth Secretariat,London. 8p.

Illustrations by Andrew Crane and RichardGabrielson.

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 March, 1987 by the Institute forResearch, Extension and Training in Agriculturewith financial assistance from the US Agencyfor International Development, SPRAD Project.

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

PRINTING: Communications Support CentreEmil Adams, Information OfficerTonu’u Sealiitu, Printer

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CAREFUL STORAGE OF YAMS

Yam Storage

Yams are an important staple foodcrop in the Pacific, Africa and theCaribbean. In the Pacific Islands themost favoured species are theWATER YAM (Dioscorea alata),SWEET YAM (D. esculenta) andPACIFIC YAM (D. nummularia).

In yam “stores” traditionally usedin the South Pacific, losses are oftenvery high and much of the harvestedyield is wasted. Losses are due to avariety of diseases, nematodes,insects, and to sprouting, but theselosses can be reduced if proper care istaken. This booklet outlines someimportant principles which canimprove the success of traditional,on-farm yam storage.

Yams are widely cultivated inmost of the Pacific Islands but theMETHODS of production and storagevary considerably from place to place.Also the TIMES for planting,harvesting and storage vary.

Occasionally, yams are leftunharvested. But the most commonpractice is to harvest the yams andstore them in a special structure.

At harvest time, many yams areimmediately eaten or taken to market.But most are stored to be eaten ormarketed during the 5 months orlonger following harvest. Some of theharvested yams will be used asplanting setts for the next season andmay be stored up to 6 months.

WATER YAM is the species mostcommonly stored. PACIFIC YAM isnot stored for eating, although someplanting setts may be stored for a fewmonths.

Storage Structures

Different Types of StoresIn the Pacific Islands, yams arefrequently stored in BARNS. These canbe simple in design like the Tongan barnillustrated here, or they can be veryelaborate like the ceremonial storagebarns of Papua New Guinea.

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In other countries, yams arestored on COVERED TABLES or hanging one byone from an ELEVATED HORIZONTAL POLE.

These ELEVATED HORIZONTAL POLES aremade of wood or bamboo and are supported 1-2metres above the ground.

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Making the Storage StructureWhatever the type of store used, thefollowing principles should beconsidered:

When Building the Structure• adequate shade• protection from rain• good ventilation• security against animals, rats,

thieves

Before the Yams are Put into TheStore• sanitation of the store• selection of sound tubers

And During the Period of Storage• regular inspection• rapid disposal of spoiling tubers• continued sanitation

These features are importantwhen building the store:

ShadeThis can be provided by either live ordead plant material or from ironsheeting.• Live Shade is obtained by building

the store under the shade ofgrowing trees, or by planting postsof fast-growing trees around thestore.

• Other Plant Material Shading canbe made from palm leaves, matsand grass thatch.

• Iron Sheeting can also be used butit will keep the store too hot unlessit is also shaded with live shade orpalm leaves.

Protection from RainIn locations having rain during thestorage season, yams should be keptdry with a water-proof roof madefrom palm leaves, grass and thatch orshaded iron sheeting.

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VentilationGood ventilation improvesstorage. The store should be in anopen area, allowing air tocirculate freely. The horizontalpole system provides excellentventilation because air cancirculate freely around the yams.Also it is easy to inspect the yamsfor insects and rotting. And ratsfind it difficult to hide in thisstore. In covered table stores andbarns, yams should be stacked sothat rotting and insect-infestedyams can be removed easily. Inbarns, yams can also be laid onsimple slatted shelving.

Secure WallsThese are needed when goats, rats andhuman thieves are likely to take thestored yams. Walls can be madestockade fashion from bamboo orwooden poles. If secure walls areneeded, then so is a tight-fitting door.

SanitationThe store, whether it is a barn,covered table or horizontal pole,must be thoroughly cleaned beforethe new crop of yams is broughtin.• All old yams and debris should

be removed from the store area,and burned or buried very deep(too deep to grow). If this is notdone, insects and rot pathogenswhich are in the old yams willmove into the new crop.

• Continue to keep the storagearea clean and tidy throughoutthe entire storage season.

• If trees are used to shade thestore, these trees should betrimmed 3 months before thestore is used for new yams, andif possible the roof of the storeremoved. This permits sunlightto penetrate to the inside and

clean the floor and dry it out.Trimming also eliminates stragglygrowth and allows the branches togrow back into a thick shade by thetime the yams are brought into thestore.

• If it is available and legal, carefullysprinkle insecticide dust around thebases of all vertical supports whichtouch the ground to control ants. ButNEVER LET YAMS TOUCH theINSECTICIDE. Be careful to warnpeople, especially children, to nottouch the insecticide-treated poles orarea.

Do not store fertilizers andpesticides in the yam store.

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Rat Control

Rats will readily eat stored yams ifthey find the store easy to enter and ifthey have plenty of cover where theycan hide. These simple measures canbe taken to greatly discourage ratsfrom entering yam stores:• Remove all piles of rubbish, wood,

pots, tools, machinery from insideand outside the store and fromunderneath tables.

• Keep a cat in the yam store.• Make rat guards to protect the store.

Rats cannot climb up metalsheeting. For a barn with outside walls,a continuous line of metal sheeting canbe nailed up around the base of theoutside wall.

Make sure the door is a tight fit.

Nail corrugated iron to the door(from ground level upwards).

Corrugated iron is nailed towalls. Overlap each sheet.

90 cm (36 inches) minimumto ground level.

Bury the corrugated ironsheet at least 15 cm (6 inches)in the ground.

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Rat-Proofing a Yam Barn

For a covered table orhorizontal pole, or any structurewith legs, tight-fitting rat guardscan be attached to the legs at aheight above the ground of atleast 90 cm (36 inches).

Remember that rat guards willnot help if wood, tools, etc. are piledup under the table, or if tools andsticks are leaning against the table,or if tree branches hang closeenough for the rats to jump downonto the store.

Rat Guard–Sleeve Type Rat Guard–Baffle Type

Sheet metalsleeve, nailed topost.

30 cm (12 inches)minimum

60 cm (24 inches) minimumfrom ground level.

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Rat poison is very DANGEROUS.Use it only if there are very many rats.At all-times keep poison away fromyams and out of reach of children andhousehold animals.

The best way to use rat poison isto put the poison inside a piece ofbamboo. Find a short piece ofbamboo which is hollow and is wideenough for a rat to crawl through. Laythis bamboo flat on the ground withthe rat poison inside.

Different Cultivars

Some species of yams store betterthan others, but also within onespecies there are some cultivars(varieties) which store better thanothers. These differences are usuallywell known by the farmers. Ofcourse, the cultivars which have poorstorage characteristics should beeaten or taken to market soon afterharvest, and NOT STORED.

Generally the better storing cultivarshave:• good resistance to nematodes• a long period of dormancy• a tendency to heal if cut or scraped• compact shapes which reduce the

risk of the tubers beingaccidentally cut during harvest.

Store Only Sound Tubers

Only sound yams (which are free ofPratylenchus nematodes, rots, cuts,insects, scrapes and bruises) should bestored. Injured yams should not bestored. They should be marketed oreaten immediately.

This means that good storagebegins in the field. Care should betaken during harvest and whentransporting yams to the store to avoidunnecessary cuts, scrapes or bruises.Such injuries increase the possibility ofrot.

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Physical injuriesYams which you want to store mustbe harvested carefully. Somecultivars have very long and unevenshapes, and when digging them out,great care should be taken not to stabthe tuber or cut the skin. So instead ofhacking carelessly at the soil, like theman in the picture, try to feel gentlyaround for the yam so that it can beloosened carefully. If cut or brokenyams must be stored, treat the cutsurfaces by dusting with cold fire ash.

TransportYams can easily be damaged duringtransport from the field by wagon ortruck. Do not clean dirt off the yamsuntil you have transported them tothe store. The dirt protects the yamsfrom scraping and bruising. Thefloor of the wagon or truck should bepadded with grass or mats or sacksand the yams piled in gently. Do notpile the yams very high. Passengersand other loads should not sit on topof the yams.

Do NOT dig yam like this.

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NematodesTwo types of nematodes are oftenfound in yam tubers, -- the lesionnematode (Pratylenchus coffeae), andthe root-knot nematode(Meloidogyne). Only the LESIONnematode (Pratylenchus) is a problemin storage. It is easy to recognize thedifference between the twonematodes.

If a yam is sound, the flesh justunder the skin is clean. But if a yamis infested with Pratylenchus, the skinhas a cracked look and the fleshimmediately under the skin is stainedyellow and black/brown. This dry rotbeneath the skin gradually spreadsinto the tuber flesh. Pratylenchusattack can be confirmed by scratchingthe skin with a fingernail or knife.

Root-knot nematode makes theyam tuber knobbly and with morehairs than normal.

Yams affected by Pratylenchus willdeteriorate very quickly whereas thosewith root-knot will generally lastthrough storage. RemovePratylenchus-infested yams from thestorage as soon as you see them. If thedamage is slight, they can be cookedand eaten. But heavily-damaged yamsmust be burned or buried very deep(too deep to grow).

Yam attacked by root-knotnematode (Meloidogyne).

Yam with Pratylenchus nematode–the flesh is yellow with brownmarkings, and the skin is cracked.

Good yam–The fleshshould be white.

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Insect AttackMEALYBUG (Planococcusdioscoreae) and scale insect(Aspidiella hartii) attack stored yams.

As they feed, these insects reducethe food reserves in the tubers. Thisboth reduces the value of the yams asfood, and also makes the yams tooweak to sprout vigorously when usedas planting setts.

Other insect pests such as theYAM MOTH (Blastobasis) andGINGER WEEVIL (Elytroteinus),damage yams in storage. They layeggs on stored yams and when theseeggs hatch, the larvae bore into theyam and eat the flesh. As the larvaefeed, they leave piles of browndroppings which can be seen on theoutside of the yam. The larvae maturein the rotten yam.

Yams infested with mealybug,scale or other insects should beremoved and eaten or taken to marketas soon as you see them. Otherwisethe insects will multiply and spread toother yams in the store.

RotFungal and bacterial rots cause muchlose in storage. Often the pathogenswhich cause these rots invade the yamsthrough wounds and nematodedamage.

REMOVE ALL ROTTING YAMSand burn them or bury deep or feed tothe pigs.

SunscaldWhen yams are being harvested, theyshould not be left in strong sunlight fora long time. This can lead to sunscald.The skin of the tuber is damaged androt can set in.

SHADE ALL FRESHLY-HARVESTED YAMS with leaves orgrass if there is a delay in movingthem out of the sun into the store.

SproutingWhen the dormancy of stored yams isfinished, they will sprout. As sproutsgrow, they exhaust the food stored inthe yam. Therefore, sprouts should betrimmed short once per week to slowdown the rate at which food isexhausted.

Mealybug attack–clustered around the head

Scale insect attack–small white scabs allover the skin.

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Curing

Stored yams will keep longer if theyare cured before they are placed in thestore. Curing helps heal wounds andtoughen the skins.

Curing should be carried outIMMEDIATELY after harvesting andtransporting yams to the storage area.Curing requires high temperatures andhigh humidity and is done under acover which traps self-generated heatand moisture.

Temperatures under this covershould be between 32-40oC.

One day of curing is enough if thetemperature is near 40oC and therelative humidity is 95%. But 2 to 4days are needed when temperaturesand relative humidity are lower.

One way to cure yams is to make astack on the ground. The stack of yamsshould be put in a lightly shaded area.If the yams still have damp soil onthem, it will help to keep the relativehumidity high.

Cover the stacked yams with grassor mats. Then place a CANVAStarpaulin over the whole stack. Thecanvas should cover the grass or mats,but be sure the canvas DOES NOTTOUCH THE YAMS. As an alternative,a simple wooden frame can be built andthe canvas draped like a tent over thepiled yams. Again, the canvas shouldNOT TOUCH THE YAMS.

Plastic sheets should not be used forcuring. Plastic will make the yams toohot. If there is no canvas tarpaulin, thenseveral layers of sacks or mats can beused.

The stack of yams should be left tocure for 1 to 4 days. NO LONGER.Check to be sure temperatures do notget higher than 40oC.

Cut-away view of yam curing in a stack on the ground.

Canvas tarpaulin or jute bags.NOT PLASTIC.

At least 15 cm (6 inches) depthof cut grass or mats is placedon top of the yams.

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Here in the Pacific, there is alsoanother way to cure yams. A truckwith a canvas tarpaulin cover makes agood curing shed. Put the baskets ofyams inside the truck and roll downthe tarpaulin on all sides and park thetruck in light shade for 1 to 4 days.

Great care is needed whenhandling the tubers after they havebeen cured so that new injuries are notmade.

Note that curing may make theyams taste a little sweeter. If thissweeter taste is not liked by the peoplein your location, then curing cannot berecommended.

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