Growing sweet potatoes in Western Australia

Introduction

The sweet potato (Ipomea batatas) is a member of the morning glory or Convolvulaceae family. It is not related to the common potato. The plant has long trailing slender stems and is perennial, but is treated as an annual in cropping.

The tuberous root has high food value, fibre and energy. It is rich in sugar and vitamin C and contains good quantities of vitamin A, vitamin B, calcium and iron.

Climate

The sweet potato is a semitropical plant that grows best between 20 and 30°C. A minimum frost-free growing season of four to six months is needed, with a minimum of cool, cloudy weather. Plant growth is restricted below 10°C and plants are physically damaged at 1°C.

Soils

Sweet potatoes grow best on well-drained sandy loams, although sandy soils produce good crops if well fertilised and watered. Heavy and swampy soils tend to produce rough, misshapen roots.

The pH of the soil measured in calcium chloride should ideally be 5.8 to 7.0.

The yield of roots declines rapidly as salt levels rise. The soil conductivity should be less than 25 to 50mS/m.

Cultivate to provide 20 to 30cm of well worked soil. Deep ripping with a tined implement may improve soil drainage and root shape.

On sandy soils, the crop can be planted on flat or ridged ground. Ridged ground facilitates machine harvesting. On heavier soils and in situations where waterlogging may occur, grow the plants on 20 to 25cm high ridges.

Rotation

Sweet potatoes should be rotated with other crops to prevent build-up of diseases such as scurf and to manage root-knot nematode. Plant sweet potatoes on the same soil only once every two to four years.

Planting material

Sweet potatoes are vegetatively propagated from cuttings. Select storage roots from healthy, high yielding plants. Good quality sweet potatoes may be bought in the markets for planting material, although market agents and growers may not know the names of the varieties.

Choose roots from a uniform consignment. They should be well formed, with bright skins and no trace of disease. To plant one hectare, you will need 400 to 500kg of seed potatoes. About 1000 tip cuttings are produced from 20kg of roots.

Preparing cuttings

New growers should make a seed bed in a warm area by planting roots end-to-end in rows 5 to 10 centimetres apart and 3 to 4 centimetres deep. In Perth, do this in August to September. Clear plastic may be placed over the plants.

In other areas, wait until the soil temperature is above 15°C. Take cuttings from the seed bed when the shoots are 15 to 20cm long. Cuttings should not be contaminated with soil. Some leaves may be removed from the base of the cutting, but this is optional.

Cuttings may be rooted in water before planting, but this is not essential. Place bundles of cuttings in 4 to 5 centimetres of water for 48 hours. Do not immerse the tip of the cutting in water or the growing point will rot. Rootlets may be just visible at planting, but should not be longer than two millimetres.

Established growers take cuttings from the best plants of the previous year’s crop. They may protect these plants with shade cloth in winter and early spring.
Do not pull slips from the old tubers as they may carry root diseases, especially scurf. It is preferable to cut off shoots (cuttings) with secateurs which have not been in contact with soil and are free of scurf.

Cuttings are normally planted by hand but a mechanical transplanter may also be used. Plant cuttings with 5 to 7 centimetres of the tip exposed and with the lower half horizontal in the ground. Space rows at 90cm, with 25 to 35cm between plants in the row. Plant closer if experience shows that a variety will produce roots which are too large at a wide spacing. Another system is to plant on ridges 1.2m apart formed by banker discs, with double rows at 0.3m and 0.5m within the rows.

Sweet potatoes can be planted at Carnarvon for most of the year, but growers should target periods of low market supply to maximise financial returns. For this purpose, plant between July and September in Carnarvon for harvest from December to February, and in March for harvesting from August to September.

In Broome, sweet potatoes are mainly harvested from December to March.

In Perth, plant from late September to late December and harvest from February/March to May/June.

Varieties

Several different types of sweet potato are available in Western Australia. New varieties are being released regularly.

Contact seed suppliers to determine the best variety for your location and cropping time.

Irrigation

Soil must be kept moist after planting to ensure good establishment of the cuttings.

On sandy soils, daily irrigation of established plants is recommended, although this will vary with temperature, wind speed and the stage of crop development. Yields and quality are seriously affected if the crop is stressed when the harvest roots begin to develop about 50 to 60 days after planting.

Water quality is important and the conductivity of water used for irrigating sweet potatoes should be less than 170mS/m.

Do not overwater as plants mature as this may cause rotting and skin cracking.

Plants are usually watered by overhead sprinkler, but may also be watered by trickle irrigation. The latter must be removed before harvesting. There have been no trials on the watering requirements of sweet potatoes in Western Australia.

Fertiliser

Sweet potatoes require less fertiliser than most other vegetables.

Compost at up to 50 cubic metres per hectare before planting will supply organic matter, nutrients and help retain moisture in the soil.

On sandy soils, apply the following rates of magnesium and trace elements before planting:

The most common trace element deficiencies are boron, manganese and iron. These elements should be applied immediately before planting and, if deficiencies are noticed in the leaves, they may also be sprayed directly on the foliage or applied to the soil after planting.

Use the following program to supply nitrogen, phosphorus and potassium:

Sandy soils

Before planting, apply:

• 200kg/ha double superphosphate

Starting one week after planting until three weeks before harvesting, apply weekly:

• 20kg urea, 25kg/ha muriate of potash

At running, apply once:

• 50kg/ha magnesium sulphate

Loamy soils

Before planting, apply:

• 500kg/ha double superphosphate, 200kg/ha muriate of potash
• 200kg/ha urea

Carnarvon

When plants start to run, apply:

• 50kg/ha urea

Two weeks later, apply:

• 50kg/ha urea

Apply a fertiliser before planting to supply phosphorus. If soil analysis before planting shows high levels of bicarbonate extractable phosphorus, apply less phosphorus. Nitrogen, phosphorus and potassium are easily washed through sandy soils by rain and irrigation. Do not overuse fertiliser, because nutrients leach into lakes and rivers leading to pollution.

Analyse soil and irrigation water before planting, plus one to two analyses of the youngest mature leaves after planting. This lets you adjust your fertiliser program and provides information on nutrients that are deficient or toxic. Some of the suggested nutrients may be deleted or reduced if they are sufficiently high in the irrigation water and soil, including sources from compost and fertilisers from previous cropping.

Weed, pest and disease control

The registration and availability of chemicals for disease, pest and weed control change regularly. Consult a trained and experienced horticultural agronomist or the Australian Pesticides and Veterinary Medicines Authority (APVMA) website for chemicals which are currently registered or have a permit for use on this crop.

The information on the label or permit for a chemical must be followed, including the directions for use, critical use comments, withholding period and maximum residue limit. Quality assurance (QA) schemes for horticultural crop production require producers to have current information on chemical registrations and permits readily available.

Weeds

Many growers control weeds mechanically. During early crop growth, shallow cultivation between rows and hand weeding will control weeds. Once plants cover the ground, the crop tends to smother the weed growth, except a few tall-growing weeds.

Pests and diseases

A range of pests and diseases affects sweet potatoes, although the severity can vary between seasons. Some of the major pests are outlined below.

Sweet potato leaf miner

The major insect pest in Western Australia is the sweet potato leaf miner. This is a small, dark reddish to black grub up to 10mm long. The adult is a small moth. The grub eats plant tissue between the upper and lower leaf surfaces, leaving a transparent papery ‘window’ in the leaf. The grub can be seen inside this window. Heavy infestations can develop quickly and severely defoliate the crop, reducing yield.

Silverleaf whitefly

Both adults and immature stages of whitefly suck sap adversely affecting plant health. The small, white adults are the first sign of infestation. Whitefly is most abundant in warmer times of the year. The underside of leaves should be checked closely to assess insect numbers and action taken early to avoid a build-up of this insect.

White-fringed weevil

Larvae of white-fringed weevil may damage tubers in the south west of WA. They are more common following pasture.

African black beetle

African black beetle causes severe damage by chewing holes in sweet potato roots and tubers may be damaged also. It is common in crops following pasture.

Rutherglen bug

Rutherglen bug can appear in large numbers in November and December. The small bugs delay plant establishment. Rutherglen bug breeds on weeds, particularly portulaca, and eradication of weeds near the crop will reduce problems.

Two-spotted mite

Two-spotted mites will attack sweet potatoes. They are usually first noticed by the appearance of the leaves, which begin to look scorched, russetty and dry. The mites are cream coloured, with two dark spots on their back. They live on the underside of affected leaves and are just visible to the naked eye. The mites suck the sap of the leaves and breed very rapidly in warm weather.

Root-knot nematode

Root-knot nematode causes malformations and cracking of storage roots. They are a soil-borne pest with a large number of hosts. If galls are present on the roots of susceptible weeds such as nightshade, or nematodes have affected a previous crop, determine a control treatment for nematodes prior to planting.

Field mice

Field mice may cause substantial damage to roots in the field.

Scurf

Scurf (Monilochaetes infuscans) is a major problem for most growers in Western Australia. It is a soil-borne fungal disease that causes greyish brown spots and large blotches on the skin of roots. It reduces marketability and keeping quality of roots.

Scurf is introduced to new properties on diseased roots used for seed. Slips taken from infected roots effectively spread the disease throughout the growing field. Once established in the soil, the disease can survive for two to three years in light soils and three to four years in heavy soils or soils rich in organic matter.

Control scurf using the following measures:

• Do not over-water.
• Plant cuttings only.
• Choose light soils.
• Do not plant sweet potatoes more than once every two to four years on the same plot of land.
• If scurf is present in the soil and a long rotation cannot be practised, reduce the disease level by fumigating with registered products.
• Do not store sweet potatoes in the ground.

Fusarium rot

The fungus Fusarium causes sweet potato roots to rot. Roots develop soft spots which gradually enlarge. It affects some varieties more than others and causes most damage in storage. In the field, the rot often starts when the roots have been damaged by cold or wet soils. It progresses rapidly within the root and under favourable conditions, the whole root may break down in a few days.

Harvesting

Roots are ready for harvesting 14 to 20 weeks after planting. Periodically, dig a few roots to check their maturity. They can be dug as soon as they are of a marketable size. Excessive size, cracking, breakdown and disease build-up may occur if harvesting is delayed too long.

Remove vines before digging the potatoes. They can be cut off by hand or slashed mechanically with a flail type pulveriser or rotary slasher.

The sweet potato is extremely sensitive to bruising. Most growers dig sweet potatoes by hand. This minimises damage to the skin but is very labour intensive. It is possible to modify a potato harvester to successfully harvest sweet potatoes, but this may reduce quality and give rise to more volunteer crops. Good yields are 20 to 30t/ha.

If frost kills the vine, dig up roots as soon as possible since quality declines quickly. Potatoes rot quickly in cold wet soils.

In Perth, sweet potatoes are often ground-stored. A plastic cover 2.5m wide is placed over the whole row in May to preserve quality. This may be removed in late August when there is less rainfall. By this means, roots may be stored up to December, but yield and quality will decrease with increasing ground storage. Roots must be harvested as soon as possible if inspection shows any signs of scurf on the roots.

Storing roots

Select sound, marketable roots. Do not wash roots intended for storage. Harvested sweet potatoes store well for one to two months under ambient temperatures but, for prolonged storage need special treatment.

They must be ‘cured’ by holding them at 29°C with a relative humidity of 90% for seven to ten days.

Curing:

• heals cuts, bruises and skinned areas to prevent decay
• keeps shrinkage and weight loss to a minimum
• improves culinary quality by converting starches to sugars.

After curing is complete, drop the temperature to 13 to 15°C to prevent sprouting. Maintain the relative humidity at 85% to prevent shrinkage. If the storage temperature drops below 10°C, the sweet potatoes will start to deteriorate. Special insulated storage sheds are required for curing and storage. A fan to ensure good air circulation during curing and storage is advisable.

Marketing

Wash and grade roots before packing. Sort potatoes into three grades:

• Small — about 3 to 5cm diameter (No. 1)
• Medium — 5 to 7cm (Special)
• Large — more than 7cm (Jumbo)

The highest price is paid for medium-sized, clean-skinned potatoes. Roots affected by scurf are downgraded in price.

Sweet potatoes are normally marketed by local growers in 36 litre plastic crates which hold about 22kg. Produce is imported from Queensland in cartons.

Arrange the roots in an orderly manner to present an attractive pack. Mark the variety name and grade on the pack. Prices are often lowest from April to August. For the rest of the year, prices may be moderate due to imports of sweet potatoes from Queensland.

Old crops

Old crops may give rise to volunteer plants in other crops. Repeated discings over summer may be required to kill old crops.

Acknowledgements

Margaret Graham, Neville Burton and John Burt authored the original versions of this material.