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.