13[7] Elsinoe [EDFS

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This leaflet is produced by the Ministry of Agriculture & Livestock, Solomon Islands, with support
from IPPSI: Improved Plant Protection in Solomon Islands, a project financed by ACIAR, the
Australian Centre for International Agricultural Research, Canberra. Authors: Helen Tsatsia, MAL &
Grahame Jackson, TerraCircle Inc.

Extension Fact Sheet 13:

Sweet Potato Scab

Common name: Sweet potato scab

Scientific name: Elsinoe batatas, but sometimes known as

Sphaceloma batatas

(the asexual stage)

Hosts: It is only known from sweet potato.

Damage

Young leaves are infected along the veins, and also the leaf stalks and stems. As vines
grow, the leaves twist, curl and tear. On older leaves, pin-point spots occur between
the veins. In severe cases (not seen in Solomon Islands), shoot-tips are killed.

Varieties differ in the amount of damage caused. The disease has been severe in Fiji
and Tonga. In Tonga, there were epidemics of the disease in the 1980s when farmers
stopped growing most varieties. Damage to the young shoots can slow early growth
and result in reduced yields. In Papua New Guinea highlands, comparisons between
healthy and diseased plants, showed a 60% difference in yield of storage roots.

Biology and Life Cycle

The fungus is taken to new gardens on planting material. As the cuttings grow, the
fungus produces very small spores in the scabby areas. These spores are spread by
rain-splash from plant to plant. They germinate, like seeds, in water on the surface of
the leaf and stem, penetrate and cause the spots and scabby lesions. Soon after
infection, more spores are produced.

It is possible that the fungus survives in crop debris, but this is not important unless
crops are planted one after the other. Most spread is from planting infected vines.

Signs and Symptoms

Look for the brown scabby marks on the leaf veins, stalks and stems. Look for torn,
cup-shaped leaves and twisted stems.

Management

Cultural control:

There is probably little that farmers can do to control this disease using cultural
methods. If the disease is severe in the crop, it might be better not to replant on the
same land, but it is doubtful that it will make any difference. This is because the
disease is spread from crop to crop in already infected planting material, and also
from spores in wind-driven rain.

If growers really want to grow susceptible varieties, because of their taste and high
market demand, for instance, then they should do the following. It is very important
that they start the crop with disease-free planting material.

•  Make a nursery – raised beds, shaded by coconut leaves – and plant washed sweet

potato roots, leaving a small gap of 1-2 cm between each;

• Collect 30 cm vines when shoots have grown, checking each one to ensure that

scab is not present;

• Plant vines in new gardens, where sweet potato has not been grown for 1-2 years.

Vigorous early growth will reduce the impact of the disease when the plants become
infected later.

Resistant varieties:

This is the most important method of control. Varieties differ in their susceptibility to
the disease. Some are very resistant. These should be selected and grown in
preference. It is for this reason that scab is not usually a problem in Solomon Islands.
Farmers avoid those varieties that show severe symptoms.

Chemical control:

Fungicides have been used to control the disease in Tonga and Papua New Guinea,
but mostly by scientists. They could be used by commercial farmers, for instance,
who want to grow a susceptible variety for the market. The recommendation is to:

•  Dip the cuttings for 15 min in mancozeb before planting;
•  Spay with mancozeb at the first appearance of symptoms;
•  Repeat at 14- day intervals, until 1-2 months before harvest, depending on the

weather.