Giant swamp taro corm rot

Radopholus similis

Asia, Africa, North, South and Central America, the Caribbean, Europe, Oceania. It is recorded from American Samoa, Australia, Cook Islands, Fiji, French Polynesia, Guam, Federated States of Micronesia, New Caledonia, Niue, Palau, Papua New Guinea, Samoa, Solomon Islands, and Tonga. On giant swamp taro, the disease has only been recorded from Guam, Palau, and Yap State (Yap island and Ulithi atoll), Federated States of Micronesia.

Radopholus similis has a wide host range. It is especially common on citrus and its relatives, on coconut and other palms, bananas, black pepper, brassicas, grasses, tea, and members of the coffee, ginger and tomato families. The nematode is associated with a serious corm rot of giant swamp taro, Cyrtosperma merkusii, in Yap (see Fact Sheet no. 257). It is also a major pest of ginger in Fiji (see Fact Sheet no. 161).

Corm rot of taro has an interesting history. Surveys in 1957/58, in the then Trust Territory of the Pacific Islands, were the first to document "a firm rot of irregular areas around the surface [of the corm] which penetrates about one inch", and control should be sought by having resistant varieties. It was thought that Pythium, an oomycete organism, caused the rot. In later reports, the problem was said to be present in Koror and Kayangel (Palau), Pohnpei and Guam. 

Plants with corm rot do not show symptoms on the leaves, except farmers say that the leaves die earlier than expected.

Externally, corms look as if insects have attacked them; holes are present, 5-20 mm diameter and 10-20 mm deep (Photos 1&2). Beneath these holes, shallow brown rots are present that occasionally penetrate to the centre of the corms. Plants with corm rot show extensive loss of fine feeder roots (Photo 1).

Radopholus similis occurs in the rots in large numbers, but relatively few are found in the roots. The nematode has a spear in its mouth which it uses to pierce the cells of the roots and corm to suck up the contents (Photo 3).

Spread of the nematode over short and long distances is on or in the corms and roots of giant swamp taro. Survival occurs in these plant parts, too. It is not known if the nematode is free-living in the soil of the swamps or pits. It is unusual to find Radopholus similis in this situation. It was thought not to survive long in flooded soil. For instance, one of the recommended ways to control the same nematode in banana is to flood fallow the soil for 8 weeks. Possibly, in the case of giant swamp taro, the nematode lives only in the corms and roots and not in the soil.

In Yap, the corm rot of giant swamp taro is a major disease. The general appearance and growth of the plants is unaffected by corm rot, although growers notice that leaves die more rapidly compared to those of healthy plants. However, the rots are deep, and a considerable amount of corm has to be removed before they are fit for human consumption.

Look for the dry corm rots, often shallow over the surface of the corms, occasionally extending as narrow channels to the centre. Look for blackened fine feeder roots on plants with corm rot. Often, roots show considerable decay, but leaves appear healthy.

CULTURAL CONTROL
Cultural control measures offer the only practical ways of reducing corm rots caused by Radopholus similis on giant swamp taro. In general, it is extremely difficult to control pathogens attacking underground storage organs of plants growing in a swamp.

Before planting:

• Clean the planting material should be done as follows: (i) remove all roots; (ii) remove all the old outer leaves; (iii) check the corm part for rots, and if present cut out the rots using a knife wiped in 1% bleach; and (iv) wash the planting material to remove soil
• Alternatively, use a hot water trreatment (see below)

After harvest:

• Cut the corms from the stem and prepare for consumption (or for planting - see above), but do not leave the rejected parts on the bunds surrounding the swamps or the giant swamp taro pits; otherwise, these pieces will be washed into the swamps or pits together with the nematodes that infect them.

HOT WATER TREATMENT
Hot water is used to disinfect Radopholus similis in banana suckers, and may be a method suitable for disinfecting planting material of giant swamp taro. However, it has not been tested. Use as follows:

• Place suckers in hot water at 50°C for 20-25 minutes, OR
• Use the method developed by IITA, the International Institute of Tropical Agriculture, for banana suckers: dip suckers in boiling water for between 20 and 30 seconds. (This method has the advantage that a thermometer is not required, only a watch with a second hand. If no watch, take 30 pebbles, place them on the floor and pick them up one at a time and put into a small container). Note, this test applies to banana and plantain only.
• Use either method with caution: Hot water treatment should be used only to grow plants that can be used as nematode-free planting material. It needs to be tested on a few plants and shown to be safe.

RESISTANT VARIETIES
At Thor village, Yap, Federated States of Micronesia, the varieties Ponape, Hawaii and Biligmao are said to show less rot than varieties Fattew, Aduvwe, Flan and Uchwo. At Dugor, variety Yurar is said to have improved resistance over Chath and Wangmil - although the rot was not a serious problem there. 

CHEMICAL CONTROL
Not appropriate for this disease.