Photo 1. Pratylenchus coffeae in yam causes a shallow rot beneath the skin; the importance of the rot is not only the loss of flesh for eating, it is also the loss of planting material. Yams infected with dry rot do not sprout or, if they do, the cutting will be infested with nematodes which will attack the roots when the setts are planted.
Photo 3. The head of Pratylenchus sp. showing the hollow spear or stylet (arrow), which is used to puncture cells in order to move through the roots, as well as to feed on their contents.
Photo 5. Banana roots with purple/black discolouration caused by Pratylenchus coffeae. Infected roots rot and plants are weak and yields are small.
Photo 6. "Toppling" is a common symptom on banana when roots are attacked by Pratylenchus coffeae. Note this banana has fallen over before the fruits have matured; a sign of nematode attack. A similar symptom occurs when bananas are infected by the nematode Radopholus similis.
Dry rot of yam, brown dry rot, banana root nematode, toppling of banana
Worldwide. Asia, Africa, North, South and Central America, Europe, Oceania. It is recorded from Australia, Cook Islands, Fiji, Kiribati, Niue, Palau, Papua New Guinea, Samoa, Solomon Islands, Tonga, and Vanuatu. The nematode is a major problem on yam (field and storage) in parts of Papua New Guinea, Solomon Islands, and Vanuatu.
In Pacific island countries, lesion nematodes cause major diseases on yam (Dioscorea alata) and banana (Musa species), and surveys have found them associated with many other crops, although specific diseases have not been identified. Many weeds are also hosts. Note; the burrowing nematode, Radopholus similis, causes a similar disease on banana (see Fact Sheet no. 257).
Roots are infected and killed, and entry into the developing tubers causes a shallow brown, dry rot, that continues in storage (Photos 1&2).
Pratylenchus has six life stages: egg, juvenile (4) and adult (Diagram). The nematodes lay eggs in the root, or in soil close by. Eggs hatch in a few days and the young nematodes, called juveniles or larvae, moult several times before they become adult. The larvae and adults enter the roots and then force their way between or through the cells using their hollow stylets or spears (Photo 3) to suck out the cell contents.
The life cycle is 2-3 weeks, depending on moisture and temperature. When the cells die, the nematodes migrate through the root (Photo 4) or tuber in search of healthier parts, or they return to the soil and search for another root. The nematodes spread short distances in soil water. They are spread long distances on yam setts and banana suckers.
Young and adult nematodes feed on roots of weeds and on the roots of other crops between crops of yam and banana. They also remain alive in stored yam tubers. When infected yam tuber pieces or banana suckers are planted, the nematodes emerge and move through the soil until they find young roots to infect.
On yam: damage to roots prevents the plant from taking up water and nutrients. When roots are damaged, plants become stunted and/or die early. Often, symptoms of the disease are not seen until the yams are harvested because: (i) in many yam gardens the foliage of different plants is mixed together, making it difficult to see individual plants; and (ii) damage done by nematodes often comes late, so symptoms of poor growth can be mistaken for early maturity.
Damage to the tubers does not stop at harvest. The nematodes continue to multiply during storage, and the brown, shallow, dry rots spread further (Photos 1&2). Often, other organisms help with the decay. Decay of the outer areas of the tubers means that shoots do not develop and, consequently, there is a loss of planting material.
On banana: Spots appear due to the nematodes feeding and breeding inside the roots; later the spots join together forming characteristic red and/or black patches - best seen when roots are split (Photo 5). The roots die and the outer parts rot. The result is a weak, stunted root system; plant growth is slow, fruiting is poor and plants readily fall over in the wind (Photo 6).
Look for signs of dry rot on yam tubers at harvest, and in storage. Look for bananas that have blown over, and inspect the roots for red and/or black patches.
There is no information on resistance among Dioscorea alata yam varieties. However, trials have found that the banana variety FHIA-01 has tolerance to Radophilus similis, so it would be worth testing it against Pratylenchus coffeae. Tolerance to Pratylenchus is also reported for Yamgambi km5. Both these varieties are in the collection of varieties held by the CePaCT lab Secretariat for the Pacific Community.
HOT WATER TREATMENT
Chemical control is not a method that can be used for the management of these diseases.
AUTHORS Helen Tsatsia & Grahame Jackson
Information (and Diagram) from Davis EL, MacGuidwin AE (2000) Lesion nematode disease. The Plant Health Instructor. DOI: 10.1094/PHI-I-2000-1030-02. Updated 2005; and from Hauser S, Coyne D (2010) A hot bath cleans all: Boiling water treatment of banana and plantain. (https://www.ctc-n.org/sites/www.ctc-n.org/files/resources/4ea6bfcd-2658-4dac-bf31-03861661b3dc.pdf). Photo 1 V Vanstone Pratylenchus penetrans: a horticulturally significant root lesion nematode. Department of Agriculture and Food, Government of Western Australia. Photo 4 John Bridge, CABI, UK.
Produced with support from the Australian Centre for International Agricultural Research under project PC/2010/090: Strengthening integrated crop management research in the Pacific Islands in support of sustainable intensification of high-value crop production, implemented by the University of Queensland and the Secretariat of the Pacific Community.
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