Photo 2. Bele wilt, Phytophthora nicotianae, showing the death of the young shoot at the top of the plant.
Photo 3. Highly magnified photo of zoospores of Phytophthora nicotianae, showing the whip-like hairs used for movement (each zoospore has two).
Photo 4. Root rot caused by Phytophthora nicotianae. Note the lack of roots, and that the lower stem has lost its bark, exposing the wood.
Bele Phytophthora wilt
Phytophthora nicotianae; previously, Phytophthora nicotianae pv. parasitica.
Worldwide. The disease has been identified on bele in Fiji, and symptoms seen on bele in Solomon Islands, although these have not been verified. Phytophthora nicotianae is also present in American Samoa, Australia, Cook Islands, Federated States of Micronesia, Fiji, New Caledonia, Niue, Papua New Guinea, Samoa, Tonga, and Wallis & Futuna, but not on bele in those countries (see Fact sheet nos. 154, 157 & 264).
Bele (aibika, island cabbage or sliperi kabis, Abelmoschus manihot). Note, this is the same water mould (or oomycete) that causes a similar disease on tomato (root and a firm dark fruit rot, see Fact Sheet no. 157), and also attacks capsicum, citrus, papaya, passionfruit (see Fact Sheet no. 154) and pineapple.
The disease is favoured by high soil temperatures, and soils saturated with water due to high rainfall or poor drainage. Waterlogged soils starve roots of oxygen that plants need for healthy growth, and the water allows the swimming spores of Phytophthora to spread, infect the roots and cause the bele to wilt (Photos 1&2).
Water moulds survive in the soil as thick-walled resting spores called 'chlamydospores'. When conditions are right, the chlamydospores germinate and produce spores called "sporangia". Yet another spore is produced inside the sporangia called 'zoospores' (Photo 3), and these are capable of swimming short distances in the water between soil particles (the thread-like processes coming from the zoospores in Photo 3 are for swimming). If the zoospores reach the fine feeder roots of susceptible plants they germinate and infect them. Infections also occur at the collar region, the part of the stem at soil level.
Once inside the plant, the water mould develops more sporangia and zoospores.
The root system rots, and the outer layers of the stem become soft and peel away exposing brown areas of rot beneath (Photo 4). Aboveground, the plants wilt, lose their leaves, and soon after collapse and die (Photo 5).
The water mould spreads in rain splash, surface water, and over long distances in soil on machinery and shoes.
The disease is caused by a species of Phytophthora; it is an oomycete or water mould, not a fungus. Although they look like fungi, Phytophthora species are related to algae. The water mould infects the roots, and stem of bele, especially at soil level, causing a collar rot. Depending on rainfall, the disease may spread quickly causing plants to wilt and die (Photos 1&2). In general, it is not a common disease and the extent of damage on bele is not high, although individual plants are killed by the root and collar attack.
Look for plants that wilt during the warmest parts of the day although the soil is still moist. At an early stage of the disease, look at the root system of wilting plants to see if the fine feeder roots are dead, and if there is any damage to the stem at soil level.
Cultural control is particularly important in the management of this disease:
None are known.
Chemical control is not a method that can be used against this soil borne pathogen.
AUTHOR Grahame Jackson
Photos 1&3 Mike Furlong, University of Queensland. Photo 2 AR Hardham, Plant Cell Biology Group, Research School of Biology, The Australian National University, Canberra, Australia. Photos 4&5 Pita Tikai, ACIAR ICM/IPM project, Solomon Islands.
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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