Photo 2. Ginger with soft rot, Pythium sp., showing drying-up of the leaves and collapse of the plants.
Soft rot, rhizome rot
Pythium spp. Pythium myriotylum is the most serious of the species on ginger.
Worldwide. In the tropics and sub-tropics. Pythium myriotylum is recorded from Australia, Fiji, New Zealand, Papua New Guinea, Samoa, Solomon Islands, and Vanuatu. In Fiji, it is highly pathogenic on ginger.
Ginger, taro (Colocasia), giant taro (Alocasia), Talo tanna or Taro Palagi (Xanthosoma), beans and capsicum are susceptible (see Fact sheet no. 44). Many kinds of seedlings are susceptible to a damping-off disease in the nursery (see Fact Sheet No. 47).
Pythium is not a fungus; it is an oomycete, related to algae. The common name is water mould. The water mould lives in the soil on roots of ginger and other crops, the remains of dead plants, on or in 'seed' (rhizomes) used for planting, and weeds. When conditions are not right for growth, the water mould produces thick-walled resting spores called 'oospores' (Diagram), and these can stay alive in the soil or in the rhizomes for a long time, waiting for a susceptible host to stimulate their germination.
The rot that occurs on ginger is typically a wet weather disease, influence by heavy rains after planting. Infection occurs when the water mould in the soil, or inside rots in the rhizome (the 'seed' or planting piece), produces spores (Diagram). Swimming spores are formed inside larger spores and, when released, find their way to the fine roots, the young buds (or 'eyes') on the rhizome, or the junction of stem and rhizome. Damage to the rhizomes (Photo 1), roots and stems causes plants to yellow, and stems to collapse (Photo 2). At this stage, stems are easily pulled from the rhizomes.
Spread of the water mould occurs when the spores swim short distance in the water between soil particles, or are carried for longer distances in rainwater through the soil or over the surface. Neighbouring plants are infected, and patches of yellowing plants develop in the field. Spread occurs over long distances in Pythium-infected rhizomes used for planting.
Pythium myrotylum causes a serious disease. In Fiji, losses from soft rot are common, with 30-100% of the fields destroyed in the wetter parts of the country. The water mould can destroy rhizomes in 1-2 weeks. Impact of the disease is reduced if the crop is grown for immature ginger, and harvested early. However, losses still occur in the crop remaining in the field for "seed", that is, planting material for the next crop.
Look for patches of unhealthy yellow, leaves. Look for dead roots and/or decay to large parts of the root system; look for rots on the rhizomes, especially at the buds.
Cultural control is particularly important in the management of this disease; in fact, it is the only practical way to manage it:
Although regular applications of metalaxyl or phosphorous acid can control soft rot, the costs involved are likely to make ginger cultivation uneconomic, and cannot be recommended.
AUTHOR Grahame Jackson
Photo 1 Robert Fullerton, Plant & Food Research, Auckland, New Zealand.
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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