Photo 1. Brown to grey area of infection covering the entire lower stem and girdling the plant caused by charcoal rot, Macrophomina phaselina. Tiny black dots are the sclerotia.
Bean charcoal rot, ashy stem blight, damping-off, bean blight
Macrophomina phaseolina. Previosuly known as Macrophoma phaseoli, Macrophoma phaseolina, Macrophomina phaseoli, Rhizoctonia bataticola, Sclerotium bataticola.
Worldwide. In the tropics and sub-tropics. Asia, Africa, North, South and Central America, Europe, Oceania. It is recorded from Australia, Fiji, New Caledonia, New Zealand, Papua New Guinea, and Solomon Islands.
Wide range of hosts in many families. In Pacific island countries, recorded from beans, capsicum, cocoa, and Caribbean pine. Curcubits are also hosts.
The fungus usually attacks stems causing a watery rot at ground level. Rot may spread up the stem and into roots. On seedlings, the spots that develop are black sunken with sharp margins, which may girdle the stem. On older plants, the spots have ash-grey centres with dark margins (Photo 1). Symptoms on seedlings and older plants often occur more on one side of the plant than the other.
Infection of the root and stem prevents the flow of water and nutrients, and plants wilt. Leaves may fall, but infection of leaves is rare. When plants die, numerous small black dots form beneath the surface of the lower stem and tap root. The dots are 'sclerotia', balls of the fungus surrounded by a black protective skin (Photo 1-3). They are so numerous that they give the plants a sooty or charcoal appearance.
Spread is mainly through movement of soil or plant debris containing sclerotia, and seed. Survival is also by sclerotia in plant debris or in soil which remain alive for up to 3 years, depending on soil conditions, less if it is wet. Hot (soil temperatures of about 27°C), and dry weather favours the disease.
Worldwide. large losses (about 5%) have been documented on soyabeans, in the USA. High losses have also been reported in chickpeas, peanut, sorghum, and sunflower. However, it appears to be a minor disease in Pacific island countries, possibly because the hot, dry conditions needed during the growing time are not common to the region.
Look for the ash-grey spots at soil level on seedlings or older plants; often, wilting, yellowing and death is more on one side of the plant. Look for the tiny sclerotia that are characteristic of this disease.
It is unlikely that fungicides will be economic against this soil borne fungus, except as a seed treatment. For seed, treats with thiram (in Australia thiram with thiabendazole) is registered for seed treatment for peas and beans.
AUTHORS Grahame Jackson & Eric McKenzie
Information from Charcoal rot of bean/tobacco (Macrophomina phaseolina). Plantwise Knowledge Bank. (http://www.plantwise.org/KnowledgeBank/Datasheet.aspx?dsID=32134); and from Diseases of vegetable crops in Australia (2010). Editors, Denis Persley, Tony Cooke, Susan House. CSIRO Publishing. Photo 1 David B. Langston, University of Georgia, Bugwood.org. Photo 2 Clemson University, USDA Cooperative Extension Slide Series, Bugwood.org. Photo 3 Paul Bachi, University of Kentucky research and Education Center, Bugwood.org.
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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