Worldwide. Probably wherever cowpea is grown. It is widespread in Oceania and is recorded from American Samoa, Australia, Fiji, Papua New Guinea, Samoa, Solomon Islands, Tonga, Vanuatu.
Mainly on Vigna species [Vigna marina (coastal weed), Vigna unguiculata ssp. sesquipedalis (yard long bean), and Vigna unguiculata ssp. unguiculata (cowpea)].
The biology and life cycle of Uromyces vignae and Uromyces apendiculatus are similar. Uromyces vignae has all five spore stages but completes its life cycle on a single species, Vigna spp. This is in contrast to other rust species that have fewer life stages, and others that have all stages, but more than one host.
The rust fungus forms brown or black pustules on both leaf surfaces, sometimes in small concentric circles. The pustules also occur on leaf stalks and pods. The symptoms are very similar to those of Uromyces appendiculatus which infects Phaseolus species (see Fact Sheet No. 217). The pustules are at first light green, later reddish-brown, with distinct yellow haloes, 1-2 mm diameter.
Spread of rust fungi is by spores blown in the wind. Some spores are specially adapted for survival and have thick dark walls that can survive a long time in the atmosphere, travelling great distances. The hardiness of the spores increases the chance of infection of new bean crops. However, in the tropics, new bean crops often overlap old ones, so that survival of rust is easily assured.
The rust can develop rapidly, resulting in severe leaf damage and defoliation. However, on yard long bean in the Pacific island countries, the rust comes relatively late in the crop and the impact seems relatively small, although there has been no research to establish the impact of the disease on yield.
Look for reddish-brown, circular pustules with yellow haloes on leaves and pods, bursting to produce, large amounts of powdery reddish-brown spores. Expert examination is necessary to distinguish between Uromyces vignae and Uromyces appendiculatus.
There are several ways to reduce the impact of rust on Vigna beans, apart from using fungicides:
There are a number of varieties with resistance to bean rust; contact agriculture authorities or seed suppliers to see if any are available in your area.
If fungicides are needed, use mancozeb. Start to spray when symptoms first appear.
AUTHOR Grahame Jackson
Photo 1 Kohler F, Pellegrin F, Jackson G, McKenzie E (1997) Diseases of cultivated crops in Pacific Island countries. South Pacific Commission. Pirie Printers Pty Limited, Canberra, Australia.
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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