Photo 1. Top and underside of a cowpea leaf showing the large reddish brown spots Pseudocercospora cruenta, mostly restricted by the veins.
Photo 2. As the spots caused by Pseudocercospora cruenta enlarge and merge, the leaves yellow and die.
Cowpea Cercospora leaf spot
Pseudocercospora cruenta. Previous names are Cercospora cruenta and Mycosphaerella cruenta (the sexual state). Note that some taxonomists consider that this fungus is the same as Cercospora canescens (see Fact sheet no. 301).
Worldwide. In the tropics and sub-tropics. Asia, Africa, North, South, and Central America, the Caribbean, Oceania. It is recorded American Samoa, Australia, Fiji, Nauru, New Caledonia, Niue, Papua New Guinea, Samoa, Solomon Islands, and Tonga.
Many legumes, e.g., Canavalia galeata (sword bean), Lablab purpureus (hyacinth bean), Mucuna aterrima (Mauritius bean), Mucuna puriens (velvet bean), Phaseous lunatus (lima bean). Phaseoulus vulgaris (French bean), Vigna marina (sea bean), Vigna radiata (mung bean), Vigna unguiculata ssp. sesquipedalis (long bean), Vigna unguiculata ssp. unguiculata (cowpea). In Africa, bambara groundnut (Vigna subterranea) is also a host.
A serious leaf disease of cowpea, but also a problem on bambara groundnut (West Africa), mung and French (or common) beans. Reddish-brown fungal spots occur, up to 15 mm diameter, circular to angular, merging, often with a yellow halo (Photo 1). Sometimes the spots are limited by the veins. Infected leaves yellow and fall prematurely (Photo 2).
On mung bean, the centes of the spots become grey or white.
Spread is by spores blown in the wind, and splashed in rain. Survival is in debris left after harvest, and on alternative hosts. In Fiji, the disease is worse during cool times with tempeatures of 20-25°C.
The disease is important in West Africa. Pod yields (numbers of pods and number of seeds per pod) are reduced by the leaf infections, and lower yields up to 40% are likely on susceptible varieties. Similar findings have been reported from the USA, where numbers of pods per plant are likely to be 25% lower on susceptible varieties. The impact of the disease has been reduced in the last 15-20 years because of the use of resistant varieties. However, new fungal strains are present, and there is always the potential for severe outbreaks of the disease.
Look for large reddish-brown spots (up to 15 mm), merging and causing leaves to yellow and fall. Look for the dark powdery spore masses on the spots on the lower leaf surfaces (Photo 3). When held to the light the older spots show a distinct ring around them. Dead areas fall out giving a shot-hole appearance.
There are resistant cowpea varieties. Screening for resistance to Pseudocercospera cruenta has also involved evaluation for resistance to Cercospora canescens. Resistance has also been found in mung bean varieties.
If susceptible varieties are grown, and weather conditions occur that experience suggests will result in disease outbreaks, spray with mancozeb. Begin after the crop has flowered and pods have started to develop. The number of sprays should be limited to 2-3 per crop.
AUTHORS Grahame Jackson & Eric McKenzie
Information from Cercospera leaf spot of cowpea Mycosphaerella cruenta. Africa Soil Health Consortium. Plantwise. (http://africasoilhealth.cabi.org/wpcms/wp-content/uploads/2015/02/38-legumes-cercospora-leaf-spot.pdf). Photos 1&3 Howard F. Schwartz, Colorado State University, Bugwood.or. Photo 2 (taken by Eric McKenzie), and used in this fact sheet, appeared previously in McKenzie E (2013) Pseudocercospora cruenta PaDIL - (http://www.padil.gov.au).
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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