Five cowpea varieties (Vigna unguiculata (L.) Walp) namely UAM 09 1055-6 and UAM 09 1051-1, IT 99k-573-1-1, IT 90k-277-2, IT 99k-573-2-1 were investigated to evaluate the fungi associated with leaf spot disease of cowpea and the severity of leaf spot disease in Makurdi (07o 45’- 7o 50’N and 08o 45’ – 08o 50’E ; 98 m) and Otobi (7° 07’ – 7°11\'N and 8° 05- 8°10\'E) in Benue State, Nigeria under natural infection. The effect of Leaf spot incidence and severity on the growth and yield of cowpea varieties were also evaluated.
Fungi associated with leaf spot lesions were isolated and identified. Diseased leaf samples from the study locations were found to be infected with Fusarium verticillioides, Curvularia lunata, Aspergillus tamarii Kite, Botryodiplodia theobromae, Aspergillus flavus Link and Aspergillus niger van Tiegh, Pythium spp, Fusarium solani, Macrophomina phaseolina and Phoma sp. Otobi field had significantly higher incidence and severity of leaf spot disease than the Makurdi field.
Cowpea variety IT 99k-573-1-1, IT 99k-573-2-1 and IT 90k-277-2 were classified as moderately resistant in Makurdi with a mean incidence of 22.23%, 16.97% and 16.67% respectively while varieties UAM 09 1051-1 and UAM 09 1055-6 were classified as Moderately susceptible and Highly susceptible with mean leaf spot incidence of 41.67% and 99.17% respectively. In Otobi, all the cowpea varieties screened were classified as Susceptible to leaf spot incidence with the exception of variety IT 90k-277-2 which was classified as Moderately susceptible to leaf spot disease.
Key words: Cowpea, leaf spot, screening, resistant, susceptible.
Cowpea (Vigna unguiculata (L) Walp) is a legume cultivated in the tropics for grain consumption as a source of cheap dietary protein and fodder for livestock. Tender cowpea leaves and immature pods are also eaten as vegetables (Dugje et al., 2009). Nigeria is the largest cowpea producer in Africa where cowpea is a source of livelihood for many subsistent farmers (Neya et al., 2015). In Nigeria, Cowpea is cultivated in the drier areas where the rainfall range is between 500 and 1200 mm/year. The extra-early and early maturing cowpea varieties also thrive in the Sahel where the rainfall is less than 500 mm (Dugje et al., 2009). Cowpea production is affected by the incidence of pests and diseases at different stages of development. Leaf spot on cowpea limits the leaf available for photosynthesis resulting in reduced yield. Although Cercospora spp have been previously known as the causal agent of leaf spot on cowpea, leaf spot on cowpea maybe caused by other fungal pathogens due to the emergence of new leaf spot fungi infecting cowpea. There is the need to identify these fungi and to screen the field response of some new cowpea varieties and existing varieties to leaf spot infection in Makurdi and Otobi in the Southern Guinea Savanna of Benue State. This study seeks to investigate fungi inducing leaf spot on cowpea in the study area and to evaluate the response of these varieties of cowpea to infection by these fungi under natural infections.
Two field trials were set up simultaneously at the Teaching and Research Farm of the Department of Crop and Environmental Protection (07o 45’- 7o 50’N and 08o 45’ – 08o 50’E ; 98 m) and National Root Crops Research Institute Otobi Sub-station Benue State, located between 7°07’ – 7°11\'N and 8° 05- 8°10\'E in the Southern Guinea Savannah of Nigeria. Two newly released varieties (UAM 09 1055-6 and UAM 09 1051-1) and three cowpea varieties (IT 99k-573-1-1, IT 90k-277-2, IT 99k-573-2-1) obtained from the Molecular Biology laboratory of Federal University of Agriculture, Makurdi were evaluated in the locations. The sites fall within the Southern Guinea Savannah agro-ecological zone of Nigeria, where the rainfall is bi-modally commencing between March/April and terminating in October/November with the highest peak in July/August. The total annual rainfall is about 2000-2500 mm and maximum and minimum temperatures of 37ºC and 21ºC, respectively, while the relative humidity is between 70-80%.
The experiment evaluated five cowpea varieties which were laid out in a Randomized Complete Block Design with three replications on a field of 17m x 13m (221m²) with plot sizes of 3m × 4m(12m2). The site was manually cultivated before planting. Two newly released cowpea varieties UAM 09 1055-6 (early maturity) and UAM 09 1051-1 and three other cowpea varieties IT 99k-573-1-1(Medium maturity), IT 90k-277-2 (Medium maturity), IT 99k-573-2-1 obtained from the Molecular Biology laboratory of the Federal University of Agriculture, Makurdi were used.
Three seeds were planted per hill at an intra row spacing of 30cm and inter row spacing of 75cm. The Cowpea seedlings were later thinned to two seedlings per hill. Single Super Phosphate fertilizer (SSP) was applied at the recommended rate of 30kg/ha at planting and NPK 15: 15:15 fertilizer at the rate of 15kg/ha at 4 weeks after planting (WAP). Weeding was done at 3 and 5 weeks after planting. Cowpea field was sprayed with Cypermethrin and dimethoate insecticide at the rate of 50 g a.i/ha at 3, 6, 7 and 8 weeks after planting.
Cowpea leaf samples with necrotic spots were collected from the two locations for isolation and identification of fungi. Small sections (3-5mm) were cut from the edges of infected leaves to contain both diseased and healthy tissues. The tissues were sterilized for 1minute in 10% Sodium hypochlorite solution after which they were rinsed in three changes of SDW and blotted dry on sterile filter papers. Potato Dextrose Agar (PDA) was prepared by adding 39g in 1litre of Sterile Distilled Water (SDW) in a conical flask. The flask was autoclaved at 121oC for 15 minutes, the media was allowed to cool, Streptomycin Sulphate added at the rate of 0.2g/L and the media allowed to solidify. The plates were then incubated on the laboratory bench at ambient conditions of light and temperature (30± 2oC) for 3 days. Pure culture was obtained by sub culturing unto fresh PDA plates.