DIVERSITY STUDY OF FUSARIUM OXYSPORUM F.SP. TRACHEIPHILUM RECOVERED FROM DISEASED COWPEA IN MAJOR PRODUCING STATES IN NIGERIA

Abstract

Cowpea Vigna unguiculata (L.) Walp is a versatile and economically important legume. The production of this crop is hindered by many disease factors in which Fusarium wilt is one of the most severe diseases. Meanwhile, Fusarium wilt of cowpea is caused by Fusarium oxysporum f.sp. tracheiphilum (FOT). Thus, this study was aimed at surveying major cowpea producing zones in Nigeria (Kaduna, Kastina, Kano, Jigawa, Oyo) for occurrence and severity of Fusarium wilt, characterized isolates based on morphological features on carnation leaf agar, and molecular properties using two specific primers on Random Amplified Polymorphic DNA (RAPD) to identify the forma specialis responsible for cowpea wilt. Pathogenicity of FOT isolates were evaluated on two cowpea varieties, using different inoculum concentrations and inoculation methods. These experiments were carried out at International Institute of Tropical Agriculture (IITA) Ibadan, Nigeria. The following data were collected; disease incidence (%) and severity (%), macroconidia and microconidia sizes (μm), shape, phialide, chlamydospores. Cowpea leaf number, plant height (cm) and laid out in a Completely Randomized Design (CRD). Data were subjected to Analysis of Variance (ANOVA) with the mean separation using Duncan multiple range test at 5% level of significance, using statistical package for social sciences (SPSS) version 17.0. Survey result revealed that 80% farmers were practicing mixed / intercropping, while 20% practiced sole cropping. Using GPS, it was revealed that the longitudes of Iseyin LGA of Oyo State were 6.5690N as the least, while 12.5820N in Malam Madori of Jigawa State was the highest. Latitude in Saki west LGA of Oyo State was 3.1380E indicating the least and 8.6680E as highest in Warawa LGA of Kano State. Whereas, 2670M was recorded in the elevation of Akinyele LGA in Oyo State being the least and 6960M in Zaria LGA of Kaduna State as the highest. Fusarium wilt incidence in surveyed states showed that Giwa LGA in Kaduna State had 95.25+ 1.34% as the highest and Kaugama LGA of Jigawa State had 51.06 +1.21 % as the lowest. Meanwhile, disease severity in Ibarapa North LGA of Oyo State was 96.18 + 0.95 % as the highest with Minjinbri LGA of Kano State had 7.01+ 1.09 % being the significantly lowest. Two hundred (200) mycoflora was found to associate with cowpea wilt. Whereas, One hundred and twenty four (124) were found to be F. oxysporum morphologically. FOT on CLA under the microscope showed that isolate FOT110 had the longest macroconidia of 8.17 μm with a dorso ventral curvature shape while, FOT80 had the shortest microconidia of 2.01 μm with a two celled oval. Wounded cowpea seedlings inoculated with FOT in the concentration of 108ml/spore at 20ml showed significant differences all through the observation period when compared to submerged cowpea seedlings. Cowpea variety Vita-7 inoculated with FOT4 through wound and submerged methods had the shortest plants height (cm) showing no significant differences 11.35 e and 10.12.. While Vita-7 and IT13K-1200-2 that were inoculated with FOT1 at 108 spore at 20 ml inoculums concentrations through submerged method had the tallest plants height 25b and 33.90a cm respectively. These prove the varying degree of susceptibility in both cowpea varieties, infection process and severity rate over time. And also that F. oxysporum causes vascular wilt once the host is penetrated, through the leaves, stems and roots to reach the xylem vessels, which it colonizes upwards, resulting in progressive yellowing and eventual death of the plant. Furthermore, molecular analysis revealed that on the basis of the first RAPD Marker FOT 1-F and 2-R; Using positive control (MH866849.1). It was revealed that 72 nucleotide sequences paired completely with the control sequence, with Oyo State having 18, Kaduna 40 as the highest paired State, while Kano and Katsina both had 7 each, showing the least in all the identified forma specialis. Meanwhile, on the basis of the second RAPD Marker FOT 3-F and FOT 4-R Using Positive Control (MH855398.1), a total of 39 nucleotide sequences paired completely with the control sequence. With Oyo State having 8, Kaduna 20 nucleotide sequences as the highest paired State, Kano 6, Katsina 4, whereas, Jigawa State had 1 nucleotide sequence showing the least in all the identified forma specialis. In comparing both primers only 30 nucleotide sequences paired to both primers as such are identified as formae specialis out of the one hundred and twenty four (124) isolates.