| dc.description.abstract | I he present study was undertaken to determine the variability among pathogenic isolates of Fusarium udum causing Fusarium wilt of pigeonpea in Kenya using various techniques, and to find out the nature of mechanical resistance in pigeonpea plants.
A survey of Fusarium wilt of pigeonpea was carried out on 86 farms located in 13 pigeonpea growing districts in Eastern, Coast, Central and Nairobi provinces during the flowering, pod setting and dry pod stages between July and September 1997. The disease was recorded in 12 districts, 8 AEZs and from 55 farms (64% prevalence). There were significant differences (P < 0.05) in Fusarium wilt incidences among the farms, districts, AEZs, altitude and cropping system, but no significant differences among pigeonpea types The wilt incidence ranged from 0 to 96.1% with a mean of 8.4%.
Inoculation techniques were assessed, and root-dip inoculation technique was found to be very effective and reliable in inoculating pigeonpea plants with F. udum isolates during pathogenicity and virulence studies under glasshouse conditions. The wilt incidence on pigeonpea plants was significantly different (P < 0.05) between the two inoculation techniques, with means of 48.1% and 14.4% for root-dip technique and colonized whole rice giain technique, respectively.
Cultural characteristics of F. udum isolates grown on PDA medium were used to classify 79 single-spore isolates into: 3 groups by aerial mycelial growth, 2 groups by mycelial texture, J groups by radial mycelial growth (colony diameter), 5 groups by mycelium colour, 3 groups by substrate colour, and 4 groups by sporulation. Macroconidial length was used to classify 79 isolates of F. udum grown on PDA medium into 3 groups.
All the 79 isolates of F. udum were found to be pathogenic to susceptible pigeonpea variety KAT 60/8. Pathogenic variability was evident among F. udum isolates resulting in significant differences (P < 0.05) in wilt incidence with a mean of 75% wilt The isolates were classified into 2 virulence groups. Seven pigeonpea varieties obtained from and recommended bv ICRISAT were used as differentials in order to determine the possible existence of races in / udum in Kenya. Twenty-one isolates were used for inoculation in a glasshouse experiment
Fusarium wilt incidence was significantly different (P < 0.05) among the isolates and varieties, with a mean of 24.9% wilt at six weeks after root-dip inoculation. Pathogenic variability was observed among the isolates. Eleven physiologic races of F. udum were identified with race 0 being dominant and independent, while the remaining 10 races were closely related.
I he Nit mutants of 79 isolates of F. udum were generated by selecting chlorate-resist ant sectors on minimal medium amended with 1.5% potassium chlorate (MMC). The nit mutants were grouped into nit\ (79% mutants), nit3 (18% mutants) and NitM (3% mutants). A total of 8 NitM tester mutants from Kenya and India were paired with nitl and/or nil3 mutants of different isolates for vegetative compatibility reactions on minimal medium (MM). The 75 isolates from Kenya, and two isolates each from Malawi and India were grouped into a single vegetative compatibility group, VCG 1.
AFLP analysis of the DNA of 56 isolates of /'. udum using 7 primer combinations with FcoR I + 2 and Mse I + 3 selective nucleotides at the 3-end of the primers revealed 326 amplified bands with 121 being polymorphic (37% polymorphism). Based on AFLP analysis three groups were identified with group I having 40 isolates, group II had 9 isolates and group HI had 7 isolates.
fhe presence of F. udum in wilt susceptible pigeonpea plants was characterized by mycelia and conidia in the xylem vessels, plugging in some vessels, disintegration of xvlcm parenchyma cells in the infected areas, and the formation of cavities due to heavy colonization in the vascular bundle, the pith and the cortex. Resistance to F. udum in the roots and stems of wilt resistant pigeonpea plants was associated with low flingal colonization, and higher occlusion due to tyloses and gels in the xylem vessels than in susceptible plants There were significant differences (P < 0.05) in the number of xylem vessels occluded by tyloses in resistant and susceptible plants with a maximum of 22.5% and 8.0% occlusion, respectively | |
