Screening Colletotrichum gloeosporioides f.sp Manihotis isolates for virulence on cassava in Akwa Ibom state of Nigeria.

Six isolates of Colletotrichum gloeosporioides f.sp. manihotis obtained from anthracnose-infected cassava stems in six different cassava-growing locations of Akwa Ibom State were examined in the laboratory for morphological and physiological differences. The isolates were then screened in the greenhouse for virulence on cassava variety NR 8083 widely grown in Akwa Ibom State. Six-week old cassava plants were inoculated with 1ml of the spore suspension of each isolate of the pathogen at a concentration of 104 conidia/ml using a sterile hypodermal needle. One injection of inoculum was made at five different internodes of each cassava stem at an ambient temperature of 300C and relative humidity of 80%. Results of laboratory examinations showed that mycelial growth on potato dextrose agar (PDA) medium was fastest (10.65mm/day) in isolate CGM 2 from Ikono and slowest (7.29mm/day) in isolate CGM 6 from Uyo. Sporulation of the isolates on PDA medium under alternate light and dark regimes was highest (9.30 x 106 spores/ml) in isolate CGM 4 from Nsit Atai and lowest (1.23 x 106 spores/ml) in isolate CGM 5 from Oruk Anam. Percentage spore germination and conidia length were highest (85.64% and 23.55µm) respectively in isolates CGM 4 from Nsit Atai and CGM 1 from Essien Udim and lowest (49.86% and 13.75µm) in isolate CGM 3 from Itu and isolate CGM 2 from Ikono, respectively. Results of greenhouse inoculations showed variations in lesion size for the different isolates. The largest (15.30mm) lesion was produced by isolate CGM 4 from Nsit Atai, followed by isolate CGM 1 (12.60mm) from Essien Udim. There were no significant (p> 0.05) differences in lesion size for isolates CGM 3 (8.63mm) from Itu, CGM 2 (8.45mm) from Ikono and CGM 6 (8.41mm) from Uyo. Lesion size was smallest (5.00mm) in isolate CGM 5 from Oruk Anam. Isolate CGM 4 of C. gloeosporioides f.sp manihotis from Nsit Atai which showed the highest sporulation density and percentage spore germination, one of the longest conidia and largest lesion size on inoculated cassava stems, appeared to be the most virulent of the six isolates of the pathogen in Akwa Ibom State during this investigation.Key Words: Fungal variability, fungal virulence, Colletotrichum gloeosporioides f.sp. manihotis, cassava, anthracnos