ABSTRACT

Survey of the cassava anthracnose disease (CAD) was conducted in the wet and dry seasons of 2006 and 2007 in six major cassava-growing areas of Akwa Ibom State. Five farms in each of the six areas were randomly selected and two diagonal transects were taken across each farm. Cassava plants at every one meter along the transects were sampled. The objective of the survey was to determine the incidence and severity of the cassava anthracnose disease. The mean disease incidence for the six survey areas was significantly (p ~ 0.05) higher in the wet season (24.10%) than in the dry season (5.92%). The highest significant disease incidence in both the wet season (36.59%) and the dry season (9.26%) was recorded at Nsit Atai. The lowest disease incidence was obtained in Essien Udim (13.63%) and Oruk Anam (4.13%) in the wet and dry seasons, respectively. Disease severity was higher in the wet season (severity score 3.23) than in the dry season (severity score 2.16). Isolates of Co//etotrichum g/oeosporio/des Isp. man/hot/s obtained from cankered cassava stems in the survey areas, showed significant (p ~ 0.05) differences in cultural characteristics, mycelial growth rate, sporulation density, spore germination, conidial length and virulence. Evaluation of the mycelial growth rate of the isolates on cassava extracts agar showed that the highest significant (p ~ 0.05) growth rate was obtained in stem extract agar (9.33mm/day) and the lowest in tuber extract agar (6.57mm/day). Investigation of latent infection by the anthracnose pathogen showed the presence of the acervuli of the pathogen on symptomless cassava stems of five varieties. Studies on the role of plant injury in C. g/oeospor/o/des Isp. man/hot/s infection showed that only in the presence of injury did the inoculum produce necrotic lesions on the inoculated cassava stems. Four microorganisms (Aspergillus sp, Curvu/aria sp, Fusar/um sp and Glioc/ad/um sp) were found to be consistently associated with the cassava anthracnose pathogen in infected cassava stems. Studies on their in vitro interaction showed that none of the associated organisms was antagonistic to the anthracnose pathogen. Screenhouse screening of ten cassava varieties for resistance to cassava anthracnose disease showed significant (p ~ 0.05) differences in lesion size and defoliation, after inoculation with virulent isolate CGM4 of C. g/oeospor/oides Isp. man/hot/s. Assessment of disease severity on a scale of 1-5, led to the selection of TMS 98/30572 and TME 419 as resistant varieties to cassava anthracnose disease in Akwa Ibom State.