Greenhouse screening of cassava cultivars for resistance to CAD
Ten cassava cultivars of which the stem cortex extracts showed varying levels of inhibition on mycelia growth, sporulation and germ tube development of C. gloeosporioides f.sp . manihotisin the in vitro test were used. Healthy cuttings (20 cm), showing no cankers were planted in pots filled with steam-sterilized The cultivars were inoculated 2 months after
soils. planting with a spore suspension of 6.2×106 spores/ml of the fungal isolate 12 FCN . Inoculation was carried out using the stem puncture technique Muimba (1982), at the inter nodal regions , followed by foliar spray of fungal suspension, using a hand sprayer until run off. In each spore suspension, a drop of Tween-80, a surface-wetting agent was added . Necrotic lesion diameters of inoculated stem parts , defoliation, wilt symptoms and disease severity were scored from 1 to 5 (1= no symptoms; 2 = development of shallow cankers on stems lower down the plants ; 3= development of successive cankers higher up the plant with cankers on older stems becoming larger and deeper; 4= development of dark brown lesions on green shoots, petiole and leaves and 5= collapse of young shoots and leaves followed by death of a part or the whole plant). The sizes of cankers on the stems were measured (length and width of spread) using a ruler. For each plant, five large cankers were measured and their mean was recorded. The cassava cultivars were arranged in a randomized complete block design in four replications, of five plants per replicate.