Resistance spectrum of selected Manihot esculenta genotypes under field conditions

Abstract

In Colombia, cassava bacterial blight (CBB) caused by Xanthomonas axonopodis pv. manihotis is a major threat to the production of cassava ( Manihot esculenta), an economically important crop. Losses to CBB range from 12 to 90%, and are most severe during wet seasons. The blight is best controlled by planting cultivars resistant to this disease. We evaluated 19 cassava cultivars for resistance in two edaphoclimatic zones (ECZs) and two crop cycles (1997 and 1998). The cultivars were planted in rows of six cuttings, replicated three times at random, and exposed to natural inoculum. Disease reaction was evaluated 4 and 7 months after planting according to a 1-to-5 scale for severity. Except in ECZ 1, in 1997, where low rainfall was unfavorable for disease development, almost all plants presented symptoms. In both ECZs, disease incidence index varied greatly between cultivars and evaluations, and, for ECZ 2, seasons. Correlations between field and greenhouse data for disease severity varied between years and were relatively low. However, most of the cultivars showing resistance in the greenhouse also showed resistance in the field. We selected several cultivars for each ECZ for use in improving the crop’s resistance to CBB.