Assessment of laboratory methods for evaluating cassava genotypes for resistance to root rot disease

Abstract

Field evaluation of six cassava genotypes for resistance to root rot disease was compared with three rapid laboratory methods (whole root inoculation, root slice inoculation, and stem inoculation) for resistance screening. Both the field evaluation and the three laboratory methods separated the varieties into resistant and susceptible groups. Genotypes 30572 and 91/02324 were resistant while 92/0247, 92/0057 and TME-1 were susceptible. One genotype (30001) was not consistent in its reaction between field evaluation and laboratory assays. In the laboratory assays with three fungal pathogens, different pathogens varied in their levels of virulence on host genotypes. With the most virulent pathogen (Botryodiplodia theobromae), the majority of the genotypes reacted in the same way across trials with the root slice and whole root assays. Due to the good correlation between the whole root assay and the field results, we recommend this for the routine assessment of cassava resistance to root rot disease and for the analysis of virulence of pathogen isolates. However, because of the advantages in terms of economy of labour, space, time, quantity of root and inoculum required, the root slice assay could be used for the preliminary screening of large cassava accessions. The selected genotypes can then be further screened with the whole root inoculation method.

Resistance to Fusarium dry root rot disease in cassava accessions

“…In general, resistance screening is conducted under field conditions; however, because the disease incidence and severity can only be assessed after harvesting, the process of selection for improved cassava accessions could be very difficult. To solve this problem, different strategies have been used for the identification of resistance in laboratory assays: inoculation of stem cuttings, root slices, and whole root (Onyeka et al, 2005).…”

“…isolates. Assessments of resistance level were conducted using the method modified from Onyeka et al (2005), with whole-root inoculation and evaluation of the final lesion area ten days after inoculation, by means of digital image analysis.…”

“…The roots were inoculated with a 20 µL drop of Fusarium sp. suspension, which was achieved by mixing all 11 isolates and adjusted to 2x10 5 macroconidia mL -1 ; a mock inoculation (only water) was also used, according to the methodology described by Onyeka et al (2005), with modifications. The samples were maintained in a moist chamber at 26°C and evaluated every two days up to ten days after inoculation, to estimate the area under the disease progress curve, and, ten days after inoculation, to quantify the final lesion.…”

Effect of canker size on availability of cassava planting materials in Nigeria

“…Cassava is a major food staple for humans, provides feed for livestock and is currently a major source of industrial raw materials for high quality cassava flour, starch and ethanol (Okechukwu et al, 2009). This new awareness, coupled with the increased food demand of the rapidly growing population has led to an expansion of cassava cultivation in many African countries (Onyeka et al, 2005).…”

Identification of levels of resistance to cassava root rot disease (Botryodiplodia theobromae) in African landraces and improved germplasm using in vitro inoculation method

“…However, the long growth cycle for cassava to develop storage roots for assessment and the nonuniform distribution of inoculum in the soil make field screening very difficult. Consequently, in vitro methods for the assessment of cassava root rot have been developed (Barragan & Alvarez, 1998;Onyeka et al, 2005b). These methods involve the inoculation of different plant parts, such as whole roots, root slices, stem cuttings and young rootlets, and disease intensity is estimated by rating the inoculated materials on a disease rating scale, or by actual calculation of the percentage of the diseased area.…”