title: Field evaluation of selected cassava genotypes for cassava brown streak disease based on symptom expression and virus load.
abstract: BACKGROUND
  Production of cassava (Manihot esculenta Crantz), a food security crop in sub-Saharan Africa, is threatened by the spread of cassava brown streak disease (CBSD) which manifests in part as a corky necrosis in the storage root. It is caused by either of two virus species, Cassava brown streak virus (CBSV) and Ugandan cassava brown streak virus (UCBSV), resulting in up to 100% yield loss in susceptible varieties.
  
  
  METHODS
  This study characterized the response of 11 cassava varieties according to CBSD symptom expression and relative CBSV and UCBSV load in a field trial in Uganda. Relative viral load was measured using quantitative RT-PCR using COX as an internal housekeeping gene.
  
  
  RESULTS
  A complex situation was revealed with indications of different resistance mechanisms that restrict virus accumulation and symptom expression. Four response categories were defined. Symptom expression was not always positively correlated with virus load. Substantially different levels of the virus species were found in many genotypes suggesting either resistance to one virus species or the other, or some form of interaction, antagonism or competition between virus species.
  
  
  CONCLUSIONS
  A substantial amount of research still needs to be undertaken to fully understand the mechanism and genetic bases of resistance. This information will be useful in informing breeding strategies and restricting virus spread.
authors.affiliation: National Crops Resources Research Institute, Root Crop Program, Namulonge, Uganda. karln1844@yahoo.com.
authors.forename: Tadeo
authors.initials: T
authors.lastname: Kaweesi
authors.affiliation: National Crops Resources Research Institute, Root Crop Program, Namulonge, Uganda. kawukisezi@yahoo.com.
authors.forename: Robert
authors.initials: R
authors.lastname: Kawuki
authors.affiliation: National Crops Resources Research Institute, Root Crop Program, Namulonge, Uganda. kyalivincent@gmail.com.
authors.forename: Vincent
authors.initials: V
authors.lastname: Kyaligonza
authors.affiliation: National Crops Resources Research Institute, Root Crop Program, Namulonge, Uganda. baguma1234@yahoo.com.
authors.forename: Yona
authors.initials: Y
authors.lastname: Baguma
authors.affiliation: Makerere University, College of Agricultural and Environmental Sciences, Kampala, Uganda. gtusiime@agric.mak.ac.ug.
authors.forename: Geoffrey
authors.initials: G
authors.lastname: Tusiime
authors.affiliation: International Institute of Tropical Agriculture (IITA), c/o ILRI, P.O Box 30709, Nairobi, 00100, Kenya. m.ferguson@cgiar.org.
authors.forename: Morag E
authors.initials: ME
authors.lastname: Ferguson
mesh_terms: D060467:Disease Resistance; D002365:Manihot / Q000235:genetics* / Q000276:immunology / Q000821:virology*; D010935:Plant Diseases / Q000821:virology*; D019176:Potyviridae / Q000302:isolation & purification*; D060888:Real-Time Polymerase Chain Reaction; D014454:Uganda; D019562:Viral Load