Relationship between Symptoms Expression and Virus Detection in Cassava Brown Virus Streak-Infected Plants

Diagnosis of Cassava brown streak disease (CBSD) has for long been based on foliar and root symptoms expression on infected plants. Variability in patterns of symptoms expression between varieties and seasons however, has meant that symptom-based diagnostics are unreliable. The current study established the relationship between symptom expression on cassava plants and the infection with Cassava brown streak virus (CBSV) using RT-PCR diagnostic tool. It was established that manifestation of CBSD-like symptoms (particularly the foliar chloroses and necrotic blotches) may not be an absolute indication of CBSV-infection. Only 67% of tested samples were both foliarly symptomatic and infected by the virus. About 22% of samples were free from CBSV despite being foliarly symptomatic and 7% were CBSV-infected but foliarly symptom less. Some CBSV-infected plants did not exhibit any foliar symptoms although had root necroses. A few CBSV-free plants were regenerated from infected cuttings in one of the four tested cultivars, Albert. Five out of fifteen (33%) plants cv. Albert were symptom less and two out of the five (40%) were CBSV-free. The findings from this study suggest that symptoms-based diagnosing for CBSV infections is unreliable. As some of CBSV-infected plants tend to be considered CBSV-free due to lack of the disease symptoms, the scenario might have contributed to unlimited spread of CBSD through latently-infected planting materials.