Studies on Variability of Cassava Brown Streak Disease Symptoms and the Relationship between Virus Infection and Symptom Expression on Cassava Crop

Diagnosis of cassava brown streak disease (CBSD) has for long been based on foliage and root symptoms expression by infected plants. Variability in patterns of symptom expression between varieties and seasons however, has meant that symptom-based diagnosis is unreliable. The current study identified three major types of foliage symptoms (chlorotic blotches, chlorotic spots, vein chlorosis) and root (brownish black necrosis, chalky necrosis, necrotic specks) symptoms associated with CBSD. The newly described CBSD symptoms includes; spotty foliage chlorosis, brown necrotic internal tissue at the base of the leaf petiole and chalky root necrosis. The relationship between symptom expression and the presence of Cassava brown streak virus (CBSV) through RT-PCR was also established. Plant parts most suitable for detection of CBSV by RT-PCR were identified. Foliage CBSD symptoms could be unreliable indication of CBSV infection. Moreover, CBSV may not be uniformly distributed within the infected plants. Up to 67% of tested samples were from plants that were both symptomatic and visually seen to be infected by CBSV. About 22% were free from CBSV despite apparently exhibiting CBSD symptoms and only 7% were CBSV-infected but symptomless. CBSV was detected in flowers, fruits, apical buds, young tender leaves, newly-opened leaves, youngest symptomatic leaves, and the tender top green portion of the stem and non-necrotic storage root tissues. The study concluded that symptom based diagnosis for CBSD should consider examination of both foliage and roots and always be confirmed with molecular detection by RT-PCR.