Characterization of natural resistance in cassava against viruses causing the cassava brown streak disease

Abstract

Cassava brown streak disease (CBSD), an emerging virus disease in Eastern Africa, is caused by viruses that pose serious threats to the crop because of destruction in roots. African cassava varieties are susceptible to either of the two viruses causing the disease, cassava brown streak virus (CBSV) and Ugandan cassava brown streak virus (UCBSV). The lack of CBSD resistance in cassava grown in Africa led the search to find and characterize potential sources of virus resistance in cassava germplasm of South America, the center of origin of the crop. Hence, a stringent virus screening protocol was developed, comprising an effective plant infection with the most pathogenic virus isolate to eliminate the susceptible lines. Infections studies with further viruses were conducted and after several rounds of screening, cassava plants were identified with resistance against both CBSV and UCBSV. Studies of virus movement and replication in resistant cassava identified genotypes with broad spectrum immunity, cassava lines with differential resistance against CBSV only, and cassava accessions that were virus-free in shoots and leaves but had virus infections restricted to the roots. High resolution in situ hybridization studies showed that viruses can translocate through the phloem of resistant cassava plants, but cannot exit the vasculature for replication in parenchyma tissues. Furthermore, evidence was provided that virus replication in roots and root tubers is independent of virus replication in aboveground tissues. Thus, roots are playing a crucial role in the cassava brown streak disease. The stability of CBSD resistance was verified by subjecting selected cassava lines to natural virus pressure under field conditions in Africa comprising mixed infections of cassava brown streak viruses and cassava mosaic begomoviruses. There was no sign that the cassava brown streak resistance was compromised by any interaction with other cassava infecting viruses. The three lines selected in this research with broad spectrum resistance against all cassava brown streak viruses and in particular the cassava line DSC 118, showing also a recovery/reversion response against cassava mosaic viruses, are so far the best resistance sources to combat the cassava brown streak disease. They can play an important role in breeding programs to control the major cassava virus diseases, CBSD and CMD in Africa