Abstract
Cassava brown streak disease (CBSD) is a severe virus disease of cassava and prevalent in the eastern regions of Africa. The disease is characterized by distinct vein chlorosis and streak symptoms on leaves and stems and necrosis of storage roots. This necrosis can encompass large areas of the root, rendering it inedible so that the entire cassava harvest can be lost. African cassava varieties are susceptible to either of the two viruses causing the disease, cassava brown streak virus (CBSV) and Uganda cassava brown streak virus, and while there are less sensitive varieties, all cassava eventually succumb to the disease. The lack of CBSD resistance in African cassava varieties prompted this search for new sources of virus resistance in the diversity of South American cassava germplasm held in the collection at International Center for Tropical Agriculture, Columbia. Our search for CBSD resistance in South American cassava germplasm accessions revealed that most of the 238 South American cassava lines infected with CBSV established systemic virus infections with moderate to severe disease symptoms on leaves and stems. Fifteen cassava accessions did not become virus infected, remained free of symptoms, and CBSV was undetected by qRT-PCR. When tuberous roots of those lines were examined, necrotic tissue was found in eight lines and CBSV was detected. The remaining seven cassava accessions remained clear of symptoms on all tissues and organs and were virus free. A broad spectrum of virus resistance also including other virus isolates was confirmed for the breeding lines DSC167 and DSC118. While detailed infection experiments with other cassava lines selected for resistance are still ongoing, this indicates that the resistance identified may also hold against a broader diversity of CBSVs. Taken together, we present the results of a comprehensive study on CBSV resistance and susceptibility in cassava germplasm accessions from South America. The virus resistance in cassava germplasm identified provides compelling evidence for the invaluable contribution of germplasm collections to supply the genetic resources for the improvement of our crops.
Highlights
MATERIALS AND METHODSCassava brown streak disease (CBSD), the most serious threat to cassava cultivation in East and Central Africa, is caused by two virus species, cassava brown streak virus (CBSV) and Ugandan cassava brown streak virus (UCBSV; Winter et al, 2010), both members of the genus Ipomovirus in the family Potyviridae (ICTV online
The genetic diversity of South American cassava is preserved in approximately 6,400 cassava germplasm accessions, 5,477 of which are kept in field and in vitro collections at the International Center for Tropical Agriculture (CIAT), Cali, Colombia
Resistance against the viruses causing cassava mosaic and CBSDs is key to cassava cultivation in Africa and, is a vital element of all breeding programs aimed at genetic improvement of this important food crop
Summary
Cassava brown streak disease (CBSD), the most serious threat to cassava cultivation in East and Central Africa, is caused by two virus species, cassava brown streak virus (CBSV) and Ugandan cassava brown streak virus (UCBSV; Winter et al, 2010), both members of the genus Ipomovirus in the family Potyviridae (ICTV online). We developed a highly efficient virus inoculation and evaluation scheme and used a severe isolate of CBSV to infect 238 cassava lines originating from South America and an additional 42 cassava varieties from Africa. Axillary buds from virus-infected plants were grafted onto 2- to 3month-old plants from each variety, which were monitored for symptom development and virus accumulation. Plantlets established from tissue culture (two to three plants/accession) were infected by bud grafting, checked for the infection status at 10 dag, and inspected for symptom development. Virus symptoms and detection of CBSV by qRT-PCR were taken as proof for susceptibility of a cassava accession, and even if only one plant became infected, the line was discarded and exempt from further testing. Monitoring and virus testing continued with symptomless and qRT-PCRnegative plants for a further 5–8 months, after which cassava lines that remained free of symptoms and virus were subjected to a second round of virus screening using five plants per cassava line (Figure 1)
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