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Additional file 1. PAL1 gene of the phenylpropanoid pathway increases resistance to the Cassava brown streak virus in cassava.

Cassava brown streak disease (CBSD) is arguably the most dangerous current threat to cassava, which is Africa's most important food security crop. CBSD is caused by two RNA viruses: Cassava brown streak virus (CBSV) and Ugandan cassava brown streak virus (UCBSV). The roles of the whitefly Bemisia tabaci (Gennadius) and farmer practices in the spread of CBSD were investigated in a set of field and laboratory experiments. The virus was acquired and transmitted by B. tabaci within a short time (5–10 min each for virus acquisition and inoculation), and was retained for up to 48 hr. Highest virus transmission (60%) was achieved using 20–25 suspected viruliferous whiteflies per plant that were given acquisition and inoculation periods of 24 and 48 hr, respectively. Experiments mimicking the agronomic practices of cassava leaf picking or the use of contaminated tools for making cassava stem cuttings did not show the transmission of CBSV or UCBSV. Screenhouse and field experiments in Tanzania showed that the spread of CBSD next to spreader rows was high, and that the rate of spread decreased with increasing distance from the source of inoculum. The disease spread in the field up to a maximum of 17 m in a cropping season. These results collectively confirm that CBSV and UCBSV are transmitted by B. tabaci semipersistently, but for only short distances in the field. This implies that spread over longer distances is due to movements of infected stem cuttings used for planting material. These findings have important implications for developing appropriate management strategies for CBSD.

Cassava is the second most important staple crop after maize in Migori County. A farm survey was conducted in Kuria East and Suna West sub-counties to determine the incidence, severity and estimated losses of cassava brown streak disease (CBSD) on cassava crops in farmers’ fields. A total of 70 farms (Kuria East = 40; Suna West = 30), with cassava crop aged 7-12 months after planning (MAP), were sampled using stratified random sampling procedure. Data were collected on CBSD incidence (foliar and root), root necrosis score, percent loss (roots) and yield traits (number and weight of fresh roots). Results showed mean foliar incidences of 58.5% and 53.9% in Kuria East and Suna West sub-counties, respectively. High foliar CBSD incidences were recorded in Kuria East sub-county with range of 54.0-74.0% observed in five local varieties; Manchoberi, Amakuria, Mwitamajera, Weite and Nyakohanda. A similar result trend was recorded in Suna West sub-county with a range of 52.1-77.5% in six varieties; Mary go round, MH95/0183 (improved), Ondielo, Nyakasanya, Nyakasamuel and Obarodak. Highest root necrosis scores and percent root loss were observed in two local varieties, Amakuria (2.4; 36.7%) and Agric IV (3.7; 40%), in Kuria East and Suna West sub-counties, respectively. Correlation analysis revealed strong positive correlation between root necrosis, root necrosis incidence and percent root loss; which implies that susceptible varieties suffer greatest loss. The estimated mean root yield loss caused by CBSD stood at 10.5% (equivalent: US 405 ha-1) and 7.2% (equivalent: US 384.30 ha-1) in Kuria East and Suna West sub-counties, respectively. The findings of this study are expected to provide impetus for the development and promotion of new high yielding, locally adapted and CBSD resistant cassava varieties.