Resistance of advanced cassava breeding clones to infection by major viruses in Uganda

Daniel Rogers Mukiibi,Titus Alicai,Robert Kawuki,Geoffrey Okao-Okuja,Fred Tairo,Peter Sseruwagi,Joseph Ndunguru,Elijah Miinda Ateka

doi:10.1016/j.cropro.2018.09.015

Abstract

Cassava brown streak disease (CBSD) and cassava mosaic disease (CMD) are two viral diseases that cause severe yield losses in cassava of up to 100%, thereby persistently threatening food and income security in sub-Saharan Africa. For effective management of these diseases, there is a critical need to develop and deploy varieties with dual resistance to CBSD and CMD. In this study, we determined the response of advanced breeding lines to field infection by cassava brown streak viruses (CBSVs) and cassava mosaic begomoviruses (CMBs). This aim helped in identifying superior clones for downstream breeding. In total, 220 cassava clones, three in uniform yield trials (UYTs) and 217 in a crossing block trial (CBT), were evaluated for virus and disease resistance. Field data were collected on disease incidence and severity. To detect and quantify CBSVs, 448 and 128 leaf samples from CBSD symptomatic and symptomless plants were analyzed by reverse transcription PCR and real-time quantitative PCR, respectively. In addition, 93 leaf samples from CMD symptomatic plants in the CBT were analyzed by conventional PCR using CMB species-specific primers. In the CBT, 124 (57%) cassava clones did not express CMD symptoms. Of the affected plants, 44 (55%) had single African cassava mosaic virus infection. Single Cassava brown streak virus (CBSV) infections were more prevalent (81.6%) in CBT clones than single Ugandan cassava brown streak virus (UCBSV) infection (3.2%). Of the three advanced clones in the UYT, NAROCASS 1 and NAROCASS 2 had significantly lower (P < 0.05) CBSD severity, incidence, and CBSV load than MH04/0300. In the UYT, only 22% of samples tested had CBSVs, and all showed a negative result for CMBs. The low disease incidence, severity, and viral load associated with NAROCASS 1 and NAROCASS 2 is evidence of their tolerance to both CBSD and CMD. Therefore, these two cassava clones should be utilized in CBSD and CMD management in Uganda, including their utilization as progenitors in further virus resistance breeding.

Highlights

Cassava brown streak disease (CBSD) and cassava mosaic disease (CMD) have persisted as major constraints to the production of cassava (Manihot esculenta Crantz) in sub-Saharan Africa (SSA), where the crop serves as a major staple food (Alicai et al, 2007; Mohammed, 2012)
Incidence and severity of CBSD in uniform yield trials (UYTs) clones were determined in four locations
CBSD incidence was low in NAROCASS 1 (0%) and NAROCASS 2 (1%), but MH04/0300 had a mean incidence of 33.1% (Table 1)

Summary

Introduction

Cassava brown streak disease (CBSD) and cassava mosaic disease (CMD) have persisted as major constraints to the production of cassava (Manihot esculenta Crantz) in sub-Saharan Africa (SSA), where the crop serves as a major staple food (Alicai et al, 2007; Mohammed, 2012). Storage roots of plants severely affected by CMD fail to bulk because their leaves become chlorotic and mottled, having impeded photosynthesis and leading to stunted growth (Mohammed, 2012). Both viral diseases are perpetuated from one season to another through the practice of farmers using stem cuttings from diseased plants as planting material (Hillocks and Jennings, 2003). The virus status of the planting materials is often unknown because symptoms may not be apparent or leaves may have withered and dropped after crop harvest This practice leads to the spread of viral diseases to new fields and often advances into disease epidemics. The year-round occurrence of high whitefly (Bemisia tabaci) vector populations in many parts of eastern Africa escalates the spread of CMD and CBSD

Methods

Discussion

Conclusion