Assessing the Degeneration of Cassava Under High-Virus Inoculum Conditions in Coastal Tanzania

Abstract

Cassava brown streak disease (CBSD), caused by cassava brown streak ipomoviruses (CBSIs), has become the most debilitating biotic stress to cassava production in East and Central Africa. Lack of CBSD-resistant varieties has necessitated the search for alternative control measures. Most smallholder farmers reuse stems from previous crops for planting in the new season. Recycling planting material in this way can lead to “degeneration” owing to the compounding effects of disease. In this study, degeneration was defined as the increase in CBSD incidence and reduction in marketable root yield over time. An experiment was established to study the rates of degeneration in selected cassava varieties Chereko, KBH2002_135, Kipusa, Kizimbani, and Mkuranga1 and cultivars Kiroba and Kikombe under high-CBSD inoculum conditions in Bagamoyo, Tanzania from 2013 to 2017. The experiment was replicated across two seasons: the first planted during the long rains (Masika) between March and June and the second planted during the short rains (Vuli) between October and December. Mean abundance of the whitefly vector (Bemisia tabaci) was much greater during the Vuli season (>19 insects per plant) than the Masika season (<2 insects per plant). CBSD shoot symptoms occurred naturally and were observed only on Kikombe, Kiroba, and Kipusa. New materials had overall lower CBSD shoot incidences (1.5%) compared with recycled materials (6.9%) in Masika, although no significant differences were obvious in Vuli. However, Masika (8.7%) had an overall lower CBSD shoot incidence than Vuli (16.5%) in the varieties that had shoot symptoms. CBSD root incidences were higher in Vuli (10.3%) than Masika (4.4%), and root yields in Masika (29.4 t/ha) were significantly greater than those in Vuli (22.5 t/ha). The highest percentage of roots rendered unusable owing to CBSD was observed in Vuli. There was significantly higher unusable root incidence in recycled materials (3.7%) than in new materials (1.4%) in Masika but not in Vuli. Overall root yield was similar between recycled and new materials in either season. Significant reductions in root yield over the course of the experiment were observed both in Masika and Vuli, whereas changes in marketable yield were significant only in Masika. Differences in the response of varieties to degeneration led to the identification of four degeneration patterns, namely “strong,” “moderate,” “mild,” and “delayed” degeneration. The strongest effects of degeneration were most obvious in the susceptible cultivar (Kikombe), which also had the lowest marketable yield in either season. Seasonal differences were a key driver of degeneration, because its effects were much greater in Vuli than Masika. To the best of our knowledge, this work reports the first study of degeneration caused by cassava viruses. [Formula: see text] Copyright © 2019 The Author(s). This is an open access article distributed under the CC BY 4.0 International license .

Genotype by environment cultivar evaluation for cassava brown streak disease resistance in Tanzania

“…In the current study, the high number (>90%) of infected cultivars in north-western Tanzania where CBSV was the most frequent virus probably confirms reports from Uganda and from laboratory studies that the virus is more virulent than its sister species, UCBSV ( Kaweesi et al 2014 ; Mohammed et al 2012 ; Winter et al 2010 ). We cannot confirm this for the current study, however, since sites in the CZ were planted during the long rainy season, which has been characterized as a low CBSD pressure season ( Shirima et al 2019 ) making comparisons difficult.…”

“…The likely reason for the low whitefly populations observed at both sites is the planting of the trial during the main rainy season. Data from trials planted in this agro-ecological zone in both the main rains (March-June) and the short rains (October-December) demonstrate that whitefly abundance and concomitant CBSD spread are much greater in the short than the long rains ( Shirima et al 2019 ).…”

“…Therefore, the high CBSD pressure recorded for some of the sites in this study meant that higher virus transmission rates were experienced at those sites. Studies have also shown that efficient transmission of pathogens or disease spread are tightly linked to prevailing environment and/or growing season ( Shirima et al 2019 ) and crop age ( Fishpool et al 1995 ; Legg (1995) ), with each having important effects on whitefly vector abundance. Data in this study demonstrated very clearly the importance of the twin factors of whitefly abundance and surrounding CBSD inoculum pressure in driving CBSD infection of initially CBSD-free trial plantings.…”

“…There has been limited progress in developing CBSD-resistant cultivars, and none of the currently available cultivars in East and Central Africa has a high level of resistance to the disease. A recent study on cassava degeneration ( Shirima et al 2019 ) points out the influence of the environment and planting season as key aspects in the successful evaluation of breeders’ material, highlighting large seasonal differences in whitefly abundance which led to contrasting patterns of disease spread. Several studies have published information on field resistance of cassava cultivars to CBSD using sets of cassava cultivars, but these did not cover multiple locations ( Kaweesi et al 2014 ; Kawuki et al 2016 ; Masinde et al 2018 ).…”

Genetic analysis and QTL mapping for multiple biotic stress resistance in cassava

“…Each genotype was grown in single-row plots of five plants with 1 × 1 m spacing (row-to-row and plant-to-plant). To augment disease pressure, the trials were planted in December which coincides with the short rains and extremely high CBSD pressure in Chambezi [60]. In addition, stakes taken from plants close to the experimental sites that clearly exhibited CMD and CBSD symptoms were planted around the evaluation trials to increase disease pressure.…”