Diversity, Distribution and Effects on Cassava Cultivars of Cassava Brown Streak Viruses in Malawi

Abstract

Cassava brown streak disease (CBSD) has emerged as a major threat to cassava (Manihot esculenta) in eastern and southern Africa. CBSD was first reported in Malawi in the 1950s, but little data on the distribution and epidemiology of the disease are available. A diagnostic survey was therefore conducted in Malawi to determine the distribution, incidence and diversity of viruses causing the disease, and to characterize its effects on local cassava cultivars. Diagnostic tests confirmed the presence of cassava brown streak viruses (CBSVs) in 90% of leaf samples from symptomatic plants. Average CBSD foliar severity was 2.5, although this varied significantly between districts. Both Cassava brown streak virus (CBSV) and Ugandan cassava brown streak virus (UCBSV) (genus Ipomovirus, family Potyviridae) were detected from sampled plants. UCBSV was widespread, whereas CBSV was detected only in the two most northerly districts. The average abundance of the whitefly vector (Bemisia tabaci) was 0.4 per plant, a low value that was partly attributable to the fact that the survey was conducted during the cool part of the year known to be unfavourable for B. tabaci whiteflies. Spearman's correlation analyses showed a positive correlation between CBSD foliar incidence and CBSD severity and between CBSD severity and CBSD stem incidence. Of the 31 cassava varieties encountered, 20-20 was most severely affected, whilst Mtutumusi was completely unaffected. Although data from this study do not indicate a significant CBSD deterioration in Malawi, strengthened management efforts are required to reduce the current impact of the disease.

The Search for Resistance to Cassava Mosaic Geminiviruses: How Much We Have Accomplished, and What Lies Ahead

“…For example, in Uganda, where the latest CMD pandemic first emerged in the 1990s, the IITA varieties were resistant to CMD, and to a lesser extent, cassava brown streak disease (CBSD), caused by cassava brown streak viruses (CBSVs) in East Africa (Ntawuruhunga et al, 2013; Pariyo et al, 2015; Patil et al, 2015). Similar successes have been recorded in other eastern and southern African countries (Ntawuruhunga et al, 2013; Mbewe et al, 2015; Masinde et al, 2016). In Zambia, eight IITA varieties outperformed local varieties significantly, resulting in increased yields, household income, and food security (Khonje et al, 2015).…”

Cassava whitefly,Bemisia tabaci(Gennadius) (Hemiptera: Aleyrodidae) in East African farming landscapes: a review of the factors determining abundance

“…Historically, CBSD was thought to be caused by two distinct viruses, cassava brown streak virus (CBSV) and Ugandan cassava brown streak virus (UCBSV), but Ndunguru et al (2015) have recently found more genetic diversity in both CBSV and UCBSV, suggesting that there may be more than two viruses involved. Both virus groups belong to the genus Ipomovirus , and family Potyviridae (Mbewe et al , 2015); however, CBSV has a five times faster rate of evolution, and is more virulent compared with UCBSV (Alicai et al , 2016). Unlike CMBs, CBSVs are semi-persistent in B. tabaci (Maruthi et al , 2005).…”

“…However, in the lake zone of Tanzania, mean annual rainfall and the length of the growing season were the most important environmental factors. Some studies note generally when numbers of B. tabaci are likely to be low in cassava fields based on the time of the year when temperatures are low and the environment is unsuitable for B. tabaci (Mbewe et al , 2015). At a finer scale, we know that micro-climate variability within a field can influence the numbers of B. tabaci found on cassava plants.…”

Cassava planting material movement and grower behaviour in Zambia: implications for disease management

“…Recently there have been a number of surveys assessing the impact and extent of CMD and CBSD in sub-Saharan Africa. Much of the work has concentrated on assessing the per-field disease incidence and severity on a regional level (Alicai et al 2007; Chikoti et al 2013; Gondwe et al 2003; Hillocks et al 1999, 2002; Mbewe et al 2015; Mulenga et al 2018; Rwegasira and Rey 2012). Conducted surveys were based on field observations, without assessing growers’ knowledge in terms of (i) capacity to identify the viral diseases of cassava, (ii) practices related to sourcing and exchange of planting material, and (iii) control strategies.…”