Current characteristics of cassava mosaic disease in postepidemic areas increase the range of possible management options

Abstract

As a case study to document the current characteristics of cassava mosaic virus disease (CMD) in postepidemic areas, surveys were carried out, in 2003 and 2004, in Siaya District of western Kenya. This was an area affected by a severe CMD pandemic in the late 1990s. Data recorded on cassava varieties were CMD incidence, severity index and number of adult whiteflies. Farmers (174) were interviewed on their understanding of the disease and their knowledge and practice of management interventions. Cassava cultivation was being reestablished, but local landraces predominated. Resistant varieties were present 13% in 2003, and 4% in 2004, of the surveyed fields. Adhiambolera was the most common variety, occurring in 35% and 40% of fields in 2003 and 2004, respectively, and had an average CMD incidence of 82% in 2003 and 73% in 2004. By contrast, the CMD-resistant variety Migyera had a low mean incidence (28% in 2003). The overall incidence for both years was 71%, consisting of 61% as a result of infection through planting diseased cuttings and 10% as a result of whitefly infection. In 2003, the total incidence was 72% and the average severity 2.7 (severity index), while in 2004 the incidence was 78% and the severity 2.6. There were significant severity variations in each division of the Siaya District during the 2 years except for Karemo and Ukwala. The abundance of whiteflies on the top five leaves of plants was low in 2003 but high in 2004, with means of 1 and 16, respectively, over the same seven divisions in both years, although this variation was thought to be because of seasonal factors. East African cassava mosaic virus-Uganda was the predominant geminivirus present in every division. Phytosanitation by farmers was minimal, as evidenced by 29% of farmers using a selection of CMD-free stems for planting and 15% using hand-roguing for CMD management. Occurrence of more than 25% CMD-free plants in 2004, moderate CMD severity and limited spread provide a conducive environment for the use of phytosanitation as a CMD control measure that can be immediately used by farmers growing their own cassava varieties.

Evaluation of Cassava Cultivars for Resistance to Cassava Mosaic Disease and Yield Potential in Central African Republic

“…Such an important difference between cropping areas already also was observed in Ivory Coast, where trials were carried out showing disease incidence that varied dramatically following the agro-climatic conditions as well as the availability of inoculum sources (Fauquet and Fargette 1990). Damara and Pissa could be areas of high and low inoculums, respectively (Ogbe et al 2003;Mallowa et al 2006). Some authors have shown that the soil type and fertility also may influence the global pressure of the disease (Hahn et al 1980;Sseruwagi et al 2003;Muengula-Manyi et al 2012).…”

A risk assessment framework for seed degeneration: Informing an integrated seed health strategy for vegetatively-propagated crops

“…For viral diseases and bacterial wilt in potato, farmer-managed trials of seed selection resulted in a ~30% yield increase, with lower disease incidence (Gildemacher et al 2011; Schulte-Geldermann et al 2012). Seed selection also increased the tuberous root yields of CMD-susceptible cassava varieties (Mallowa et al 2006). In parts of western Kenya where CMD is in its post epidemic phase, there is a resurgence of local landraces that are CMD-susceptible, partly because farmers choose the most vigorous plants as seed sources (Mallowa et al 2006).…”

“…Seed selection also increased the tuberous root yields of CMD-susceptible cassava varieties (Mallowa et al 2006). In parts of western Kenya where CMD is in its post epidemic phase, there is a resurgence of local landraces that are CMD-susceptible, partly because farmers choose the most vigorous plants as seed sources (Mallowa et al 2006). Farmers may decide against roguing when diseased plants produce usable yield, limiting the practical usefulness of roguing (Legg et al 2015; Mallowa et al 2011; Sisterson and Stenger 2013).…”

“…In our evaluation, under low external inoculum (e.g., when crop fields are isolated from each other), and when 10% of the cuttings underwent reversion, incidence-altering management was better for managing degeneration than rate-altering management strategies. In post CMD epidemic areas (where there is reduced inoculum), seed selection along with reversion and natural cross protection by mild viruses together allow farmers to cultivate locally preferred, CMD-susceptible varieties (Mallowa et al 2006).…”

“…However, specialized programs designed to produce healthy seed are rarely used by smallholder farmers in low-income countries (Thiele 1999; Thomas‐Sharma et al 2016). In many low-income countries, 80-95% of seed is routinely obtained from informal seed sources with poor or unknown seed health status (Mallowa et al 2006; McGuire and Sperling 2016; Thomas‐Sharma et al 2016). Thus, one focus for the application of this new risk assessment framework is the context of low-income countries, especially food-security crops such as banana and plantain, cassava, potato, sweetpotato, and yam.…”