Dual begomovirus infections and high Bemisia tabaci populations: two factors driving the spread of a cassava mosaic disease pandemic

Colvin, John, Omongo, C.A., Maruthi, M.N., Otim-Nape, G.W. and Thresh, J.M. (2004) Dual begomovirus infections and high Bemisia tabaci populations: two factors driving the spread of a cassava mosaic disease pandemic. Plant Pathology, 53 (5). pp. 577-584. ISSN 0032-0862

Abstract

A cassava mosaic disease (CMD) pandemic currently affects much of East and Central Africa. To understand the factors
driving the pandemic’s continued spread, complementary data sets were collected from cassava plots, planted with
healthy cuttings, at eight sites along a north–south transect in southern Uganda, through the pandemic’s leading
edge. Data were collected on virus incidence, symptom severity, populations of the whitefly vector,
Bemisia tabaci
, their
infectivity and ability to transmit different viruses. In 1996, 6 months after planting, CMD incidences were highest at
sites 1 and 2, then decreased progressively until site 6, and remained low at sites 7 and 8. The largest
B. tabaci
populations
also occurred at northernmost sites, 1–3. In 1997, CMD incidence increased significantly at sites 5–8 and this was
associated with significant increases in the
B. tabaci
populations. The pandemic’s spread was also associated with significant
increases in the percentage of dual infections of
East African cassava mosaic virus
-Uganda and
African cassava
mosaic virus
, which caused the severest symptoms and the greatest reduction in leaf area. Whitefly adults collected from
within the pandemic area were infective, whereas those collected ahead of the pandemic were not. The transmission rate
of
African cassava mosaic virus
from plants with dual infections was significantly less than that of
East African cassava
mosaic virus
-Uganda, which may explain the latter’s predominance within the pandemic. These results show that the
arrival of
East African cassava mosaic virus
-Uganda into areas affected previously only by
African cassava mosaic virus
,
has resulted in novel virus/vector/host–plant interactions that drive the pandemic’s continued spread.

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