BOX 5-3 Implications for Epidemiology  Epidemiologic models can
be useful for evaluating the potential of various management
strategies to control the spread of plant pathogens (Jeger and
Chan, 1995). In the case of insect-transmitted pathogens, such
models suggest that effects on vector fitness that lead to
higher rates of population growth on infected plants could
significantly affect rates of pathogen spread (Holt et al.,
1997; Zhange et al., 2000). One model of the whitefly-
transmitted African cassava mosaic virus is based on substantial
field data (Holt et al., 1997) which showed that the rate of
spread was sensitive both to vector population dynamics
(abundance, birth rate, mortality) and to virus transmission
rates (inoculation and acquisition). When virus infection of
hosts leads to increased vector fecundity, spatial aggregation
of vectors is promoted (Zhang et al., 2000). Models predict that
vector aggregation should have a dual effect. Within the
infected crop, it should reduce the effective contac