Incidence, Distribution and Characteristics of Major Tomato Leaf ...

Incidence, distribution and characteristics of major tomato leaf curl and mosaic virus diseases
whereby x is distance from source of infection; P is the probability, and exp is the
exponential factor. Earlier Vanderplank (1963), determined virus spread with time by:
dN/dt = KN (Nmax – N) (2),
whereby N is number of plants infected; t is the time; dN is the difference between
number of plants infected at a particular time and another; dt is the difference in time; K
is a constant factor; and Nmax is the maximum number of plants infected.
Plumb and Thresh (1983) and Raccah (1986) also studied virus spread in time and space
in relation to prevailing weather conditions. They found that there was direct influence
of weather conditions on the rate at which viruses spread.
According to http://www.apsnet.org (2003), if disease progress is a monocyclic epidemic,
and is linear, the slope of the disease progress curve is constant. Furthermore, disease
progress in a monocyclic epidemic is proportional to the amount of initial inoculum. We
can calculate the slope of the disease progress curve, and describe a monocyclic epidemic
with linear disease progress curves using the differential equation:
dx/dt = QR (3),
whereby dx is an infinitesimally small increment in disease severity; dt is an
infinitesimally small time step; Q is the amount of initial inoculum; and R is a
proportionality constant that represents the rate of disease progress per unit of inoculum.
Since Q and R are both constant during the course of an epidemic, the slope, dx/dt, is
constant, and disease progress is linear. R has a value that represents the "average" for the
whole epidemic, a value that depends on many factors such as aggressiveness of
pathogen, host susceptibility, environmental conditions, etc.
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