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Cultivar
Mixtures
Effect of cultivar
mixtures on epidemic development
The development of a
plant disease epidemic is a function of the initial inoculum, the rate of
disease development and the duration of crop growth (see
APSnet
Epidemiology topic).
Cultivar mixtures affect the epidemic factors as follows:
a) The initial inoculum
level can be reduced. Compared to a pure-line susceptible population, the
initial inoculum is lowered when a given race or strain is not completely
virulent on all genotypes in the mixture.
b) The rate of secondary
infection can be reduced. As the proportion of susceptible tissue
available for a given race of the pathogen is lowered, the number of new,
secondary infections by this race is reduced, which results in a decrease
in the observed ‘apparent infection rate.’
Leonard’s (1969)
classic formula describes this second effect based on a single pathogen
genotype in a simple mixture of one susceptible and one immune plant
genotype:
X'/X0 = mn
. X/X0
X = Proportion of
infected tissue in a population composed only of the susceptible
genotype
X'= Proportion of
infected host tissue for the susceptible genotype in the mixture
X0= Proportion of
host tissue initially infected
m = Proportion of
susceptible plants in the mixture
n = Number of
generations of successful infection and reproduction by the pathogen
Although this formula is
a simplification of reality (for example, it doesn’t consider the
spatial configuration of host genotypes and the pattern of inoculum
dispersal), it summarizes the ‘mixture effect’ as a function of m and
n. It predicts that the disease suppressing effects of the mixture
will increase linearly with an increasing proportion of resistant plants
(lower m) but exponentially with more cycles of pathogen infection
and reproduction (higher n). Thus, mixtures are most effective
against pathogens with polycylic disease cycles (see
APSnet
Epidemiology topic).
Empirical results in
maize (see figure below) show that mixtures of resistant and susceptible
genotypes slow the rate of increase of common maize rust (measured as the
cumulative number of pustules per susceptible plant) compared with pure stands
of susceptible genotypes. The table at the bottom of this page shows other
examples of cultivar mixtures in which some degree of disease suppression
was achieved.
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Cumulative
numbers of common maize rust pustules on susceptible plants in
pure stands and in mixtures of 25% susceptible and 75% resistant
plants (adapted from Mundt and Leonard, 1986).
Click
image for an enlarged view. |
| Examples
of degree of disease suppression achieved through the use of
cultivar mixtures. |
|
Plant |
Pathogen |
Reference |
Trait |
Disease reduction |
|
Maize |
Rust |
Mundt and Leonard, 1986 |
Pustules/plant |
50% |
|
Snap beans |
Bean rust |
Mundt and Leonard, 1986 |
AUDPC |
30-60% |
|
Wheat |
Stripe rust |
Mundt, 1994 |
Severity |
14-64% |
|
Wheat |
Leaf rust |
Mundt, 1994 |
Severity |
45% |
|
Barley |
Scald |
Mundt et al., 1994 |
Severity |
12% |
|
Barley |
Scald |
Newton et al., 1997 |
Severity |
11-50% |
|
Barley |
Powdery mildew |
Newton et al., 1997 |
Severity |
0-20% |
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Rust, caused by Uromyces
appendiculatus, on the upper leaf surface of a bean leaf.
(Courtesy R. Hall) Click image for
an enlarged view. |
 |
Uredinia of the common rust
pathogen, Puccinia sorghi, on corn. (Courtesy C. De Leon) Click image for
an enlarged view. |
 |
Typical leaf symptoms of barley
scald. (Courtesy L. Jackson) Click image for
an enlarged view. |
 |
Cottony mycelial growth with dark,
spherical cleistothecia of Blumeria graminis, the powdery
mildew fungus, on barley. (Courtesy D. Mathre) Click image for
an enlarged view. |
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