Agriculture Reference
In-Depth Information
the components grown as pure stands as the disease approaches the mean carrying
capacity on the components. Depending on the carrying capacity of the mixture and
the duration of the epidemic, disease on the mixture may eventually equal the mean
of that on the pure stands as Sitch and Whittington (1983) showed for powdery
mildew (caused by
Erysiphe polygoni
)
of swede (
Brassica napus
). However, the
early delay in epidemic development in the swede mixture, at a crucial time for plant
development, was sufficient to limit yield loss due to powdery mildew.
All diseases start from initial foci but they are often divided into 'focal' and
'non-focal' depending on the rate of new infections and dispersal relative to
symptom development, the number and proximity of the original foci, and the
source of the initial inoculum e.g. volunteers, crop residues or an incoming spore
cloud.
Using focally-inoculated oat crown rust as an example, Mundt and Browning
(1985b) investigated different sizes of 'host genotype units' (GU) to determine
whether blocks of pure cultivars would be as effective as an intimate mixture in
containing the initial infection and spread of the pathogen. Experiments (Mundt and
Browning, 1985b; Mundt and Leonard, 1986) and computer simulations (Mundt
et al.
, 1986; Mundt and Brophy, 1988) indicated that the number and the size of the
GUs are important parameters but intimate mixing may not be essential for optimal
restriction of disease. Where several different pathogens are expected, the best
planting arrangement will depend on the pathogen with the shallowest dispersal
gradient.
Species mixtures have the added advantage that the pathogens involved are
specialised to one of the components with virtually no possibility of selection of
races able to attack more than one. Some species mixtures have been used widely
for many years, for example in temperate and tropical, short-term and long-term
pastures. Cereal species mixtures for feed production were grown on more than 1.4
Mio ha in Poland and have been shown consistently to restrict diseases (Czembor
and Gacek, 1996). More recent experiments in Poland also demonstrate possible
positive effects on weeds in cereal and cereal-legume species mixtures (Gacek,
personal communication; see also Bulson
et al.
, 1990). In Switzerland, the 'maize-
ley' system (i.e. maize planted without tillage into established leys), which is
promoted to reduce soil losses and nutrient leaching, has been shown to reduce smut
(caused by
Ustilago maydis)
and attacks by European stem borer (
Ostrinia nubilalis)
and aphids (Bigler
et al.
, 1995).
Examples at different scales
Substantial reductions of several air-borne foliar diseases have been reported in
experiments with multilines, cultivar and species mixtures of cereals (Wolfe and
Barrett, 1980; Chin and Husin, 1982; Alexander
et al.
, 1986; Koizumi and Kato,
1987; Wolfe, 1987; McDonald
et al.
, 1989; Finckh and Mundt, 1992a,b; Czembor
and Gacek, 1996, among others, see Finckh
et al.,
2000 for review). Cultivar
mixtures and multilines are being used on a commercial scale in the US, Denmark,
Finland, Poland, Switzerland (Table 10.2) and Colombia (coffee), to control,
