Allelochemicals Biologica... - Name

IMPACT OF PATHOGENS ON PLANT INTERFERENCE AND
ALLELOPATHY
3.1. The Impact of Pathogens on Competition
Several pioneering publications on plant competition suggest that pathogens might
alter the balance of competition in mixed communities in favour of the resistant
components (de Wit, 1960; Harper, 1977). Since then, there have been numerous
reviews and theoretical interpretations of the impact of pathogens in natural
communities and on competition (Chilvers and Brittain 1971; Burdon, 1982; 1987;
1991; Dinoor and Eshed, 1984; Gates et al., 1986; Alexander, 1990; Ayres and Paul,
1990; Clay, 1990; Paul, 1990; Dobson and Crawley, 1994; Jarosz and Davelos, 1995;
Alexander and Holt, 1998; Mattner 1998), but empirical experimentation is less
extensive. Empirical studies have usually involved binary mixtures of a host and a
non-host, grown under optimal conditions of nutrient and water availability, and
inoculated with a single, copious and homogenous dose of inoculum. With few
exceptions, these studies show that the influence of a host-specific pathogen reduces
the vigour of the host, rendering it less able to compete with a neighbouring non-host
species (Burdon, 1987; Ayres and Paul, 1990; Mattner, 1998). As such, the combination
of the ‘burden of a parasite’ and competition can have a devastating effect on a plant.
For example, Groves and Williams (1975) demonstrated that the combined effects of
competition from subterranean clover (Trifolium subterraneum) and rust infection
(Puccinia chondrillina) reduced the yield of skeleton weed (Chondrilla juncea) by
94%. This combined effect was more marked than the action of either the competitor
(yield was reduced by 70%) or the rust (yield was reduced by 51%) alone. Similarly,
Friess and Maillet (1996) found that infection by cucumber mosaic virus reduced the
vegetative yield of its host purslane (Portulaca oleracea), with this effect intensifying
when infected plants were in competition with healthy plants. In mixtures of common
lambsquarters (Chenopodium album) and corn (Zea mays) or beetroot (Beta vulgaris),
foliar infection by Ascochyta caulina reduced the competitiveness of its host
(lambsquarters) and increased the yield of non-host crops. In corn, infection negated
the effects of competition from lambsquarters altogether, highlighting its potential as
a biological control agent (Kempenaar et al., 1996). Despite pathogens reducing the
competitiveness of their hosts, the effect of infection is often greater on a neighbouring
non-host than on the host itself. For example, in mixtures of groundsel (Senecio
vulgaris) and lettuce (Lactuca sativa), rust (Puccinia lagenophorae) increased the
biomass of lettuce (the non-host) markedly, while only reducing that of its host
groundsel marginally (Paul and Ayres, 1987a).
Competitive stress for limited resources intensifies as plant density increases
(Shinozaki and Kira, 1956). Consequently, the effect of a pathogen on the
competitiveness of its host is most devastating at high densities. For example, rust
reduced the competitive ability of groundsel in mixtures with healthy groundsel (Paul
and Ayres, 1986) or with lettuce (Paul and Ayres, 1987a) more so at high densities
than at low densities. Similarly, Ditommaso and Watson (1995) demonstrated that
anthracnose (Colletotrichum coccodes) was more detrimental to the growth of
velvetleaf (Abutilon theophrasti) in mixtures with soybean (Glycine max) at high
plant densities. Conversely, the performance of the non-host, relative to the infected
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