Allelochemicals Biologica... - Name

88
SCOTT W. MATTNER
acids. Sodium salts of myristic, palmitic, oleic and stearic acids suppressed the growth
of clover at concentrations as low as 5 ppm (Takahashi et al., 1993). Similarly, in a
study examining the allelopathic effects of a number of crop and pasture species,
Halsall et al. (1995) found that aqueous extract from the dried shoots of perennial
ryegrass suppressed the germination, radicle elongation, nodulation and seedling root
elongation of subterranean and white clover. The magnitude of this inhibition increased
as the concentration of the extract increased.
Crown rust, caused by Puccinia coronata f.sp. lolii, is the most devastating fungal
disease of ryegrass, with epidemics regularly occurring between spring and autumn
in temperate regions worldwide (Mattner and Parbery, 2001). Severe epidemics reduce
ryegrass tillering by 20-38% (Lancashire and Latch, 1966; Mattner, 1998), leaf
emergence by 60%, leaf area by 62%, root growth by 75% (Mattner, 1998), and increase
the rate of leaf senescence by up to 184% (Lancashire and Latch, 1966; Trorey, 1979;
Plummer et al., 1990; Mattner, 1998). Losses of herbage yield in ryegrass from rust
have been as great as 94% (Critchett, 1991), with seed yield losses ranging from 12-
36% (Hampton, 1986; Mattner 1998). Furthermore, rust infection reduces forage
quality (Isawa et al., 1974; Trorey, 1979; Potter, 1987) and palatability to grazers
(Cruickshank, 1957; Heard and Roberts, 1975).
As would be expected by the devastating effect that rust has on ryegrass growth,
most studies show that rust reduces the competitiveness of ryegrass with non-host
plants such as clover. For example, in mixed swards of ryegrass and clover, Lancashire
and Latch (1970) found that rust reduced ryegrass yield by 84% and increased the
yield of clover by 87%. Furthermore, the proportion of clover in the rusted sward
increased from 24% at the beginning to 80% at the termination of their experiment.
Thus, their study pointed to a lowered competitiveness of rusted ryegrass. In mixtures
of rust resistant and susceptible ryegrass, Potter (1987) found that rust reduced the
yield of the susceptible cultivar and increased that of the resistant one, concluding
that rust reduced ryegrass competitiveness. Similarly, crown rust infection in swards
of ryegrass and cocksfoot (Dactylis glomerata) reduced ryegrass composition from
30% to 15%, and was more marked in rust susceptible than resistant cultivars (Trorey,
1979). However, in a series of experiments, Mattner (1998) reported an anomaly to
the results of this previous research.
In pot studies consisting of 50:50 mixtures of ryegrass and clover grown over a
range of plant densities, Mattner (1998) found that rust reduced the yield of ryegrass
by an average of 41%. However, interference from rusted ryegrass suppressed clover
biomass by up to 47% compared with interference from the more productive, healthy
ryegrass. The onset of the suppression of clover by rusted ryegrass was rapid, occurring
as early as 6-13 days after inoculation, which according to some growth parameters
was earlier than the effects of rust on ryegrass itself. The suppression of clover by
rusted ryegrass was greatest at low plant densities and diminished or disappeared as
density increased. In a separate trial, rusted ryegrass again suppressed clover growth,
even after the removal of infected tissue by cutting and after the death of the ryegrass.
In this instance, ryegrass killed by infection, with a competitive ability of virtually
zero, inhibited the growth of clover more than living plants of healthy ryegrass. In