Control of plant disease through soil solarization - Disease Control in Crops - page 202

was nevertheless reduced later, suggesting delayed mortality (Assaraf et al., 2002). These
studies suggest that assessing the level of pathogen populations shortly after exposure to
a control agent does not necessarily refl ect the full control potential, or the potential of the
weakening phenomenon induced by sublethal treatment.
Various studies have shown stimulation of populations of benefi cial microorganisms
(e.g. biocontrol agents and plant-growth promoters) in solarized soils. These include
microorganisms such as Trichoderma spp., fl uorescent pseudomonads, Bacillus spp.,
Talaromyces and others (Elad et al., 1980; Stapleton & DeVay, 1982; Greenberger et al.,
1987; Gamliel & Katan, 1991; Tjamos et al., 1991; Stevens et al., 2003). Moreover,
frequently (but not in all cases), solarized soils become more suppressive to pathogens,
which is apparently connected with the aforementioned long-term effect (Kassaby, 1985;
Greenberger et al., 1987; Freeman et al., 1990; Gamliel & Katan, 1993). An additional
mechanism might be partial or complete nullifi cation of fungistasis in the absence of the
host, thus exposing the vulnerable germinating propagules to the antagonistic action of
soil biota (Greenberger et al., 1987). An analogous situation was described by Cohen
et al. (2004) with the invasive plant Acacia saligna . They found that solarization was very
effective in reducing the population of this plant, despite the fact that its seeds are highly
tolerant to elevated temperatures. A possible explanation is that solarization nullifi ed the
dormancy of the seeds and consequently, the heat-sensitive germinating seeds were killed
upon exposure to the high temperatures induced by solarization, essentially by 'suicidal
germination'.

10.4.2 Induced resistance as a mechanism of disease control

Most of the studies on mechanisms of disease control by solarization concentrate on
either the direct effect on the pathogen, namely, thermal killing, or the indirect effect
via stimulation of microbial antagonistic activity in the soil as detailed above. However,
another indirect effect, via induced resistance in the plant, should also be considered
(Katan, 1981). In recent years, many studies have shown that certain biocontrol agents
induce resistance and consequently disease reduction. There are studies which demon-
strate physiological, including hormonal, changes in plants growing in solarized soils
(Grunzweig et al., 1993, 2000). In these soils, plant growth is frequently stimulated and
mineral nutrient levels are increased, as detailed further on. These effects have also the
potential to affect plant resistance. SH has been shown to reduce some foliar diseases
despite the fact that only the roots are in contact with the solarized soil (Hassan & Younis,
1984; Daelemans, 1989; Stevens et al., 1992, 1996; Lopez-Herrera et al., 1994; Levy
et al., 2005). The works of Stevens et al. (1996) and Levy et al. (2005) indicate that
reduction of foliar diseases in solarized soil is connected with induced resistance. Since
only the roots are exposed to the solarized soil, signals may move upward in the plant,
causing physiological changes (Grunzweig et al., 1993). Solarization frequently stimu-
lates rhizobacteria, such as fl uorescent pseudomonads and Bacillus (Stapleton & DeVay,
1984; Gamliel & Katan, 1991; Stevens et al., 2003), which are potential inducers of
resistance (see Chapter 4). These can also indirectly contribute to induced resistance in
the solarized soil.
It can be concluded that SH can affect disease incidence via a variety of mechanisms,
beyond its direct effect on the pathogen.