Agriculture Reference
In-Depth Information
The perennial weeds
Cynodon dactylon, Sorghum hlepense
and
Convolvulus arvensis
are
also less sensitive to solarization (Elmore, 1991). Moreover, some annual weeds, such as
Melilotus sulcatus
Desf., are not controlled by solarization (Cohen & Rubin, 2007).
It should be emphasized that since SH is climate-dependent, it is not surprising that
different results pertaining to the effectiveness of control of a certain pathogen are reported
from different climatic regions.
10.4
Mechanisms of control and plant-growth
improvement
Mechanisms of pathogen and disease control
10.4.1
The fi rst studies on SH were already showing that disease and pathogen control could be
obtained, even under situations that are considered marginal from a climatic point of view.
This indicated that pest control by solarization is more than merely physical killing of
pests at elevated temperatures, and that additional processes, for example, biocontrol, are
involved. The long-term effect of solarization reported in various studies (e.g. Katan
et al.,
1983; Tjamos & Paplomatas, 1988; Satour
et al.,
1989; Stevens
et al.,
2003) supported the
notion of a shift in the soil equilibrium that is suppressive to the pathogen, which could
be connected to the physical, chemical and biological processes occurring in the soil dur-
ing and after solarization. Many studies have shown that biological control processes are
induced or stimulated in the solarized soil, thus contributing to pathogen control.
The reduction in disease incidence occurring in plants grown in solarized soils, as with
any soil treatment, results from the effects exerted on each of the three living compo-
nents involved in the disease - the pathogen, the surrounding soil microorganisms and
the host plant, as well as on the physical and chemical environment which, in turn, affects
the activities and interrelationships of these biotic components (Cook & Baker, 1983).
Although these processes occur primarily during solarization, they may continue to vari-
ous extents and in different ways after removal of the polyethylene sheets and planting.
SH may affect inoculum density, inoculum potential or both. It is not possible to cover
all of the studies related to biocontrol processes induced by solarization. Below are just
some typical examples.
The 'weakening' effect is an important mechanism involved in solarization and
possibly other disinfestation methods. Thus, propagules of the pathogen may survive
sublethal heating (and therefore, inoculum density will show no initial major reduc-
tion), but the partially damaged (weakened) propagules become more vulnerable to
the biocontrol processes and microbial activities taking place in the soil. Consequently,
inoculum density of the pathogen will show later a more rapid decline. This has been
shown with various pathogens, including
F. oxysporum
f. sp.
niveum
(Freeman & Katan,
1988),
Sclerotium
rolfsii
(Lifshitz
et al.,
1983),
Armillaria
mellea
(Munneke
et al.,
1976),
Sclerotinia
minor
(Philips, 1990),
V. dahliae
(Tjamos & Fravel, 1995),
F. oxysporum
f. sp.
ciceri
(Arora
et al.,
1996) and others. Solarization and organic amendments had a
stronger effect on disease severity by
M.
phaseolina
than on inoculum density, possibly
indicating a weakening effect (Ndiaye
et al.,
2007). Flow-cytometric, physiological and
micoscopic studies on the viability of sublethally heated conidia of
Fusarium
showed that,
although apparently not affected when examined shortly after heating, their population
