Agriculture Reference
In-Depth Information
conidia in vitro when different volumes of ACT prepared from immature compost
were diluted in sterile water by adding 25, 15, 5, 1.67, 0.5 or 0 µl tea to a total vol-
ume of 50 µl. Diluted ACT was then mixed with 50 μl of 1 × 10
5
B. cinerea
conidia
ml
−1
sterile water. After incubation, the mean germination proportion of conidia
was less than 0.1 at all dilutions of ACT and 0.97 in water. However, the effect of
diluting compost tea for crop protection needs to be tested under field conditions.
9.5
Mechanisms of Action
Disease suppression by biological control agents has been explained by mainly bi-
otic mechanisms; including, direct antagonism through production of antibiotic or
lytic compounds (Cronin et al.
1996
; Fogliano et al.
2002
), parasitism (Stewart
2001
), competition for nutrients or niche (Kredics et al.
2004
), siderophore-medi-
ated competition for iron (Diánez et al.
2006
) and inducible plant defence mecha-
nisms (Droby et al.
2002
; Reglinski et al.
2005
; van Loon et al.
1998
). Mechanisms
of action for species of
Trichoderma
, for example, include parasitism of pathogenic
fungal mycelia, antibiosis and/or production of cell wall degrading enzymes such as
chitinases and β-1,3-glucanases (Elad
1994
; Metcalf
2002
; Stewart
2001
). Myriad
mechanisms of action might be associated with any particular compost tea, with
the relative contribution of each mechanism varying depending on environmental
conditions, including the physiological state of the host plant.
The presence of microorganisms in compost, and hence teas, has been demon-
strated to be a contributing factor to the level of pathogen or disease suppression
observed (Haggag and Saber
2007
; Koné et al.
2010
; McQuilken et al.
1994
; Palmer
et al.
2010b
; Pane et al.
2012
; Scheuerell
2004
). Some authors postulate that the
total bacterial count is associated with the efficacy of NCT (Al-Mughrabi
2006
;
Cronin et al.
1996
; Diánez et al.
2006
); however, a number of reports do not support
this hypothesis. Assuming there is some minimum number of culturable microor-
ganisms needed for disease suppression, the absolute number does not appear to
be related to the level of disease suppression (Palmer et al.
2010b
; Scheuerell and
Mahaffee
2004
,
2006
). An alternative hypothesis is that specific microorganisms
are important for disease suppression, rather than microbial abundance per se. If so,
then toxic metabolites might be induced by specific microbial taxa in the presence
of the plant pathogen (Martinez et al.
2006
).
Inhibition of pathogen spore germination by compost tea has been reported pre-
viously; for example, NCT prepared from spent mushroom compost reduced the
germination percentage of conidia of
Venturia inaequalis
, the cause of apple scab
(Cronin et al.
1996
). Palmer et al. (
2010b
) found that ACT prepared from immature
compost inhibited the germination of
Botrytis cinerea
conidia to a greater extent
than ACT in which microorganisms were removed by filtration, even when ACT
was diluted 1 part in 10 with water. The large difference in activity between unfil-
tered and filtered ACT suggested that maximum inhibition of
B. cinerea
required
close proximity between microorganisms in ACT and the pathogen. There was no
