Agriculture Reference
In-Depth Information
(Ketterer, 1990; Scheuerell & Mahaffee, 2002). However, compost extracts and teas have,
in other studies, been shown to have little or no effect on plant pathogens or disease
(Al-Dahmani
et al.
, 2003; Al-Mughrabi, 2006). Research also suggests that different
composts, preparation and application methods affect the effi cacy of the fi nal product
(Weltzein, 1989; Urban & Tränkner, 1993; Elad & Shtienberg, 1994; Yohalem
et al
.,
1994; Brinton
et al.,
1996; Cronin
et al
., 1996). The main factors affecting disease sup-
pression are discussed below.
5.8.1.1
Aeration
Most commercially available compost tea brewers are designed to produce ACTs rather
than NCTs, but there is considerable debate over whether aeration produces a tea which
is more effective in suppressing plant pathogens. Ingham (2003) states that NCTs are
less effective than ACT's, but the majority of the scientifi c literature which demonstrates
suppression of plant pathogens concerns NCTs. Few studies have directly compared the
effects of ACTs and NCTs made from the same compost source, although those that
have done so conclude that aeration of the tea has no effect on disease control (Mahafee
& Scheuerell, 2006). The popular literature also contains repeated references to the fact
that NCTs are often phytotoxic, but again, there appears to be no documented evidence
to support these claims.
5.8.1.2
Compost source and age
The nature of the compost used to produce compost teas (including feedstock, production
method and age) has been shown to affect the disease-suppressive properties of the teas.
It is diffi cult to draw clear conclusions from past work. For example, some workers found
that teas made from composts based on animal manures were more effective than those
made from composts based on undigested vegetable matter. However, others demon-
strated that those made from composts based on undigested vegetable matter were equally
effective. Scheuerell & Mahaffee (2006) examined the effects of compost teas made from
30 different composts and found that disease suppression was associated with the particu-
lar batch of compost and not necessarily the feedstocks used to create the compost.
The impact of compost age on the disease-suppressive properties of compost teas
has been studied extensively, but it is diffi cult to draw meaningful conclusions from the
results. Several workers have suggested that the ideal time to use a compost to make
compost tea depends on its maturity (which has a signifi cant impact on microbial numbers,
microbial diversity and the presence of benefi cial microorganisms). The time which com-
post takes to reach maturity in turn depends on the feedstocks used and the composting
method. However, defi nitions of compost maturity vary, which makes interpretation of
the literature diffi cult.
5.8.1.3
Fermentation time
For both NCTs and ACTs, disease-suppressive properties have been found to increase
with fermentation time to a maximum, then decline (Ketterer, 1990; Ketterer & Schwager,
1992). It is thought that the optimum fermentation time is likely to depend on the compost
