Agriculture Reference
In-Depth Information
the production system has been shown twice to produce teas that meet the United States
Environmental Protection Agency guidelines for recreational water, otherwise, a 90-day
harvest interval applies when they are to be used on non-grain food crops. In practice,
these standards are likely to eliminate the use of compost teas produced with added nutri-
ents on food crops, since few producers are likely to be prepared to pay for the necessary
testing. Given the apparent importance of added nutrients to compost tea effi cacy (based
on recent experimental results), considerable further work is needed to determine whether
human pathogen re-growth in compost teas can occur and to what extent. Methods to
prevent re-growth are also needed in order to ensure consumer safety and use of compost
teas as part of integrated disease control strategies. There is continuing debate in Europe
over whether compost extracts and teas require to be registered with national pesticide
authorities such as The UK Pesticides Safety Directorate or whether their use should be
regulated through national food standards bodies.
5.9
Mechanisms involved in the suppression/control
of plant disease using composts and compost
extracts/teas
Composts
5.9.1
Four mechanisms have been described for the activity of biocontrol agents present in
composts against soil-borne plant pathogens. They are:
(a)
successful competition for nutrients (including primarily C, N, Fe), oxygen and
infection sites;
antibiosis;
(b)
(c)
parasitism and predation;
(d)
induced systemic resistance.
Most reports of disease suppression suggest that microbiostasis (i.e. competition and/
or antibiosis) and hyperparasitism are the principal mechanisms. Competition results
when there is a demand by two or more microorganisms for a resource; competition
for that resource may lead to disease control, where a non-pathogen successfully out-
competes a plant pathogen. For example, competition seems to be the main mechanism
whereby
F. oxysporum
is controlled by
Trichoderma harzianum T-35
(Sivan & Chet,
1989). Siderophores, such as low molecular weight ferric-specifi c ligands, are produced
under iron-limiting conditions by some benefi cial microorganisms to ensure that they
secure suffi cient iron. This in turn can limit iron availability for some pathogens such as
Pythium
spp. and can result in reduced disease incidence.
Antibiosis occurs when the production of specifi c and/or non-toxic specifi c metabolites
or antibiotics by one organism has a direct effect on another organism. The biological
control of several plant pathogens is known to be mediated at least in part through anti-
biosis. For example, the toxin 'gliotoxin' has been shown to cause the antagonism of
Gliocladium virens
against
Pythium ultimum
(Roberts & Lumsden, 1990). Weller
et al.
(2002) found that take-all decline in barley, caused by
Gaeumannomyces graminis
var
tritici
was related to a build-up of fl uorescent pseudomonad bacteria, which produced the
antifungal metabolite 2,4-diacetylphloroglucinol.
