Agriculture Reference
In-Depth Information
evidence for the presence of water-soluble antibiotics in filtered ACT, even though
it inhibited germination of
B. cinerea
to a significant degree. Other studies also
found that filtration of ACT or NCT did not completely eliminate the ability of
the tea to suppress various plant pathogens (Diánez et al.
2006
; Marín et al.
2013
).
Diánez et al. (
2006
) provide some evidence to support the hypothesis that sidero-
phores are released by microorganisms to capture iron in the media, so that this
element is no longer available for other species.
The phyllosphere is a nutrient limiting environment (Lindow and Brandl
2003
).
Practitioners claim that nutrients in compost tea might help sustain microbial popu-
lations after application. When ACT was applied to grapevines in the field, the num-
ber of culturable microorganisms on the surface of Chardonnay leaves 10 days post
application remained higher than observed on leaves 30 min pre-application (Evans
et al.
2013
). At a second vineyard site in the same study, the number of culturable
bacteria and yeast declined to pre-application levels by 21 days after application
of ACT to leaves of grapevine variety Riesling, whereas the number of culturable
fungi remained higher at this time. These observations do not explain why microbi-
al numbers were sustained for a period of time, but they do indicate that populations
augmented by compost tea can decline with time and in a manner that is analogous
to a fungicide decay curve.
There is limited evidence to suggest that compost teas induce host resistance to
disease (Haggag and Saber
2007
; Siddiqui et al.
2009
; Zhang et al.
1998
). Koné
et al. (
2010
) treated tomato seedlings with various types of NCT, inoculated them
with
Botrytis cinerea
, and observed control of grey mould in the greenhouse for up
to 9 weeks. In another study involving preventative treatment, Evans et al. (
2013
)
applied ACT to leaves of Cabernet Sauvignon grapevines in pots 7 days before in-
oculation with conidia of
Erysiphe necator
. This treatment reduced mean powdery
mildew severity on the three youngest expanded leaves (at inoculation) to less than
1 %; mean severity on non-treated, inoculated leaves was 15 %. It was suggested
that either the active components of the compost tea persisted on the treated plants
and/or host resistance to a necrotrophic or biotrophic pathogen was induced. The
latter mechanism would need to be investigated using appropriate methods, such as
those described by Magnin-Robert et al. (
2007
). Such experiments must eliminate
or isolate the possibility of direct toxicity by organic or other chemicals, such as
phenolic compounds (Hoitink et al.
1997
). The effect of nutrients in compost tea
and changes in leaf nutrient status, especially calcium or silicon, or osmotic poten-
tial, could also be investigated in relation to levels of disease suppression observed
(Segarra et al.
2007
).
The effect of nutrients in compost tea on spore germination and growth of plant
pathogens in the infection court remains unclear. Harper et al. (
1981
) found that ni-
trate or ammonium forms of nitrogen supported abundant growth of
Botrytis cinerea
in vitro. If these forms of nitrogen are present in compost tea and persist on plant
surfaces, then there is potential for stimulation of saprophytic growth of
B. cinerea
on plant surfaces. It is postulated that a highly pathogen-suppressive compost tea
might be associated with no or very low levels of nitrate in the tea. Absence of ni-
trate in compost tea, recorded for the disease suppressive ACTs as tested by Palmer
et al. (
2010b
), suggests that nitrogen is present mostly in organic form, including
