Agriculture Reference
In-Depth Information
5.8.1.6
Application frequency
Decisions as to whether compost teas can be part of an economically viable integrated
disease control strategy depend on the frequency of applications required to produce
disease-suppressive effects. However, the effects of frequency of application of compost
teas on disease suppression have not been studied. Most studies carried out to date have
used spray intervals similar to those used for synthetic pesticides.
5.8.1.7
Use of adjuvants
Various adjuvants have been studied with the aim of improving the disease-suppressive
properties of compost teas. The four main types of adjuvants which have the potential for
use include:
(a)
spreaders
- which reduce the surface tension of spray droplets, thus allowing them to
spread evenly over leave surfaces rather than lying in beads.
stickers
(b)
- which enhance the ability of compost teas to adhere to plant surfaces.
protectants
(c)
- which protect microbes from stresses due to desiccation, UV light and
so forth.
nutrients
(d)
- which can be used as food substrates for the benefi cial microorganisms in
the extracts/teas.
The majority of researchers working in this fi eld have recorded increased disease
suppression where specifi c adjuvants have been added to compost extracts/teas prior
to application. Adjuvants which have been shown to enhance the disease-suppressive
properties of compost teas include a range of spreaders and stickers including methyl
cellulose (Sackenheim
et al.
, 1994; Scheuerell, 2002; Scheuerell & Mahaffee, 2004) and
various nutrients including molasses, bouillon, rape seed oil and casein (Scheuerell &
Mahaffee, 2002; Mahaffee & Scheuerell, 2006). Further work is required to determine the
best combinations of adjuvants to use for specifi c situations.
5.8.2
Problems with the use of compost extracts/teas
One of the most controversial issues associated with the use of compost teas at present, is
their potential to propagate and spread human pathogens contained in the feedstock. For
example, Welke (1999) detected faecal coliform and Salmonella populations in the source
compost, the NCT fermentation and on samples of broccoli and leek growing in a fi eld
and sprayed with the NCT. Evidence has shown that pathogens can grow during the pro-
duction of both ACTs and NCTs. However, the indications are that pathogen growth is not
supported when ACTs or NCTs are prepared without fermentation nutrients (Ingram &
Millner, 2007). Further work is required to ensure that the production and use of compost
teas and extracts can be guaranteed not to propagate and spread human pathogens on to
food intended for human consumption.
The issue was recently addressed by the United States National Organic Standards
Board (US NOSB, 2004). They set out rules for the safe production of compost tea
which, if followed, mean that compost teas produced without added nutrients can be
applied without restriction. Compost teas made with added nutrients can be used only if
