The use of composts and compost extracts in plant disease control - Disease Control in Crops - page 115

those responsible for general suppression cannot be transferred as easily (Baker & Cook,
1974). Examples of pathogens suppressed in this way include Rhizoctonia solani and
Sclerotium rolfsii (Chung & Hoitink, 1990; Gorodecki & Hadar, 1990). These pathogens
both produce large propagules known as sclerotia which do not rely on exogenous C
sources for germination and infection. During suppression, the sclerotia are colonized
by specifi c hyperparasites (mainly Trichoderma spp.) and their inoculum potential is
reduced. Suppression of damping-off caused by Rhizoctonia solani is variable, mainly
due to the random nature of colonization of compost by effective biological control agents
after peak heating (Hoitink et al. , 1997). The location of the compost pile in relation to
naturally occurring biological control agents (e.g. in forests and agricultural systems) is
important in this context.
Microorganisms that produce antibiotics and those that induce systemic resistance in
plants (to specifi c pathogens) represent other examples of specifi c suppression.

5.9.2 Compost extracts and teas

Most workers acknowledge that several modes of activity are involved in disease
suppression following application of compost extracts and NCTs. To date, there is no
published work that has determined the mechanisms involved with ACTs. The effects of
compost extracts/teas appear mainly to be associated with live microorganisms, since the
activity of sterilized or micron-fi ltered compost extracts has in some cases been shown to
be reduced against test pathogens (Weltzein & Ketterer, 1986). In a few cases however,
activity has been unaffected following sterilization/micron fi ltration (Yohalem et al .,
1994; Cronin et al ., 1996).
The main living active agents in compost teas are thought to be bacteria in the genera
Bacillus and Serratia and fungi in the genera Penicillium and Trichoderma , although
other genera are involved (Brinton et al ., 1996). Very little is known about importance
of total microbial numbers or species diversity in relation to the effi cacy of compost
extracts and teas. Induced resistance, antibiosis and competition are thought to be the
main means by which live microorganisms bring about disease suppression from NCTs.
For example, germination of Sphaerotheca fuliginea conidia was not inhibited when
treated with NCT in vitro . However NCT-treated cucumber leaves demonstrated indica-
tors of induced resistance including increased papilla formation, lignifi cation and necrotic
reactions when S. fuliginea began to infect (Samerski & Weltzein, 1988).
Several studies have demonstrated that antibiosis can be partly responsible for patho-
gen or disease suppression, based on work which shows no loss of suppressive activity
due to NCTs when the teas are sterilized or micron fi ltered prior to application (Elad &
Shtienberg, 1994; Yohalem et al ., 1994; Cronin et al ., 1996). It is known that many of the
microorganisms present in compost extracts and teas can produce compounds that are
toxic to other microorganisms. For example, some chemicals produced by Pseudomonas
spp. (e.g. siderophores) exert a powerful chemical effect against other organisms (Potera,
1994). Antibiotics known to be produced by Bacillus subtilis and others, can inhibit
growth and germination of many fungal species (Brinton et al. , 1996). NCT has also
been shown to have an in vitro mycolytic effect on microspores and chlamydospores of
Fusarium oxysporum f. sp. cucumerinum (Ma et al ., 2001).