The use of composts and compost extracts in plant disease control - Disease Control in Crops - page 108

Many attempts have been made to develop techniques for predicting the disease-
suppressive properties of composts based on the presence of specifi c microbial antagonists
or the level of microbial activity, but prediction of disease suppressiveness in composts
is rarely possible, and no one technique can be used to predict the suppressiveness of a
compost to all diseases. The presence of specifi c antagonists, such as Trichoderma spp.
and/or Gliocladium virens in a compost is no guarantee that that compost will be suppres-
sive to diseases caused by Rhizoctonia solani , despite the fact that these fungi are known
to be important biological control agents of R. solani. Several workers have used a simple,
rapid enzyme assay to determine microbial activity based on the rate of hydrolysis of fl uo-
rescein di-acetate (Boehm & Hoitink, 1992; Ryckeboer, 2001). Results obtained can be
reliably used to determine the suppressiveness of potting mixes to Pythium diseases, but
it is not possible to predict the level of suppression of diseases caused by other pathogens
such as Rhizoctonia solani using this technique.
Several approaches have been used to monitor changes in the soil microbial community
structure following amendment with compost. These techniques include for example,
physiological profi ling using Biolog® microplates (Riddech et al ., 2002; Borrero et al .,
2006) or analysis of phospholipid fatty acids (PLFAs) (Sundh & Rönn, 2002), plating
on selective media, for example, for fatty acid metabolising microorganisms (McKellar
& Nelson, 2003), DNA-based techniques such as analysis of terminal restriction
fragment-length polymorphism (T-RFLPs) (Michel et al ., 2002) and denaturing gradient
gel electrophoresis (DGGE) (Calvo Bado et al ., 2002). These approaches may lead to an
improved understanding of which changes in microbial communities are associated with
increases in disease suppressiveness within composts.

5.7.4.5 Inoculation of composts with biological control agents

Compost-amended potting mixes can be produced which provide consistent natural
suppression of damping-off and root diseases caused by Phytophthora and Pythium spp.,
but variable suppression is obtained for other diseases. It is possible to improve the sup-
pressiveness of potting mixes to other pathogens such as Rhizoctonia and Fusarium spp.
by amending the compost with specifi c antagonists including Trichoderma spp. (Nelson
et al ., 1983; Cotxarrera et al ., 2002), Verticillium biguttatum and non-pathogenic Fusarium
oxysporum (Postma et al ., 2003; Cotxarrera et al ., 2002). The composts produced can
be up to 3 times as suppressive as the unamended, naturally suppressive compost, but
the compost into which antagonists are introduced must provide an appropriate envi-
ronment for growth and reproduction of the antagonist if disease suppression is to be
enhanced (Postma et al ., 2003). For example, the degree of decomposition of the compost
was important in determining the degree of suppression of Rhizoctonia damping off in
container media (Nelson et al ., 1983).

5.8
Compost extracts or teas have been tested for their ability to suppress a wide range of
foliar diseases in glasshouse and fi eld-grown edible and ornamental crops, with vari-
able results. Much of the early work was carried out on plant pathogens cultured in vitro
or in detached leaf assays, but there are plenty of recent examples where the suppres-
sion of diseases on whole plants has been recorded. Examples of diseases controlled in

Effects of compost extracts/teas on plant disease