Agriculture Reference
In-Depth Information
The amount of N present in composts has been shown to affect disease suppressiveness.
Phytophthora dieback and fi reblight (caused by
Erwinia amylovora
) are two examples of
diseases which are increased as a result of excessive N fertility (Hoitink
et al
., 1986).
Fusarium diseases tend to be increased by compost amendments that are high in ammo-
nium N in particular. This means that they can be enhanced in fi eld soils or container
media amended with sewage sludge composts. Sewage sludge composts have a low C:N
ratio and release predominantly ammonium N (Kato
et al
., 1981). In contrast, composts
with high C:N ratios immobilize N and suppress Fusarium diseases if colonized by
appropriate microorganisms (Trillas-Gay
et al.,
1986).
The total salt content in composts has been shown to affect the biological control of root
rot caused by
Phytophthora
spp. on soybean (Hoitink
et al.
, 1993). Composted municipal
solid waste applied 4 months ahead of planting (to allow for leaching of salts) increased
soybean yield and controlled the root rot. Application of the same compost just prior to
planting decreased soybean yield in comparison to the control.
Composts based on tree bark release toxic compounds (natural fungicides) that lyse
zoospores and sporangia of
Phytophthora
spp. (Hoitink & Fahy, 1986). As the decom-
position level of composts increases, the role of these natural fungicides in the overall
suppressive effect on the pathogen decreases and the contribution of the biocontrol agents
gradually increase.
A lack of oxygen around plant roots, for example due to an abundance of small parti-
cles/pores in the soil/growing medium/compost mix or to the presence of rapidly decom-
posing compost can predispose plants to attack by root pathogens such as
Phytophthora
spp. (Hoitink & Fahy, 1986).
5.7.4.4
Biological attributes of the compost
Very little information exists on the biology of compost-amended soils. A signifi cant
amount of literature now exists on the antagonists involved in suppression of plant patho-
gens in compost-amended media however, and this can be studied with a view to extending
the use of disease-suppressive composts in fi eld soils. The biology of composts depends
primarily on the feedstocks used, the nature of the composting process (in particular
the temperatures generated and their duration, the moisture and oxygen content) and on
colonization of the compost after peak heating.
Fate of plant pathogens and biocontrol agents during composting
Disease-suppressive composts should by defi nition contain no plant pathogens. Eradication
of plant pathogens present in the original feedstock occurs during the composting process
as a result of exposure to high temperatures, release of toxic products during or after the
self-heating process and microbial antagonism in the sub-lethal outer temperature zones
of piles/windrows or later during curing. Most plant pathogens are killed by 30 minutes
exposure to 55
o
C (Hoitink & Fahy, 1986). A few plant pathogens such as tobacco mosaic
virus, the clubroot pathogen (
Plasmodiophora brassicae
) and some
forma speciales
of
Fusarium oxysporum
are less sensitive to heat and highly controlled in-vessel compost-
ing systems may be required if feedstock material is likely to be contaminated with such
pathogens.
