Allelochemicals Biologica... - Name

64
ANA LUISA ANAYA
of two fungal antagonists of soil-borne diseases was evaluated after their inoculation
in potting soil and in compost produced from different types of organic waste and at
different maturation stages. The fungal antagonists Verticillium biguttatum, a
mycoparasite of Rhizoctonia solani, and a non-pathogenic isolate of Fusarium
oxysporum antagonistic to Fusarium wilt, survived at high levels (10 3 -10 5 CFU g -1 )
after 3 months incubation at room temperature in green waste compost and in potting
soil. Their populations faded-out in the organic household waste compost, especially
in the matured product. In bioassays with R. solani on sugar beet and potato, the
disease suppressiveness of compost increased or was similar after enrichment with V.
biguttatum. The largest effects, however, were present in potting soil, which was very
conducive for the disease as well as the antagonist. Similar results were found in the
bioassay with F. oxysporum in carnation where enrichment with the antagonistic F.
oxysporum had a positive or neutral effect. Postma et al. (2003) foresee great potential
for the application of antagonists in agriculture and horticulture through enrichment
of compost or potting soil with antagonists or other beneficial micro-organisms.
All soils are suppressive to phytonematodes to some degree. The degree of
suppressiveness to them or other soilborne pathogens in a soil can be enhanced not
only by infesting soil with selected microorganisms, but by the use of appropriate
cropping systems and the application to soil of specific organic amendments or chemical
compounds. Conducive cropping systems such as monoculture can reduce soil
suppressiveness to the point where the soil is not resistant to plant parasitic nematodes
(Wang et al., 2002).
Inorganic fertilizers containing ammoniacal nitrogen or formulations releasing
this form of N in the soil are most effective for suppressing nematode populations.
Anhydrous ammonia has been shown to reduce soil populations of Tylenchorhynchus
claytoni, Helicotylenchus dihystera, and Heterodera glycines. The rates required to
obtain significant suppression of nematode populations are generally in excess of 150
kg N/ha. Urea also suppresses several nematode species, including Meloidogyne spp.,
when applied at rates above 300 kg N/ha. Additional available carbon must be provided
with urea to permit soil microorganisms to metabolize excess N and avoid phytotoxic
effects. There is a direct relation between the amount of “protein” N in organic
amendments and their effectiveness as nematode population suppressants. Most
nematicidal amendments are oil cakes, or animal excrements containing 2-7% (w/w)
N; these materials are effective at rates of 4-10 t/ha. Organic soil amendments
containing mucopolysaccharides (e.g., mycelial wastes, chitinous matter) are also
effective nematode suppressants (Rodriguez-Kábana, 1986).
Vargas-Ayala and Rodriguez-Kábana (2001) established a field microplot trial to
evaluate nematode population dynamics in a rotation program utilizing nematodesuppressive
and non-suppressive legumes, and nematode-host and nonhost grass
species. The rotation treatments consisted of velvetbean (Mucuna deeringiana) or
cowpea (Vigna unguiculata) during the first year, followed in winter by oat (Avena
sativa), wheat (Triticum aestivum), rye (Secale cereale), rye grass (Lolium sp.), clover
(Trifolium sp.), hairy vetch (Vicia villosa), lupine (Lupinus sp.) or fallow. Rotation in