Examples of
Use of Cover Crops for Enhancement of Nematophagous Fungi
History
Earlier
studies by Linford (1938) and Cooke
and Godfrey (1964) demonstrated that incorporation of cabbage leaves
into the soil enhanced nematophagous fungi. Later it was found that legume
crops tend to enhance nematophagous fungi better than other crops. Root-knot
nematode numbers were suppressed when soil amended with alfalfa was inoculated
with Arthrobotrys conoides (Al-Hazmi et al., 1992). The efficient nematode
trapping fungal species, A. dactyloides
and Monocosporium ellipsospora,
appeared only in microplots amended with alfalfa (Mankau, 1968). When alfalfa
meal was incorporated into the soil, suppression of root-knot nematodes by
nematode-trapping fungi (NTF) increased. Pea rhizosphere enhanced the densities
and species diversity of nematode-trapping fungi better than white mustard or
barley (Persmark and Jansson, 1997). The population density of the NTF and formation of
conidia traps, structures that can overcome fungistasis were much higher in the
pea rhizosphere than the root-free soil (Persmark
and Nordbring-Hertz, 1997).
Screening cover crops for
nematophagous fungi enhancement
Three
cover crops were evaluated for their NTF enhancement ability in a greenhouse
(Wang et al., 2001). Soil from a pineapple field was either amended with
chopped leaf tissues of sunn hemp (Crotalaria juncea), rapeseed (Brassica
napus), marigold (Tagetes erecta) or pineapple (Ananas comosus)
at 1% (w/w) and compared with soil treated with 1,3-dichloropropene (1,3-d) or
bare soil. Three months after cowpea seedlings were planted into these soils,
NTF numbers were higher in treatments receiving leaf amendments as compared to
1,3-d treated soil or bare soil (Fig. 1).
However, only soil treated with sunn hemp had higher propagules of
parasitic NTF (the most efficient nematode trapper) than 1,3-d and bare soil.
Therefore, among the cover crops tested, sunn hemp is recommended cover crop
for NTF enhancement.
Fig.
4-1. Nematode-trapping fungal population densities in soil treated with sunn
hemp (Crotalaria juncea, Cj), rapeseed (Brassica napus, Bn) or
marigold (Tagetes erecta, Te) at 1% (w/w) were compared with soil
amended with pineapple leaves (Ananas comosus, Ac), treated with
1,3-dichloropropene (1,3-d), or remained bare (BS). Columns with same letters
were not different according to Waller-Duncan k-ratio t-test (P £ 0.05) (Wang et al., 2001).
Other
potential legumes tested for NTF enhancement were cowpea and velvetbean (Mucuna
deeringiana). A soil collected from South West Florida Research and
Education Center, University of Florida, Immokalee, FL was amended with chopped
leaf tissues of sunn hemp (SH), cowpea (CP) or velvetbean (VB) at 1% w/w for 7
days in plastic pots and assayed for nematophagous fungal population densities.
Soil amended with higher C: N ratio crop biomass such as black oat (Avena
sativa) and soil without amendment (BS) were included as a control. At 21
days after plating (dap), sunn hemp enhanced Dactylaria eudermata, a NTF
forming three-dimensional nets as compared to the control (P < 0.05). Cowpea enhanced the abundance of Catenaria
anguillulae (P < 0.05), which
is a zoosporic forming endoparasitic fungus that was found to attack many
species of plant-parasitic nematodes in Florida (Esser and Schubert, 1983). In
contrast, effect of velvetbean on nematophagous fungi was not significant (Fig.
4-2). Both sunn hemp and cowpea enhanced Harposporium anguillulae (P
< 0.05), but this fungus mainly only infect free-living nematodes, with a
mouth cavity larger enough to ingest the fungal conidia of H. anguillulae.
Fig. 4-2. Number
of propagules of nematophagous fungi per g of soil treated with leaf amendment
of sunn hemp (SH), cowpea (CP), velvetbean (VB), or black oat (BO) or
non-amended bare soil (BS). Means are average of 4 replications at 7 (red bar)
and 21 (blue bar) days after plating (dap). Means followed by the same letter
were not different among the treatments at each dap according to Waller-Duncan
k-ratio (P £ 0.05) test
(Wang unpublished data, 2003).