Koon-Hui Wang
University of Florida, Department of
Entomology and Nematology, P.O. Box 110620, Gainesville, FL 32611-0620, U.S.A.
(last updated on January, 2002)
Modes
of nematode suppression by cover crops can be categorized as providing a
nonhost or a poor host environment for nematodes (Rodriguez-Kabana et al.,
1988), producing allelochemicals (Halbrendt, 1996), and enhancing nematode antagonistic flora and fauna
(Linford, 1937), These modes of
action need not be mutually exclusive. An ideal cover crop should exhibit more
than one mechanism involved in nematode suppression. Suppression of Rotylenchulus
reniformis by Crotalaria juncea (sunn hemp) was used as an example
to demonstrate the mechanisms of cover crop in nematode suppression. This
article is a summary a published paper (Wang, 2002)
A
poor host for nematodes is a plant in which females of nematode does not
produce eggs abundantly. The low egg production could be due to slow female
development or low fertility. Crotalaria
juncea is a poor host to R. reniformis because egg production and
female development rate of R. reniformis in C. juncea
were lower than that in cowpea, Vigna unguiculata, a good host of R.
reniformis (Table 1, Fig. 1, Fig. 2).
Table
1. Rotylenchulus reniformis vermiform stages and eggs number in Vigna
unguiculata and Crotalaria juncea 8 weeks after inoculation (Wang et
al., 2002).
|
Crop |
Vermiform
stages /250cm3
soil |
Eggs/g root |
|
Vigna unguiculata |
2,276z
a |
1,082 a |
|
Crotalaria juncea |
66 b |
31
b |
z Values are means of 5
replications. Means followed by different letters in a column were different
according to Waller-Duncan K-ratio (K=100) t-test.
Allelopathy
is a plant-plant or plant-microorganism biochemical interaction (Rice, 1984). Several cover crops were known for their ability to
produce allelopathic compounds against plant-parasitic nematodes. For examples,
Tagetes spp. produces a-terthienyl, Crotalaria spp.
produces monocrotaline (Gommers and
Bakker, 1988; Fassuliotis
and Skucas, 1969), Brassica
napus produces glucosinolates that are nematicidal when reacted with
myrosinase after crop incorporation (Brown
et al., 1991). However, allelopathic effect of these cover crops
is nematode specific. Rotylenchulus reniformis is more vulnerable to the
allelopathic compound released from C. juncea than B. napus and T.
erecta (Fig. 3). Percentages of R.
reniformis remaining active in C. juncea leachate were lower than
that in pineapple leachate (Ananas comosus) as well as water and sand
leachate (Fig. 3).
Incorporation
of cover crops into soil as organic amendments had long been known to enhance
nematode-antagonistic microorganisms (Linford,
1937; Linford et al., 1938). However, not all the nematode-antagonistic fungi
respond to organic matter. Nematode-trapping
fungi that form constricting rings, and the nematode endoparasitic fungi
were often isolated from soil with higher organic matter (Gray, 1985). Cooke (1963) divided the nematode-trapping fungi into saprophytic
and parasitic groups. Saprophytic nematode-trapping fungi form
three-dimensional-network traps in response to the presence of nematodes and
are regarded as inefficient nematode-trappers. Parasitic nematode-trapping
fungi have low saprophytic ability, but form traps spontaneously. This group
consists of fungi that form constricting rings, adhesive knobs, or adhesive
branches, and are more effective nematode-trappers than the saprophytic group (Jansson and Nordbring-Hertz, 1980). High organic matter and moisture increase the
parasitic nematode-trapping fungal populations and may stimulate the trap
formation of saprophytic nematode-trapping fungi (Gray, 1985).
During
the C. juncea growing period and after C. juncea incorporation, a
niche was created that favored free-living nematodes (Fig. 4). In the presence of nematodes, trap
formation is induced in nematode-trapping fungi (NTF). In a soil where
parasitic NTF were more abundant, C. juncea amended soil enhanced
parasitic NTF more efficient than most of the other organic amendments except B.
napus amendment (Fig. 5A). Among the most
commonly found NTF detected in C. juncea amended soil were Monocosporium
ellipsospora and Arthrobotrys dactyloides. Both of these fungi were
found to be effective against M. javanica and were formulated for
nematode biocontrol (Jaffee and
Muldoon, 1995; Stirling
and Smith, 1998). In a repeated test, parasitic NTF population was
low, C. juncea amendment enhanced higher population densities of
saprophytic NTF than the non-amended soils (Fig. 5B).
Parasitized nematode eggs were only detected in C. juncea amended soil
when eggs were plated on water agar and incubated for 2 weeks (Fig. 5C). Percent of R. reniformis
vermiform stage parasitism was higher in C. juncea and Ananas comosus
amended soil than unamended soils (P<0.05, Fig.
5D). Soil treated with 1,3-D or left bare suppressed the activities of NTF
as well as fungal parasitism on eggs and vermiform stages of R. reniformis (Fig. 5A, B, C, D). The presence of parasitic
nematode-trapping fungi and egg parasites might explain the longer period of R.
reniformis suppression in intercycle and intercrop field trials (Wang et
al., 2002, in press).