Agriculture Reference
In-Depth Information
the ability to migrate within the root or back
into soil.
Root-lesion nematodes puncture and migrate
through root epidermal and cortical cells (Color
Plate 19a). Tissue degradation results in lesions
that favor greater colonization by root-rotting
fungi. These activities reduce the ability of roots
to produce branches and absorb water and nutri-
ents. Cortical degradation and reduced branching
(Color Plate 19b) often are not visible until plants
are 6 or more weeks old, and these symptoms are
often confused with root rots caused by
Pythium
and
Rhizoctonia
. Interactions of root-lesion nem-
atodes, fungal pathogens, other plant-parasitic
nematodes, and insect pests have been reported
(Lasserre et al., 1994; Taheri et al., 1994; Smiley
et al., 2004a,b).
Foliar symptoms are nonspecifi c (Van Gundy
et al., 1974; Orion et al., 1984; Doyle et al., 1987;
Thompson et al., 1995; Smiley et al., 2005a,b).
Intolerant plants with roots heavily damaged by
root-lesion nematodes may exhibit poor vigor,
yellowing and premature death of lower leaves,
stunting, reduced tillering, and reduced grain
yield and grain quality. Damaged wheat plants
are less capable of extracting soil water and
exhibit stress and wilting earlier than undamaged
plants as soil moisture becomes limiting for
plant growth. Plants that become infested while
growing under drought stress are more likely
to suffer yield loss (Nicol and Ortiz-Monasterio
2004).
Pratylenchus
species associated with wheat are
not strongly restricted by soil type and may attain
damaging population levels even in the very driest
(250 mm annual precipitation) rainfed wheat-
producing regions. Large populations have been
detected throughout the depth of root growth in
deep soils (Taylor and Evans 1998; Thompson et
al., 1999; Ophel-Keller et al., 2008).
Pratylenchus
species can survive in an inactive, dehydrated
state (anhydrobiosis) in roots and soil during dry
conditions (Glazer and Orion 1983; Talavera and
Vanstone 2001). Individuals entering host roots
after emerging from anhydrobiosis multiply more
rapidly than individuals that have not been sub-
jected to dormancy. Populations of
Pratylenchus
often decline during long fallow periods between
crops but high rates of survival have also been
reported (Orion et al., 1984; Talavera and
Vanstone 2001).
Causal organisms
Pratylenchus neglectus
(Rensch) Filipjev
Schuurmanns & Stekhoven and
P. thornei
Sher &
Allen often occur as mixtures in the same soil.
Both species are parthenogenic, with males gen-
erally being rare or absent. In contrast, species
such as
P. penetrans
(Cobb) Filipjev and Schuur-
mans Stekhoven are amphimictic, with popula-
tions having both males and females.
All species of
Pratylenchus
retain a vermiform
body shape (Color Plate 19a) with many being
about 0.5 mm long and 0.02 mm in diameter. Life
cycles range from 45 to 65 days depending on
species and environmental variables. Females
deposit about one egg per day in root tissue or in
soil. First-stage juveniles molt to second-stage
juveniles within the egg. One second-stage juve-
nile emerges from each egg about 1 week after the
egg was deposited. Two additional molts within
35-40 days result in the adult stage. All juvenile
and adult stages are parasitic. The number of
nematodes in root tissue increases exponentially
through the growing season.
Identifi cation of
Pratylenchus
to the species
level is an essential prerequisite for most control
strategies. However, identifi cation is diffi cult
because the few morphological characteristics of
taxonomic value for differentiating
Pratylenchus
species are, without exception, replete with large
ranges of intraspecifi c variation, often including
overlapping ranges and shapes. Therefore, proce-
dures to differentiate species based on compara-
tive morphology (Loof 1978; Filho and Huang
1989; Handoo and Golden 1989) are always diffi -
cult and can be unreliable. Modern techniques are
based on detection of differences of proteins or
DNA (Ibrahim et al., 1995; Ouri and Mizukubo
1999; Uehara et al., 1999; Andrés et al., 2000;
Waeyenberge et al., 2000; Carta et al., 2001; Al-
Banna et al., 2004; Carrasco-Ballesteros et al.,
2007; Castillo and Vovlas 2007). The PCR or
