Sedentary Endoparasites: Appearance

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Sedentary Endoparasites Appearance
Numerous plant-parasitic nematode forms have
evolved toward a more intimate relationship with
plants, in which they induce the plant to form
specialized feeding cells for their nourishment.
The nematode has a secure, protected spot in
which to feed, and can obtain enormously more
nutrition for enhanced egg production. All
females forming highly specialized feeding sites
swell to many times their size as juveniles the
extra space is taken up with extra-long gonads.
These pictures represent five genera that have
evolved along these lines. Meloidodera (1) note
equatorial vulva (V). Globodera (2) body
spherical, forming cyst. Heterodera (3) body
lemon-shaped, forming cyst. Meloidogyne (4) body
spherical, no cyst. Rotylenchulus (5) body
kidney-shaped, no cyst.
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Sedentary Parasites Feeding Sites (1)
Multinucleate giant cells
Uninucleate giant cell
Nurse cells
Syncytium
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Sedentary Endoparasites Feeding Sites (2)
Giant Cells Cells in the plants vascular
system that swell to many times their original
size. Giant cells have dense cytoplasm and
numerous nuclei. The nuclei divide
synchronously. Giant cells of this sort are
produced by root-knot nematodes. Cystoid and
some reniform nematodes produce uninucleate giant
cells (see previous slide). Syncytium Cell
walls in the plants vascular cells break down
partially to form what is functionally a single
huge cell. The cytoplasm of a syncytium is dense
and has numerous nuclei. Syncytia are produced
by cyst nematodes, some reniform nematodes, and
false root-knot nematodes (Nacobbus). Nurse
Cells A group of cells within the plants
cortex that have dense cytoplasm and no vacuole.
They do not increase in size and do not fuse.
Nurse cells are produced by the citrus nematode
(Tylenchulus).
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Sedentary Parasites Feeding Sites (3)
N
Abbreviations B Broken cell walls N Nematode
N
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Cyst nematodes Globodera
Females of Globodera are spherical, without a
protruding vulval cone. Their life cycle is
identical to that of Heterodera, except that eggs
in dried Globodera cysts will still be
viable. Two species of Globodera are of major
world importance, both on potato G. pallida
(pallid potato cyst nematode) and G.
rostochiensis (potato cyst nematode or golden
nematode). Both species have caused extensive
damage to European potato crops. Vigorously
enforced quarantine and eradication efforts have
prevented its spread in the U.S., where it is
confined to Long Island and a few farms in
upstate New York. Both species were introduced
into Europe from South America in the 1800s on
potato tubers. The only notable Globodera in the
U.S. is G. solanacearum (tobacco cyst nematode),
found from Connecticut to North Carolina.
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Cyst Nematodes Heterodera
Cyst nematodes in the genus Heterodera are among
the worlds most significant plant pathogens.
Females of these cyst nemas are lemon-shaped. As
the female ages, her body fills with eggs, after
which it transforms into a leathery cyst that
protects the eggs from many environmental
stresses. The eggs have a diapause that may last
for up to 12 years. The most significant
Heterodera spp. are H. avenae (cereal cyst n.,
CCN) H. glycines (soybean cyst n., SCN) H.
schachtii (sugar beet cyst n., SBCN) H. trifolii
(clover cyst n., CCN) Damage to German sugar beet
fields in the 1860s led to the discovery and
study of SBCN, the beginning of modern economic
nematology and nematode management.
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Heterodera schachtii Sugar Beet Cyst Nematode
Heterodera schachtii is a major problem on crop
plants in the Chenopodiaceae (beet, sugar beet)
and Brassicaceae (cabbage, mustard, rapeseed,
etc.). Heavy infection of roots reduces growth
of all plant parts. Like most cyst nematodes,
SBCN is found mostly in cooler areas of the
temperate zones.
Cysts are ideal little containers for moving
nematodes around in irrigated fields, as in this
California cabbage field. Although plants are
only infrequently killed from SBCN attack, growth
is greatly retarded and marketable heads are not
produced. In root crops such as sugar beet, the
storage root is small and contains far less sugar
than is needed for successful commercial
operation.
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SBCN Damage to Sugar Beet
Symptoms of SBCN on sugar beet combine two
undesirable traits. One is the
over-proliferation of fine roots from most of the
storage root. The result is a small storage root
with low sugar content, commercially unsuitable
for processing. Feeding by the nematode also
reduces the size of the storage root, which makes
harvesting and processing too costly.
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SCN Damage to Seedling Soybean
SCN can be devastating to soybean fields if
initial infestation is high. The shape of the
damage is an indication that tillage machinery
has tended to move the nematode in the direction
of the rows, a common general indication of
nematode damage in crops. Surviving plants may
compensate somewhat for their dead brethren by
having more space and less competition for water
and nutrients.
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SCN Races and Resistance
Each population of SCN can be characterized as a
particular race, based on ability to reproduce on
standard soybean lines with different genes for
resistance. Successful deployment of resistance
leads to markedly improved growth and yield (top
picture). The effects of different resistance
genes is shown below. The Arkansas isolate, a
Race 3 population, has minimal effect on this
soybean cultivar. The North Carolina isolate, a
Race 2 population, causes severe chlorosis and
stunting of the same cultivar, even at only half
the initial inoculum of the Arkansas isolate.
Accurate assessment of the predominant nematode
races in a field is essential for successful
management of SCN. Even with wide deployment of
numerous resistant cultivars, SCN still causes
about a 2 reduction in the U.S. crop.
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Spread of Known SCN Infestations
Since 1954, when SCN was first found in Tennessee
and North Carolina, this nematodes known range
has spread inexorably over most of the soybean
production range in North America. Thought
originally to have been spread on farm equipment,
it is now believed that SCN may have been widely
distributed but rare, and probably confused with
SBCN. It was also thought that cold winters
would prevent it from moving north of southern
Illinois. Not true!