Biology Reference
In-Depth Information
Ditylenchus destructor
Potato Rot Nematode
.
Feeding on underground stem structures of
a large number of plants but important on potato,
especially in Idaho and Wisconsin. Discolored
spots on tubers progress to a gray or brown
decay. The tissues have a granular appearance;
they dry and shrink and the skin may crack.
Invasions continue in storage, sometimes with
complete destruction of tubers.
Ditylenchus dipsaci
Stem and Bulb Nema-
tode
. An internal parasite of bulbs, stems, leaves,
rarely roots, causing Eelworm Disease of Narcis-
sus, Ring Disease of Hyacinth, Onion Bloat, Stem
Disease of Phlox. The name
dipsaci
covers many
strains and probably more than one species. The
type was found in 1857 on Fuller's teasel. The
nematodes are thought to release a pectinase dur-
ing feeding, which results in a dissolution of the
middle lamella and the production of large
intercellular spaces. They injure, besides hya-
cinth and narcissus, grape-hyacinth, tulip,
galtonia, garlic, shallot, and onion, and cause
a stem disease of alfalfa and many flowers
besides phlox.
The strains of hyacinth and narcissus are not
reciprocally infective, although the hyacinth
strain does infect onions. Hyacinths have yellow
flecks or blotches on the leaves, which are often
twisted, short, and split. In narcissus there are
pustules or blisters, called spikkels, in leaves,
which can be felt when the leaf is drawn through
the fingers. Nematodes in such pustules probably
enter leaves as they push up through the soil.
Bulbs badly diseased at planting produce no
foliage, or a few leaves that are premature,
twisted, and bent.
When leaves are dry, nematodes are inactive;
but when the foliage is moist and decayed, they
revive and pass down into the soil or the neck of
the bulb. They enter bulb scales, move down to
the basal plate, and then enter the base of other
scales. Infected scales are brown, and, since there
is little lateral movement of nematodes, the cut
surface of a bulb shows one or more brown rings
contrasting with healthy tissue. Eggs, larvae, and
adults are all present in the brown areas. Male and
female adults are wormlike, up to 1.9 mm long.
Infective larvae issue in large numbers in whitish
tufts in a break between basal plate and scales,
and work through the soil to invade adjacent
plants.
Theyarealsospreadinirrigationwater,on
tools, and by animals. Some winter in weed
hosts, some in seed of composites. In moist soil
they die in a year or so, but they have been
recovered from plants after 5 or 6 years. The
strain on phlox attacks campanula, sweet wil-
liam, evening primrose, goldenrod, schizanthus,
anemone, foxglove, and orchids. The leaves
are very narrow, crinkled, and waved, often brit-
tle, with a tendency to lengthen petioles. Stems
may be swollen near the top or bent sidewise;
plants are stunted, often fail to bloom, may
die prematurely. The nematodes enter through
stomata of young shoots and work upward
as the stems develop. They infest seed of
phlox and other composites, and may be so
disseminated.
In onions the inner bulb scales are enlarged,
causing a split onion that seldom flowers and
sometimes rots at the base. Seedlings are twisted,
stunted, covered with yellow spots. On plants
grown from sets, a slight stunting and flaccid
condition of outer leaves is followed by leaf-tip
necrosis and continued stunting. The larvae may
live long in infested soil and may be carried in set
onions.
Control
Commercial growers routinely treat
narcissus bulbs in hot water, 4 h at 110
to
112
F. All infected plants, parts, and debris
should be removed from fields and destroyed;
a 2-to 4-year rotation may be tried. Take up and
burn infested phlox or similar plants. Put new
plants in a new location or in fumigated soil.
Ditylenchus
(Sychnotylenchus) gallicus On
elm.
Ditylenchus iridis Probably a strain of
D. dipsaci
, on bulbous iris. Mildly infected plants
dry up prematurely and have poor root systems.
Heavily infected plants are stunted, having
few if any roots, and the bulbs decay before
harvest. Treat bulbs with hot water as for narcis-
sus, but soak only 3 h and as soon after curing as
possible.