Rush skeletonweed or Skeletonweed
[Chondrilla juncea L.][CHOJU][CDFA List: A] Photographs
Map of Distribution
SYNONYMS:Skeleton weed; naked weed; gum succory; devils-grass;
or biennial, with rigid, wiry flowering stems to 1 m tall, milky
sap. Plants exist as basal rosettes until flowering stems develop
at maturity and rosette leaves whither. Persistent flower stems can hinder
harvest machinery. Several forms (biotypes) occur, differing in leaf width,
branching pattern, and flowering time. Characteristics can vary between, but
rarely within populations since all reproduction is by clones (vegetative
and seed apomixis). Plants are highly competitive for water and nutrients.
Rush skeletonweed is also a significant problem in several other countries,
particularly Australia. Introduced from southern Europe. The biocontrol agents
skeletonweed gall midge (Cystiphora schmidti), skeletonweed gall mite
(Eriophyes chondrillae), and rush skeletonweed rust (Puccinia chondrillina)
can control some infestations. All are established in California.
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SEEDLINGS:Cotyledons spatulate to oval. First leaves elliptic with backwards
pointing teeth. Require a continuous moisture supply for up to 6 weeks to
develop a persistent root system.
PLANT:Rosette leaves oblanceolate, 4-12
cm long, 1-5 cm wide, prostrate, and typically lacking hairs. Margins often
purple-tinged and irregularly shallow-lobed, with lobes often pointing
backwards towards the leaf base. Lobes opposite one another. Terminal
lobe more or less sharp-pointed. Rosettes produce 1 or more flowering stems
with numerous branches. Upper stems mostly lack hairs, but typically
have dense, bristly, downward pointing hairs at the base. Stem leaves
often absent or bract-like, but when present resemble reduced rosette leaves.
and UNDERGROUND STRUCTURES: Taproot slender,
deep, persistent, with short lateral branches along the length. Taproots become
somewhat woody with age and can penetrate soil to depths of 2-3 m or
more. Most lateral roots are short-lived, non-woody, and less than 8 cm long,
but a few lateral roots near the surface can become rhizome-like
and grow laterally for 15-20 cm before turning downwards. Adventitious
buds near the top of the taproot and on major lateral roots can
produce new rosettes. Roots are easily fragmented, with pieces as small
as 1-2 cm producing new rosettes from depths to 1 m.
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FLOWERS:July until flowering stems killed by frost (fall or winter).
Flower heads axillary or terminal, sessile or short-stalked, and solitary
or in interrupted spike-like clusters of 2-5. Each flower head consists of
7-12 bright yellow ligulate flowers, strap-shaped with 5-lobed
corollas 12-18 mm long, and phyllaries (bracts) cylindric as a unit
and in 2 unequal rows, the outer much smaller than the inner. Receptacle
lacks small bracts (chaff) among the flowers. Temperatures of at least 15
ºC are necessary to induce flower production.
and SEEDS:Achene body oblong, tapered
at both ends, lacking hairs, pale to dark brown, and 3-4 mm long, with
many lengthwise ribs, pointed tubercles near the top, and to
6 small scales at the apex, surrounding the point of beak attachment.
Beak slender, 5-6 mm long, not including the pappus which consists
of many equal, fine, white bristles about 5 mm long.
CHARACTERISTICS:Persistent rigid stems
with clusters of flower head bracts (and sometimes seeds) on
old stems distinguish rush skeletonweed from dandelion (Taraxacum
officinale Wigg.) and Brassicaceous weeds such as mustards (Brassica
spp.) and radish (Raphanus spp.).
HABITAT:Disturbed soils of roadsides, croplands, especially irrigated
grain fields, semi-arid pastures, rangelands, and residential properties.
Grows best on well-drained, sandy or gravelly soils in climates with cool
winters and hot, relatively dry summers without prolonged drought. Tolerates
a wide variety of environmental conditions, including rainfall less than 250
mm (10 in) to more than 1200 mm (~50 in), cold winter areas, and continental
climates. Severe infestations are less common on heavy clay soils.
DISTRIBUTION:Uncommon. North Coast (ce & cw Mendocino Co.), Cascade
Ranges (ne Shasta Co.), northern Sierra Nevada (s Plumas, Sierra, Nevada,
Placer, El Dorado, Calaveras cos.), Central Valley (n Sacramento, e Yolo,
Fresno cos.), San Francisco Bay region (sc Napa, n Santa Clara cos.), South
Coast Ranges (s Monterey, San Luis Obispo cos.), and South Coast (Los Angeles
Co.); to 600 m (2000 ft). Previous infestations now eradicated occurred in
Tehama, Butte, Solano, San Mateo, Madera, Santa Barbara, and San Diego cos.
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PROPAGATION/PHENOLOGY:Triploid. Reproduces only by clones produced vegetatively
from adventitious buds on roots and asexually by apomictic seed.
Seeds primarily disperse with wind, but also by water, animals, and human
activity. Seeds can be highly viable, with ~90% germination the first year
and are short-lived under field conditions, to ~2% the third year but often
less than 6 months. Seeds lack dormancy and can germinate within 24 hours
under optimal conditions. Fresh seeds germinate without light and at a temperature
range of 7-40 ºC (optimum 15-30 ºC). Seed germination and new bud
growth begin in fall after first rains in mild winter areas or early spring
in colder climates. Seedling emergence is reduced in water saturated or heavy
clay soils and during drought conditions. First year plants on deep sandy
soil can produce viable seed earlier. Can develop from rosette to seed maturity
in 1 month. Flowering stems are produced in early summer. One plant can produce
15,000-20,000 seeds per season.
SURVIVAL:Moderate soil disturbance, such
as grain cultivation alternated with grazing on a yearly basis, encourages
infestations by dispersing rootstocks. Under moist conditions, shallow burial
of seed by hooves of grazing livestock appears to promote seed germination.
Increasing nutrient levels of poor soils discourages infestation by increasing
competition from other vegetation.
SPECIES:Rosette leaves of rush
skeletonweed and dandelion share the following characteristics:
leaves without hairs, leaf lobes pointing backward and opposite one
another, milky juice exuded when torn. Unlike rush skeletonweed, dandelion
has unbranched, leafless, hollow, non-persistent, fleshy flowering
stems and seeds without small scales at the apex. In addition,
dandelion is typically found in turf and gardens. Chicory (Cichorium
intybus L.) is similar to rush skeletonweed and dandelion, but has
rosette leaf lobes pointing outwards or forwards and not always
opposite, and basal leaves with a few rough coarse hairs.
Prevention: Rush skeletonweed
is likely to establish along roadsides and right of ways, and spread to the
surrounding areas. In rangeland areas, proper grazing management and fertilization
will help prevent its establishment. Where infestations are present, cattle
and sheep will readily graze the rosettes and shoots until the stems become
lignified. Grazing will reduce seed production, and few viable seed will pass
through a ruminant digestive system. Subterranean clover (Trifolium subterraneum),
an important forage source, forms dense stands, which prevent skeletonweed
seedling establishment. Wheatgrass, however, has failed to prevent reinfestation
after herbicides were used to control initial stands.
Mechanical: Rush skeletonweed
may proliferate in wheat-fallow areas where fallow cultivation is used. Tillage
will effectively eliminate seedlings and older plants. However, new plants
will rapidly reestablish from severed rootstocks as small as 2 cm and from
a depth of 120 cm. Mowing will reduce the number of viable seed produced.
However, the plants will persist due to vegetative reproduction. Mowing after
initial seed set is not recommended and will likely increase seed dispersal.
Biological: Three organisms
have been released for control of skeletonweed; the skeletonweed gall midge
(Cystiphora schmidti), skeletonweed gall mite (Eriophyes chondrillae), and
skeletonweed rust (Puccinia chondrillina). The gall midge has four to five
generations per year and attacks all known biotypes of skeletonweed. Females
insert eggs below the surface of rosette stems and leaves. The larvae feed
on the leaves and stems at the site of egg deposition. This results in the
formation of galls which are circular, slightly raised, approximately 3 mm
in length, and yellowish to maroon in color. Infested plants show reduced
vigor and early senescence, with decreases in viable seed production and stem
size and length. Heavily infested stands may appear a purplish color from
a distance. This midge and the mite are available for distribution in California.
One of the best strategies for use is collecting stems with galls from an
infested patch, removing any seed heads or flowers, tying the stems into teepee
bundles, and placing them in the center of uninfested patches.
The gall mite has several generations per year and has been the most effective
agent released for skeletonweed control. The mites attack shoot buds when
plants bolt in the spring, and continue to form galls until the fall. Infested
plants exhibit deformed shoot buds, and produce few to no viable seed. The
mite also reduces vegetative reproduction by reducing carbohydrate reserves
and preventing new rosettes from establishing from the original plant. This
mite is available for use in California. After seed head and flower removal,
gall infected stems may be placed against new uninfested plants, and colonization
The rust has multiple generations per year and has demonstrated considerable
success in California. Large cinnamon to brownish pustules form on leaves
and stems of rosettes and mature plants alike. These open lesions result in
stunted, deformed plants, with reduced branching and floral bud production.
Infected plants produce fewer viable seed. This rust is available in California,
and can be spread by placing infected stems in patches of healthy plants.
To improve the chances of infection, spray uninfected plants with water before
placing the stems or time infected stem placings with expected periods of
heavy dew. It should be noted that different biotypes of rush skeletonweed
have shown resistance to this rust in Australia. The narrowleaf biotype is
susceptible, while the intermediate and broadleaf types appear to be resistant
to the fungus. This biocontrol agent is the first example of an exotic fungus
successfully used in the classical biocontrol of a weedy plant. It may, however,
take up to four years before results are highly visible.
Chemical: There are few
herbicides registered for control of rush skeletonweed. Single treatments
rarely provide long term control, and repeat applications have been proven
more effective. Picloram (Tordon) and metsulfuron methyl (Escort) have been
effective in controlling rush skeletonweed, but are not currently labelled
for use in California. Spring herbicide applications will control rosettes
and bolting plants. However, new rosettes may form from rootstocks. Tank mixes
of clopyralid (Transline) and MCPA or 2,4-D have been shown to be more effective
than MCPA or 2,4-D alone. Glyphosate will control rosettes, but is nonselective
and will kill any desirable vegetation. New rosettes will emerge, and fluorish
without competition. Table one lists herbicides, rates, and timings for effective
control of rush skeletonweed.
Table 1. Herbicide recommendations
for rush skeletonweed control in rangelands
Integrated weed management:
Compatibility of herbicide programs, competitive vegetation, and biocontrol
agents is critical for effective management of rush skeletonweed. Complete
kill of above ground shoots with a herbicide will greatly reduce establishment
of the rust Puccinia chondrillina. Likewise, all the currently labeled herbicides
may kill or injure important legumes. Reestablishment strategies for legumes
following herbicides has not been well documented. An integrated approach
of using the rust fungus with subterranean clover has been shown to be compatible
and effective in reducing rush skeletonweed populations.
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