Alfalfa Dwarf—History and Current
Status
Linkage with Glassy-Winged
Sharpshooter and Pierces Disease in Grapes
Alexander Purcell
Division of Insect Biology. University of California.
Berkeley, CA 94720-3112
For
more information on Xylella, go to: http://www.cnr.berkeley.edu/xylella
History of Alfalfa Dwarf:
1928-37 Alfalfa dwarf (AD) described in California by
a USDA pathologist in southern California, J. L. Weimar, who later showed that
the disease was graft transmissible and had a pattern suggesting spread by
insect vectors. The incidence of dwarf was somehow worsened by presence of
water, but soil and water from beneath diseased plants was not infective.
Weimar published photographs of “bacteria-like bodies” that in 1978 were proved
to cause the disease. He concluded that a virus was more likely the pathogen
but did not rule out bacteria as a cause; he simply could not consistently
culture one or prove that they were. His experiments suggest possible root
graft transmission [never followed up by others since].
1938-48 Hewitt and others at UC showed that
xylem-feeding leafhoppers (sharpshooters) transmitted the “virus” that caused
both AD and Pierce’s disease (PD) of grape. The spread of PD was not reduced by
removing diseased vines or vector control within vineyards, but appeared to
come mostly from immediately outside vineyards. Alfalfa fields were a major
source of PD. Esau (UCD) showed plugging of xylem was most characteristic
internal symptom. UCB entomologists characterized insect transmission and
ecology. Freitag (UCB) used insect vectors to show that the PD “virus” had an
amazingly broad host range (75 of 100 plant species tested).
1974-79 D. L. Hopkins in Florida and Goheen et al.
(UCD) report an association of Pierce’s disease with uncultivable bacteria and
heat or antibiotic therapy typical for bacteria. Mircetich et al. (UCD) show
that the same bacterium probably caused almond leaf scorch (previously called
“golden death” in Mohave desert orchards). Davis et al. (UCB) culture the
causal bacterium, later named Xylella
fastidiosa (Wells et al.). The bacterium causes almond leaf scorch,
Pierce’s disease and alfalfa dwarf. Purcell proposes mechanism of vector
transmission as from the foregut ; demonstrates freezing therapy of grape;
introduces insecticidal control of vectors in riparian vegetation for coastal
vineyards.
1980-92
Purcell (UCB) shows that only spring infections of grape with X. fastidiosa were likely to remain
diseased the following year; summer inoculations resulted in infections that
did not survive the winter. A “new” citrus disease is described in Argentina.
This disease spread to Brazil, where it was named citrus variegated chlorosis
(CVC) and is now the main cause of disease losses in orange. Observed cases of
CVC in Brazil rose from 3 trees in 1987 to >2 million in 1992 [now about
70-90 million, about a third of crop]. CVC moved throughout Brazil in a few
years, carried on nursery plants.
1993-96 Hill et al. (UCB) show that X. fastidiosa does not move in most of
its plant hosts and that incredibly few bacteria (<100) per insect are
needed for optimal transmission to grape (Purcell et al., UCB & UCR)
demonstrate that a new strain of X.
fastidiosa causes oleander leaf scorch (OLS) disease in southern
California. The spread of OLS is linked to a newly established vector, the
glassy-winged sharpshooter (GWSS), that had become abundant on citrus and
irrigated ornamentals. Vanderlei Rodas, Brazilian grower demonstrates to his
skeptics that pruning and tree removal control CVC if clean nursery stock is
planted.
1997 GWSS appears in Temecula Valley in significant numbers
in citrus and grape.
1998 GWSS populations explode, and scattered cases of PD
show up in Temecula.
1999 Another big year for GWSS in Temecula, but the
incidence of PD there exceeds 90% in numerous vineyards. California grape industry becomes alarmed
and prompt CDFA and UC to form Task Forces. GWSS is found in southern Kern Co.
citrus and grape, but growers reported that GWSS has been there for at least
several years. No PD has been confirmed in Kern county in recent memory.
2000 The complete genome of X. fastidiosa is sequenced in Brazil (Simpson and 99 other
authors). This previously obscure
pathogen becomes the first non-viral plant pathogen to be completely sequenced
(cost: $12 million). Three months later a DOE facility in Walnut Creek
completes the raw sequences for 2 other strains of X. fastidiosa, which will be “finished” in 2001, along with at
least 3 other strains. Scattered cases of PD and ALS are found in Kern Co. for
the frist time in over 50 years. USDA, CDFA, and commodity groups fund major
expansion of research funding and efforts to “contain” [=delay] GWSS from
spreading northward. Expanded and intensified searches reveal GWSS infestations
in Fresno, Bakersfield, Porterville, Sacramento, Chico, and Brentwood. Eradication is attempted in the last 3
sites.
Symptoms
of Alfalfa Dwarf
The
main symptom is stunted regrowth after
cutting. This stunting may not be apparent for many months after initial
infection. Leaflets on affected plants are smaller, often slightly darker
(bluish) color) but not distorted, cupped, mottled or yellow. Taproot is normal
size, but slicing it diagonally or horizontally down the root reveals wood with
an abnormally yellowish color, with fine dark steaks of dead tissue scattered
throughout the wood. In recently infected plants the yellowing is mostly in a
ring beginning under the bark, with a normal white-colored cylinder of tissue
inside the yellowed outer layer of wood. The inner bark is not discolored, nor
do large brown or yellow patches appear as in bacterial wilt (Clavibacter insidiosum). Dwarf disease
progressively worsens over 1-2 years after first symptoms and eventually kills
plant. Noticeable dwarfing requires 6-9 months after inoculation in the
greenhouse, probably longer in the field.
Diagnostic
confirmation
Several methods are now
available in research labs. Commercial labs in Davis, CA (AgriAnalysis) and
Indiana (AgDia) provide serological detection of Xylella in plant samples.
AgDia sells a detection kit.
Culture of X. fastidiosa from fresh tissue (1-4
days). Cultures take about a week to confirm. Contamination of cultures may
obscure results.
ELISA – serological test
available at commercial labs. Low sensitivity but usually adequate for alfalfa
except for some infected plants that do not show symptoms.
PCR – a molecular technique
that amplifies genes unique to Xylella.
Most sensitive method and may soon be available in commercial labs.
Microscopy of sap is quick
and cheap if you have a small centrifuge and > 40X microscope. Serological methods can be used to confirm
that the bacteria you see are X.
fastidiosa.
Distribution
AD
has been rarely reported since the 1950s. In the 1920s and ‘30s, dwarf was
reported as widespread and destructive in southern California. It was apparent
in parts of the San Joaquin Valley in the late 1930s and early 1940s, but never
north of southern Madera County. In recent years AD has been either uncommon or
not noticed. Improved weed control with herbicides may have reduced overall
vector populations, leading to less AD. Another explanation may be that AD may
have been overlooked.
The causal
bacterium (Xylella fastidiosa)
A Gram-negative, rod-shaped bacterium
that strictly colonizes the xylem (water-conducting) cells. Related to Xanthomonas. Grows slowly in a variety
of media, including simple, chemically defined media. Temperature range for
growth is 16-33 C. Dies slowly in plants below 10 C, rapidly above 35 C. PD
strains colonize most plant species at the site of inoculation but are systemic
and reach high populations in only a few plant species. PD strains in
California cause disease in grape, almond, alfalfa, and some weeds (poison
hemlock, umbrella sedge, et al.). A new strain that does not infect grape
causes oleander leaf scorch disease. Other strains outside California cause
diseases of peach, plum, elm, oak, sycamore, maple, mulberry, sweet orange,
Asian pear (Taiwan) and coffee. Limited to the Americas except for pear in
Taiwan and grape in Kosovo (1996 report).
Vector
Transmission
Characteristics Vectors can
transmit X. fastidiosa immediately
after acquisition (no latent period) but retain transmission ability for life
unless they molt. Molting stops
transmission until the insect feeds on another infected plant. This indicates
that the bacteria are transmitted for the foregut, perhaps even from inside the
needle-like mouthparts. Few live
bacteria (perhaps less than 100 per insect!) are adequate for efficient
transmission. ALL xylem sap-feeding insects are probably vectors. Meadow
spittlebug (Philaenus spumarius) is an
efficient vector.
California’s Central
Valley Vectors
Green sharpshooter (GSS, Draeculacephala minerva) is the most
common and widespread vector in alfalfa and definitely important in spreading
alfalfa dwarf. Breeds and feeds on
grasses and sedges but readily feeds on alfalfa and is fairly efficient in
transmitting X. fastidiosa to alfalfa. Flies mostly about 30 minutes after
sunset for 30-40 minutes. Attracted to lights at night.
Red-headed sharpshooter
(RHSS, Carneocepahala fulgida) occurs
mostly on Bermuda grass, but feeds on many other grasses. Disperses farther and
occupies drier habitats than GSS, otherwise flight activity is similar to GSS.
Glassy-winged sharpshooter
(GWSS, Homalodisca coagulata) is the
most infamous and recent vector in California.
It has been seen in alfalfa near large breeding habitats like citrus
groves. Much more dispersive than any
other native Californian sharpshooter, this insect can change its host plant
daily or more frequently, depending on xylem sap chemistry. Feeds and breeds on Johnson grass, wild
sunflower, and other weeds, but its main breeding sources in California so far
have been citrus and irrigated ornamentals.
Food plant range is enormous; includes conifers, eucalyptus, and almost
any woody plant near main breeding sources.
Often feeds on dormant trees or vines in winter, but the range of plants
on which it lays eggs is much narrower. Its role as a vector for spread of
alfalfa dwarf is not yet known.
Control of Alfalfa
Dwarf
Prevention
of grass weeds in or immediately near alfalfa fields from the beginning of
stand establishment is probably the key.
Insecticides that can be used on alfalfa are not long-lasting in their
effects on sharpshooters. The
traditional GSS and RHSS vectors need grasses to breed. A simple guideline for
traditional vectors is “No grasses, no sharpshooters”. Grass weeds along
ditches, ponds, and even roads sometimes can provide habitats where
sharpshooters breed and persist throughout the year. A sweep net is useful for detecting adult sharpshooters in weeds
or alfalfa fields. Bermuda grass, water grass, and cultivated fescues and
perennial ryegrass are favored hosts for breeding.
What to do about GWSS? We don’t know yet how big a threat GWSS will
be for alfalfa. Further evaluation of
this insect in alfalfa is warranted.
There are a
number of unanswered questions about this disease
Why
has AD never been found north of Fresno-Madera?
Is there mechanical transmission of X. fastidiosa? [Even a small transmission rate would be important after a couple
of years.] Spread by root grafts? Hay?
How durable are infections made in summer months
compared to those established during spring?
How do populations of X. fastidiosa change during the year?
How resistant or tolerant are the main alfalfa
cultivars to AD? New varieties?
Are alfalfa strains of X. fastidiosa genetically distinct from grape strains?
How big a threat will GWSS be for alfalfa?
For more information and references go to http://www.cnr.berkeley.edu/xylella