Invasive
Diseases
There
are several diseases that are currently on alert.
Watch out these diseases and look into your plants
in nurseries and home yards to make sure that your
plants are not infected by them. The following
description will help you to identify each disease.
If you find or suspect any of these diseases, please
report to the nearest office of Nevada Department of
Agriculture with properly collected samples.
1. Daylily rust
Daylily rust is caused by the fungus Puccinia
hemerocallidis. The fungus was first identified
in Georgia in 2000, and subsequently found in
Alabama, Florida, South Carolina, and Connecticut.
Several varieties can be infected by this rust
fungus, and the most susceptible variety is "Pardon
me'. It is known that perennial Patrinia is
an alternate host of this fungus. Currently 6
Patrinia species are sold and grown in the U.S.
as ornamental plants. The fungus needs these
alternate hosts to complete its life cycle, which
could result in severe infection and high risk of
spreading.
 
Fig. 1. Rust appearance
on leaves of two varieties of daylily plants
(Pictures used here with permission and for helping
inspectors and the public to recognize the disease
only)
The fungus produces uredinial/telial stage
on daylily in the Liliaceae family, and spemagonial/aecial
stage in herbaceous
perennial as an alternate host. The uredinial spore
stage is often referred to as the "repeating" stage,
and it is the spore stage that can re-infect the
same host plant. The urediniospores of daylily rust
are bright orange and are produced on both the upper
and lower leaf surfaces in pustules. Symptoms range
from bright yellow spots to streaks on 'Pardon Me'.
Figure1 is showing varied symptoms on daylily
leaves.
2.
Plum Pox virus
2. Plum
Pox virus
 
Fig. 2. Symptoms on
fruits and leaves showing the diagnostic color
pattern on fruits and mosaic pattern on leaves.
(Pictures adopted from other public sources and for
helping inspectors and the public to recognize the
disease only)
The first confirmation of presence of plum
pox virus (PPV) in Adams County, Pennsylvania in
October, 1999 indicated that this dreadful disease
has been established in North America. This virus is
capable of causing diseases on many plant species
including peaches, plums, apricots, nectarines,
almonds, sweet and sour cherries. Over 20 different
aphid species transmit this virus in a stylet-borne
manner. Movement of nursery stock carrying this
virus is a major means for long-distance spreading.
Symptoms in stone fruits may vary with
cultivar, plant age, nutrient status of plants, or
temperature. Different strains of PPV cause varying
symptoms and severity. Diagnostic symptoms include
mild light green discoloration bordering the leaf
veins and/or yellow to light green rings. Infected
fruits may become deformed or irregular in shape and
develop necrotic or brown dead areas.
3. Sudden
oak death
3. Sudden oak
death
 Fig.3.
Death of entire oak trees caused by Sudden Oak Death
(SOD) disease. (Picture adopted from other public
sources and for helping inspectors and the public
recognize the disease only)
Sudden Oak Death (SOD) is a serious disease
that occurs in several counties of California. It
also occurs on the tan oak trees in the Brookings
area in the Southwestern-most corner of Oregon. The
pathogen is Phytophthora ramorum, which
causes crown dieback or wilting, and stem bark
lesions or cankers. The fungus is known to infect
tan oak, coast live oak (Quercus agrifolia),
California black oak (Q. kelloggii) and
rhododendron. Other organisms such as bark beetles
may be involved in the later stages of the tree
death. However, the fungus alone can kill trees.
4. Potato
wart disease
4. Potato wart
disease
 Fig.4.
Warty appearance on a potato plant (Picture adopted
from other public sources and for helping inspectors
and the public recognize the disease only)
Recent discovery of potato wart disease in
Prince Edward Island (PEI), Canada provides another
disease alert. Potato wart disease is caused by the
soil-borne fungus Synchytrium endobioticum,
which infects cultivated potato and a number of wild
Solanum species. Symptoms are usually expressed on
tubers and stolons (underground stems), therefore
the disease is often not noticed until the tubers
are lifted. On infected tubers, the eyes develop
into characteristic warty, cauliflower-like
swellings. When formed underground, they are the
same color as the potato skin, but gradually darken
with age, if exposed to light, they turn green.
Infected plants may occasionally produce symptoms
above ground including decline of growth vigor and
small, and greenish-yellow warty growths at the stem
base.
5.
Pierce's disease of grapevine
5. Pierce's
disease of grapevine
Fig. 5. Symptoms on grapevine infected by the
bacterium Xylella fasitidiosa (Picture
adopted from other public sources and for helping
inspectors and the public recognize the disease
only)
Pierce’s disease is a destructive disease on
grapevine and is caused by the bacterium called
Xylella fastidiosa. This disease is present
mainly in the southeastern and southwestern regions
of the United States and also in Central America.
Although Pierce’s disease has been in California
since 1880’s, it has been relatively limited in
certain areas because the bacterium is only
transmitted by local sharpshooter species that don’t
fly far away from their native habitats. In 1989, a
newly introduced sharpshooter called glassy-winged
sharpshooter (GWSS) was found in Orange and Ventura
counties of California. In difference from those
native sharpshooters, GWSS feeds on a wider range of
plants, reproduces readily on grapevine, and moves
much faster, which implies that GWSS is a more
efficient vector of Pierce’s disease. In response to
the introduction of this dangerous vector,
California State has formed a task force to combat
Pierce’s disease and the new vector.
Pierce’s disease is named after the person
Newton B. Pierce. As a California’s first
professionally trained plant pathologist, Newton B.
Pierce described and characterized this disease as
“California vine disease”. Although he was
specialized in bacteriology, the bacterium
Xylella fastidiosa as the pathogen of this
disease was not figured out during that time,
partially because this bacterium could not be
isolated from the infected tissue and cultured on
common nutrient media. In 1973, it was found that
the xylem vessels of grape plants showing symptoms
of Pierce’s disease were filled with fastidious
xylem-limited bacteria, later confirmed to be
Xylella fastidiosa. After the bacterium was
proven to be the pathogen of this disease, various
researches were carried out by scientists in an
effort to find a special nutrient medium that could
allow the bacterium to grow in vitro. Currently,
this bacterium can grow on specially formulated
nutrient media.
Symptoms of Pierce’s disease may vary with
cultivars and susceptibility of the plants. In
general, symptom shows up on leaves first as a
sudden drying and scalding of margin area of the
leaf while the other part of the leaf remains green.
Scalded areas continue to move toward the central
area of the leaf until the entire leaf becomes
scorched or dead. Infected leaves may detach from
the distal end of the petioles, leaving the bare
petioles attached to canes. On canes, the bark
tissue matures unevenly, showing islands of brown
bark surrounded by green bark. In the spring,
infected plants are delayed in growth. In the later
season, plants show dwarfed vines and shrunk or
dried small fruits. Yellow to brown streaks may be
shown in the current-season wood of infected vines.
Root system is also hurt by the bacterium. Infected
plants may die within months or years depending on
the tolerance of the plants. The expression of these
symptoms is due to the occupation of the xylem
vessels by the bacterial cells and some matrix
material produced by both bacteria and the plant,
which severely blocks the water and mineral
transportation system in a plant.
Pierce’s disease can be transmitted by
grafting. However, in most cases, the disease is
transmitted by xylem-feeding insects such as
sharpshooter leafhoppers and spittlebugs. The
sharpshooters can acquire the bacterium by feeding
on the infected plants for less than 2 hours, and
then obtain an ability to transmit this bacterium
for life long. However, the adults do not pass the
bacterium to their progeny, which means the new
generations of sharpshooters do not carry the
bacterium unless they feed on the infected plants.
In California, there are four types of sharpshooters
that can transmit Xylella fastidiosa. They
are green sharpshooters, blue-green sharpshooters,
red-head sharpshooters and glassy-winged
sharpshooters . Obviously, there are significant
differences in morphology and color among these
sharpshooters.
6.
Potato virus Y-NTN strain (PVY-NTN)
6. Potato virus
Y-NTN strain (PVY-NTN)
Potato virus Y is a common virus on
potato and is transmitted worldwide by at least 30
species of aphids. Potato plants infected by this
virus may show severe mosaic, leaf-drop streak, and
potato vein banding mosaic. Losses caused by PVY
renage from 10% to 80%. There are three different
strains of this virus: O strain (PVY-O), N strain (PVY-N),
and C strain (PVY-C). PVY-O occurs worldwide. PVY-N
was not found in North America until 1990 when it
was detected in eastern Canada. PVY-C has not been
found in North America.
PVY-N is called tobacco veinal necrosis
strain. Since its first positive detection in
eastern Canada, strict quarantine against
dissemination of this strain through infected seed
stocks has been enforced. Survey for this strain has
also been initiated in some states. Currently, this
strain is known to occur in Europe and some areas of
eastern Canada and the northeastern part of the
United States.
PVY-NTN is a special type of PVY-N, which
causes potato tuber necrosis. It is not known
previously to occur in North America and is
considered as an quarantine organism by Canada,
Mexico and the United States. In 2001, PVY-NTN has
been detected from two potato fields in the Cuyama
Valley of Santa Barbara County, California.
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