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VIRUS:
Most
plant viruses are made up of nucleic acid (genetic
material, usually RNA, more rarely DNA) in a
protein coat. They are what is known as "obligate
parasites", in other words they can only gain
nourishment from another living organism in order
to grow and reproduce (a minority of fungal plant
diseases such as rusts
and powdery mildews are also obligate parasites).
Once a virus has entered a plant, it is able to
replicate by inserting its genetic material into
plant cells and using them to produce more of
itself. Symptoms
of viral infection may be latent (no visible affect
on the plant), may reduce growth (common), or
result in any of several other symptoms including
visible marks on foliage or flowers such as
distortion, yellowing, mosaics, spots, streaks, and
mottling. There is generally no treatment
available for an infected plant. Some
virus infected daylilies
were recently (2005) discovered and removed from
different nurseries by the Michigan Department of
Agriculture. These plants had been imported for
sale and it isn't known whether other similarly
infected plants have been available at nurseries or
garden centers in other regions. If they have, then
these plants present a risk to other daylilies and,
perhaps, other plants growing in their
vicinity. The
virus in question is the Tobacco Ringspot
Virus (often abbreviated to TRSV), which can
infect a number of different plants. However, there
are different strains of this virus that may have
variable host ranges and we do not, at this time,
know which strain has infected these daylilies or
whether it is a strain that already occurs in North
America. This
virus is typically transmitted mechanically (such
as during division of plants), and by vectors such
as dagger nematodes, grasshoppers,
melon/cotton and green peach aphids,
onion thrips,
spider
mites
and tobacco flea beetle, which introduce the virus
by feeding on infected plants and then subsequently
on healthy ones. Some of these are known pests of
daylilies and could therefore spread the disease.
In some plants, this virus can also be transmitted
by pollen and seeds. Daylily
gardeners are advised to watch out for the yellow
blotches of this virus as illustrated below, but be
aware that other conditions like pest, disease and
weather damage, nutrient imbalances, and chemical
injury such as from pollution or sprays may
resemble virus symptoms. Currently the only way to
determine for sure if a plant has a virus is to
have it tested by a diagnostic service. If
you suspect a daylily in your garden may have this
disease, remember that once a plant is infected
with a virus there is generally no treatment and
the plant will likely be infected for the remainder
of its lifetime. Any divisions taken from that
plant are also likely to be infected, as are any
other plants subsequently divided with the same
tools if those tools are not adequately disinfected
between plants. Also, there is the risk of pests
carrying the disease to healthy daylilies. Until we
know whether the virus is pollen and/or seed
transmitted in daylilies, it is safest not to use
any suspect plants for hybridizing. The
usual recommendation is to destroy virus infected
plants. However, it can be difficult to
differentiate virus symptoms from other disorders
visually, plus we do not know at this time if any
infected daylilies found their way to other
nurseries, garden centers or gardens in North
America. Expert advice is therefore recommended
before determining a course of action regarding
suspect plants. It is always prudent not to
purchase any daylily that has an abnormal
appearance. |
