University of Vermont Extension
Department of Plant and Soil Science
Fall News
Article
CANNAS AND
VIRUSES
Dr. Leonard Perry, Extension Professor
University of Vermont
Cannas are a tender perennial,
popular and grown by gardeners for their large attractive leaves and
large
flower spikes. Being tender, their roots
need to be overwintered indoors in cold climates. Within the last
ten years, virus diseases
have become widespread in cannas throughout the world, so if you
have these
perennials you may not want to overwinter them but rather discard
them this
fall after tops are killed by frost.
If you're not familiar with the
canna, it is the only genus in the Canna family. Although tropical,
most of the 100 or more cultivars
(cultivated varieties) have been
developed as hybrids in temperate
climates. There, they are grown outdoors in summer for their bold
foliage,
coarse visual texture, tropical effect, and flower spikes in colors
usually from
red to orange to yellow. Leaves are
green, reddish bronze, or striped. Depending
on cultivar and conditions they may range from 2 to 6 feet or more
tall. They prefer full sun, or at least 6 to 8
hours a day, and warm temperatures. They
may tolerate some drought but many also can be grown as aquatic
plants. Although tops will die back if frozen, the
roots (rhizomes) may survive in the ground if mulched where air
temperatures stay
above zero degrees (F).
Where temperatures freeze outdoors,
plants are cut back after frosted, and then dug, or allowed to dry
in their
pots, and held overwinter in a non-freezing location. Then water in
spring, increasing the amount
as growth resumes. This is the process
that has been used until recently when at least three viruses have
been found
overwintering in such plants, and spreading through plants
worldwide. This has resulted in many
growers no longer selling cannas, and many collections having been
destroyed. Larger and reputable specialty growers, and
some botanical gardens such as Longwood in Pennsylvania
(longwoodgardens.wordpress.com)
have obtained or selected virus-free plants and are propagating from
these.
Two of the viruses, the bean yellow
mosaic and canna yellow streak, are in the potyvirus group which is
not passed
along in seeds. So plants with these
could be grown from seeds and be clean of virus, until reinfected.
The canna yellow mottle virus is in the
badnavirus group which can be passed through seeds-- unusual for
viruses-- so
continues the disease and its spread.
What you'll see with virus-infected
cannas first are short, light green streaks in leaves parallel with
the
veins. Close inspection shows lots of
speckling. These streaks grow larger,
then turn to dead (necrotic) streaks, with lighter colored streaks
in the
flower petals. As more of these dead
veins appear, leaves turn a more general yellow (chlorotic) and the
plants will
be less vigorous. These symptoms are
more easily seen in green than bronze cultivars. It is easy to see
in some such as 'Phaison'
with light green streaks against the red instead of the usual pink
striping. In
other cultivars, such as 'Pretoria', it is difficult to see with the
usual
yellow streaks appearing more sandy in color. If you're not sure,
watch the
leaves as they unfold in spring. If they
have such streaks or blemishes they are likely infected.
The only effective solution so far
for infected plants is to destroy (not compost) them in trash bags,
or to bury
or burn them. Destroying infected plants will slow the spread of
this disease,
and help prevent future "clean" plants from becoming infected.
Although
you may not see aphids on cannas, these can spread viruses. If a
low level of virus, you may not see
symptoms or they may be mild and plants may continue to grow
normally.
If you suspect a virus infection, and have
more than a few plants severely infected, you may
want to have a laboratory test done as several other problems such
as nutrition
may mimic virus symptoms. Check with
your local state university diagnostic lab to see if they can do
such virus testing
(www.npdn.org), or tell you where
to send
samples.
Underwatering or root stress may
result in brown leaves. Spider mites may
cause speckling and eventual browning. Blackened
leaves, beginning with rust-colored spots on undersides, is likely
canna rust
disease-- most prevalent under hot and humid conditions.
Then, when buying future cannas,
make sure the sources indicate if the plants are virus free and
discuss how
they have ensured this. Even if you buy
virus-free plants, they may get infected from sick plants nearby.
If you like the appearance of cannas, but
avoid or discard them due to virus disease, consider some of the
other
large-leaved tropical plants as alternatives such as ornamental
bananas, elephant
ear's, and ornamental gingers.
Return to
Perry's Perennial
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