What
it is and what you can do to stop it...
Of all of the
agricultural pests and diseases that threaten citrus
crops, citrus canker may be one of the most devastating.
Citrus canker is a highly
contagious disease caused by the bacterium Xanthomonas
axonopodis pathovar citri. An infestation can
destroy entire crops, but the disease poses no health
risk to humans or animals.
Severe infection may
produce a variety of effects, including defoliation,
dieback, severely blemished fruit, reduced fruit
quality, and premature fruit drop.
Appearance and Strains
Citrus canker symptoms
appear on the fruit, leaves, and twigs of infected
plants, and typically consist of small, round,
blister-like formations called lesions. Lesions usually
become apparent about 7 to 14 days after infection. As
the lesions mature, the epidermis or outer layer of
tissue ruptures, producing a craterlike spot lined with
tan-colored tissue that is usually surrounded by an
oily, water-soaked margin and a yellow ring that looks
like a halo. On fruit, the lesions appear scab-like or
corky. On leaves, old lesions sometimes fall out,
leaving behind a scattering of round holes.
There are at least three
distinct strains or types of citrus canker, as
determined by serological tests and DNA analysis. The
"A" strain affects members of the plant family
Rutaceae, including most citrus species and hybrids,
especially grapefruit, lime, sweet lime, and trifoliate
orange. The "B" strain of citrus canker
affects lemons in Argentina, Uruguay, and Paraguay.
However, Mexican or key lime, sour orange, Rangpur lime,
sweet lime, citron, and occasionally sweet orange and
mandarin orange can also be affected. The "C"
strain affects only Mexican lime growing in Brazil.
Other, unrecognized strains may also exist. The current
and all previous U.S. infestations have been associated
with the "A" strain.
Infection
Like most bacterial plant
diseases, citrus canker is greatly influenced by
temperature and moisture conditions. The disease is most
likely to develop when heavy rains occur during a period
when the mean temperature is greater than 68 degrees F.
(20 degrees C.).
Spread
Citrus canker spreads
swiftly over short distances by wind driven rain,
flooding, air currents, insects, birds, and human
movement within groves. Overhead irrigation may also
play a role in spreading the bacteria.
Movement of infected
plants, seedlings, propagative material, and fruit is
the primary means of spreading the canker pathogen over
larger distances. Contaminated clothing, tools, packing
boxes, and other items associated with harvesting and post harvest
handling of fruit are also potential sources
of infection.
Canker bacteria survive
and multiply primarily in naturally occurring lesions.
Bacteria may also survive in crevices in the bark
tissues of citrus trees. Bacterial populations appear to
decline rapidly in soil. It has been suggested that the
bacterium may survive at low population levels on citrus
hosts without symptoms developing, and it may also
survive for short periods of time on some weeds and
grasses. Neither of these survival mechanisms has been
proven, however.
Identification and
Control
When an area is suspected of
being infested with citrus canker, fruit or leaf samples
are sent for testing by State and Federal laboratories.
In countries where citrus
canker is an established, ongoing problem, control of
the disease is primarily achieved through a combination
of tactics, including the production and use of
disease-resistant plant varieties, use of protective
sprays, and phytosanitary measures (use of certified
nursery stock). Outbreaks of citrus canker may also be
reduced when windbreaks are constructed in windy areas.
When environmental
conditions are favorable for the spread of the disease,
chemical control measures are not entirely effective.
However, materials containing copper (bordeaux mixture,
copper hydroxide, basic copper chloride, copper
oxychloride, and tribasic copper sulfate) are the most
effective bacterial sprays for protecting leaves and
fruit. These materials can reduce the incidence of
disease, but they will not eliminate established
infections. Extensive use of copper may also cause
phytotoxicity problems in treated groves.
Pruning infected shoots
or plant parts during late summer and autumn can reduce
the risk of infection the following spring. Defoliation
of canker-affected trees can also further reduce
infection risk.
In the United States,
quarantining areas affected by citrus canker is still
practical. Eradication of infected and adjacent trees is
the most effective means of protecting commercial citrus
from the disease. Once positively identified, diseased
trees in commercial groves are uprooted, placed in a
pile, and burned. Surrounding, disease-free trees are
destroyed as well, as an added precaution. In
residential areas, diseased trees and surrounding,
exposed trees are cut down or removed.
Areas where trees have
been destroyed must be kept free of citrus sprouts and
seedlings. Movement of citrus fruit budwood and other
plant parts is prohibited from property on or adjacent
to sites where infected plants are located. All
clothing, tools, and equipment used in infested areas
must be properly disinfected.
For More Information
If you have questions about
citrus canker, call the central office of APHIS' Plant
Protection and Quarantine (PPQ) unit at (301) 734-8645,
or contact the nearest regional office of PPQ:
Central Region (210)
548-2750
Northeastern Region (609) 757-5070
Southeastern Region (601) 863-1813
Western Region (916) 857-6241
Source: USDA Animal
and Plant Health Inspection Service (APHIS)
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