CITRUS - Diseases of Fruit Crops in Australia - page 91

6 CITRUS

Citrus species

Edible Citrus (Rutaceae) has been cultivated in Australia
since its introduction with the First Fleet in 1787. In a
relatively short time, the Australian citrus industry has
grown to a value in excess of $400 million annually, more
than 10% of the total value of fruit production in
Australia. Currently, approximately 32 000 ha of citrus
are planted in Australia. The orchards occur in the
Riverina region of New South Wales (28%), the Riverland
region of South Australia (24%), the Murray Valley region
of New South Wales and Victoria (23%), Queensland
(15%), and 10% occur in other regions, such as Bourke
and Narromine (New South Wales), Kununurra (Western
Australia), the New South Wales east coast and the
Northern Territory.

experience a range of climates. This provides the
opportunity to produce quality citrus from a diverse
array of varieties. The varieties of Australian-grown
citrus include (in descending order of production)
Navels, Valencias, mandarins, lemons, limes and
grapefruit. Lemons, limes and grapefruit dominate
plantings in the north; Navels and Valencias in the
south; and mandarins are planted in the intermediate
regions.
The wide range of climatic conditions and cultivated
citrus varieties is largely beneficial in Australia; however,
it has also created the opportunity for several citrus
pathogens to become problematic. This chapter provides
details of many of the citrus diseases present in Australia,
as well as some that require strict quarantine to prevent
their arrival.

The 22 degrees of latitude separating the southern-most
and northern-most production areas of Australia

BACTERIA

CITRUS CANKER - BIOSECURITY THREAT

Cause
Citrus canker, caused by the bacterium Xanthomonas
citri subsp. citri , is a serious disease of citrus
worldwide. The disease is known also as 'Asiatic
canker', 'true canker' or 'cancrosis A'. The taxonomic
nomenclature of X. citri subsp. citri has undergone
several changes and some of its previous names have
included X. smithii subsp. citri, X. axonopodis pv. citri
and X. campestris p.v. citri .

The diseases caused by these species however are not
considered to be as economically important to Australia
as that caused by X. citri subsp. citri .

Symptoms
Typical citrus canker lesions first appear on the lower
surface of leaves as small, pinpoint, water-soaked and
slightly raised spots, which expand, thicken over time
and protrude from both leaf surfaces. As the lesion
develops, the tissue becomes spongy or corky, and the
colour changes from tan or brown, to grey or white,
usually surrounded by a greasy, water-soaked margin and
a yellow halo. Lesions often occur in clusters in
areas where water pools, such as along leaf margins or
at leaf tips. Lesions can develop a crater-like centre, which
may fall out to create a shot-hole appearance in the leaf
blade. The lesions are often associated with leaf miner
feeding tracks, because feeding wounds provide an access
point for bacterial cells to infect the plant tissue.

The bacterium causing Asiatic canker has a wide host
range and affects almost all citrus species and varieties
and some related genera. This includes some native
Australian citrus species (e.g. Citrus glauca or desert
lime), which have been shown experimentally to be
hosts of the pathogen.

In addition to X. citri subsp. citri , the bacteria X. fuscans
subsp. aurantifolii and X. alfalfae subsp. citrumelonis can
infect and cause similar disease symptoms on citrus.