CITRUS - Diseases of Fruit Crops in Australia - page 95

STUBBORN - BIOSECURITY THREAT

Cause
Prokaryote (mollicute) Spiroplasma citri .

Symptoms
Stubborn is not usually lethal; however, when young
trees are infected, they can be severely stunted and
unproductive. Symptoms are less obvious in mature trees
where the disease is limited and spreads slowly through
the tree.
Several symptoms are associated with the disease.
Foliage is often bunchy, dense and abnormally upright,
because leaves are shorter, broader, thicker and cupped;
shoots may have shortened internodes; and growth may
develop from multiple axillary buds. Leaves often have
variable chlorotic patterns or mottling. Under very hot
conditions, leaves may have irregular-shaped, blunted or
heart-shaped yellow tips. Premature leaf drop can occur
on severely affected branches.

Fig 6.9 Symptoms of stubborn in leaves, fruit and seed. Note the
overall reduction in size between diseased (left) and healthy
tissues (right).

The host range for this organism includes members of
the genus Citrus as well as other members of the
Rutaceae family (e.g. Fortunella spp.). Orange,
grapefruit, mandarin and mandarin hybrids are most
susceptible to the disease. Some varieties, like Poncirus
trifoliata , can have symptomless infections. Spiroplasma
citri has been detected in several noncitrus hosts,
including members of the Amaranthaceae,
Chenopodiaceae, Brassicaceae and Plantaginaceae
families. The pathogen is known to cause brittle root
disease of horseradish in the United States and sesame
yellows in Turkey and Iran. Often, weed species are
infected in the orchard with the disease occasionally
being transmitted to citrus.

Shoots and blossoms can develop out of season. Heavy
out-of-season f lowering can lead to fruit of varying
ages growing on the same tree, but yield is often
greatly reduced due to premature fruit drop or
mummification of fruit on the tree. Infected fruits
may be small, lopsided or acorn-shaped (thick rind at
base and thin rind at stem end), and they may not
colour at the stem end as they mature (colour
inversion). Seed abortion is common and grapefruit
and sweet orange fruit may have a blue or grey albedo
and develop a dense or cheesy peel. Fruit may also
have an insipid or bitter f lavour.

Source of infection and spread
Spiroplasma citri infects the phloem sieve tubes of its hosts,
persisting in trees as they decline. Typically, the organism
is distributed unevenly and at low levels in infected trees. It
is graft transmissible, and the rate of transmission depends
on tissue type, host species and the environment.
Spiroplasma citri can be cultured from the seed coat, but
seed transmission is not known to occur. The disease is
known to be spread naturally by several species of leaf
hopper, including Neoaliturus haematoceps, Circulifer
tenellus and Scaphytopius nitridus . While these leaf hopper
vectors are not present in Australia, other phloem-feeding
insects in Australia could possibly transmit the pathogen.
Insect vectors become infective 10-20 hours after
acquiring the pathogen via feeding and remain infective
throughout their lives. Evidence suggests that transmission

Fig 6.8 Symptoms of stunting and reduced productivity caused
by citrus stubborn (right) compared to a healthy tree (left).