Page 8 - Vol.38-No.9

CROP PROTECTION

CITRUS IS A VERSATILE AND

ADAPTABLE LONG-LIVED

ORCHARD CROP

•Sub-tropical: including Florida, Brazil
and Transvaal (South Africa) with cool dry
winters and rainy hot summers – between
20° and 28° latitude
•Tropical: alternating wet and dry sea-
sons but no cold period, and including the
DR. TERRY MABBETT West Indies, West Africa and South East

Asia – within 20° of the equator.
Citrus encompasses all species belonging Most of the internationally-traded bulk
to the genus Citrus together with cultivars, citrus has traditionally been grown in re-
hybrids and rootstocks and other genera gions with classical Mediterranean-type Limes are now mostly grown in the
like Poncirus (trifoliate orange) and For- climates: e.g. Spain, Southern California, tropics but are seen doing well here
tunella (kumquat). All belong to the plant Australia and Cape Provinces in South Afri- in the Mediterranean-type climate of
South Australia
family Rutaceae and all are related through ca, but production has increasingly spread surface area of 200 square metres. There
possession of transparent citrus oil glands into sub-tropical and tropical regions of are 400-500 stomata per mm2 of leaf
and flowers with an annular disc.
the world including Central America and surface and essentially confined to the
Most edible citrus species including Cit- South America, The West Indies, East and abaxial (lower) leaf surface. Adaxial (up-
rus sinensis (sweet orange), C. reticulata West Africa and South East Asia. per) leaf surface has a thick cuticle wax
(mandarin), C. limon (lemon) and C. au- ‘bloom’ and hardly any stomata.
rantifolia (lime) originate in eastern Asia, Citrus under pressure Citrus leaves are particularly prone to
including China and South East Asia, but Crops grown outside of their comfort infection during periods of ‘flushing’ when
the regions where citrus is now planted zones (natural distributions and centres of trees produce a mass of soft vulnerable
and grown on the largest scale and yield- origin) experience higher pest and disease leaves all at the same time. Flushing is
ing by far the heaviest crops of fruit are pressures from a wider range of insect and a normal response in citrus to seasonal
far removed from the citrus tree’s natural mite pests and plant pathogens, and citrus changes in rainfall and temperature. Risk of
distribution and centre of origin. is no exception. Trees are under greater insect pest attack and infection by micro-
physiological stress and less equipped to bial pathogens is highest when leaf flushes
withstand those insect pests and plant appear during periods of heavy rainfall and
pathogens not usually encountered and
others which are normally tolerated with-
out economic loss in yield.
Higher insect pest and disease pressures
in the wet humid tropics where conditions
facilitate easier infestation or infection,
faster pest and disease development and
wider pest and disease dissemination will
place extra burdens on citrus, compared
Citrus is most vulnerable to leaf attack
by insects and foliar infection by with citrus crops in sub-tropical regions
pathogens when the trees are carrying where citrus originated. Tropically-grown
flushes of new leaves.
citrus possesses a higher density of leaf
Three distinct zones for citrus cultivation stomata than does citrus grown outside
can now be identified according to climate. the tropics, thus offering more entry points
They are: for fungal pathogens like Mycospaherealla
•Mediterranean type: including Mediter- citri (greasy spot) which infect the leaves
ranean coastal areas, Southern California via the stomata.
South African Cape and coastal New South Citrus trees present an easy target and
Wales (Australia) with cool, rainy winters provide good host material for plant pests Sweet orange originated in Asia but
and hot dry summers – between 30° and and pathogens. Evergreen citrus carries up the bulk of oranges are now grown in
40° latitude to 200,000 leaves per tree with a total countries like Spain and South Africa
with Mediterranean-type climates.
6 Vol. 38 No. 9