Agriculture Reference
In-Depth Information
Fig 6.26 Dieback symptoms caused by mal secco.
Fig 6.28 Wood discolouration in a stem affected by mal secco.
The discolouration is caused by gum production in the xylem
tissue in response to invasion by the fungus.
twigs, exposed woody surfaces or debris. For infection
to occur, spores need 40 hours of moisture at
temperatures in the range of 14-28°C. The optimum
temperature for disease development is 20-25°C. The
development of mal secco is accelerated by cultural
practices that increase the growth of the trees (e.g. the
use of nucellar lemon clones, heavy pruning and high
rates of nitrogen fertiliser).
Fungal spores disperse over short distances by wind-
blown rain, birds, insects or animals. Infected twigs and
branches can remain infectious in the tree canopy for
several weeks and when buried in the soil can harbour
viable fungal propagules for even longer (longer than four
months). Infected material on the ground is a source of
inoculum for root infections. The disease spreads over
long distances by moving infected propagation material
and plants. Fruit, roots, stems and shoots can harbour
symptomless infections.
Phoma tracheiphila
survives in
lemon seed as mycelium; however, developing seedlings
are not known to be infected, suggesting the disease may
not be readily seed-borne.
Fig 6.27 Mal secco-affected stem covered with small, black
fungal fruiting bodies.
sectioned, the affected tissue has an orange-reddish or
salmon-pink colouration, which is brown in older tissue.
This is caused by gum production in the xylem vessels.
When infection starts in the roots, the progression of
symptoms is rapid and fatal. It is called 'mal
fulminante'. Deep infections that extend into the hard
wood of the trunk and large branches, causing a brown
discolouration of the tissue, are referred to as 'mal nero'.
Trees can be infected in this manner with no external
symptoms until the pathogen reaches the functioning
xylem.
Phoma tracheiphila
attacks trees of any age but is
more severe on young citrus trees.
Importance
Mal secco is the most destructive fungal disease of
lemons worldwide. It reduces the quantity and quality of
fruit produced in affected orchards and limits the use of
susceptible cultivars. The disease currently appears to
be confined to citrus-producing countries in the
Mediterranean Basin and Black Sea region, Asia Minor
and parts of Africa.
Source of infection and spread
Fungal spores (conidia) are water-borne and enter the
plant through wounds in above or below-ground tissue
to cause infection. Young tissue is particularly
vulnerable to infection. Spores may develop on infected
