Mal secco

Scientific name

[Fungus] Phoma tracheiphila (Petri) Kant. & Gik. (syn. Deuterophoma tracheiphila Petri)

Other common names

“mal fulminante” a rapid fatal form that is apparently caused by root or stem infection

“mal nero” referring to internal wood browning

Disease cycle

Conidia are produced in pycnidia found on withered twigs and from hyphae growing on exposed wood or debris, including branches, leaves, and fruits. Most probably the pathogenpathogen:
an organism, usually a microorganism, which causes disease by intimate association with its host
can also be transmitted to other trees through contaminated pruning tools. Twigs and leaves lying on the soil may be a source of inoculuminoculum:
a.) the pathogen or its part that causes infection of the host b.) small amount of a microorganism that is transferred to a substrate or a culture medium in order to propagate it
for infection through wounded roots. Leaves infected by the fungus fall to the ground during autumn and spring, and the fungus within the leaf tissue are able to sporulate at temperatures ranging from 10 to 25°C (55 to 77°F).

Under natural conditions, the inoculuminoculum:
a.) the pathogen or its part that causes infection of the host b.) small amount of a microorganism that is transferred to a substrate or a culture medium in order to propagate it
can be dispersed by wind and rain; P. tracheiphila penetrates the hosthost:
an organism that is infected with or fed upon by a pathogenic or parasitic organism
through wounds via both conidia and myceliummycelium:
mass of hyphae that constitutes the vegetative parts of a fungus (the conspicuous part in most cases is the fruiting body of fungi).
. Infection usually occurs during the winter. Although the optimum temperature for pathogenpathogen:
an organism, usually a microorganism, which causes disease by intimate association with its host
growth is about 25°C (77°F), optimum temperature for symptom expression and xylemxylem:
the plant vascular tissue that conducts water and dissolved minerals upwards from the roots to the rest of the plant
colonization is 20 to 22°C (68 to 72°F). Infection occurs between 14 and 28°C (57 and 82°F), whereas at temperatures above 28°C (82°F), fungal growth ceases and symptoms are not expressed. A unique characteristic of this fungus is its ability to produce a- sexual spores called phialoconidiaphialoconidia:
(sing. phialoconidium): a conidium that develops from a phialide.
. These are spores formed on the myceliummycelium:
mass of hyphae that constitutes the vegetative parts of a fungus (the conspicuous part in most cases is the fruiting body of fungi).
in the plant xylemxylem:
the plant vascular tissue that conducts water and dissolved minerals upwards from the roots to the rest of the plant
released into the transpiration flow. When a sporespore:
a.) small, walled reproductive body produced by non-flowing plants, algae, fungi and some protozoans; usually microscopic and resistant to desiccation and heat b.) dormant, nonreproductive body formed by certain unicellular organisms, especially bacteria, in response to harsh environmental conditions
reaches a location where it can germinate it forms new hyphae causing disease.

Symptoms

Typical symptoms include veinal chlorosischlorosis:
an abnormal yellowing or whitening in green plant tissue due to insufficient chlorophyll production
, leaf wilt; red coloration of the xylemxylem:
the plant vascular tissue that conducts water and dissolved minerals upwards from the roots to the rest of the plant
and diebackdieback:
progressive death of shoots, branches, and roots that generally begins at the tip and works back to the main body of the plant
of twigs and branches; in many cases diebackdieback:
progressive death of shoots, branches, and roots that generally begins at the tip and works back to the main body of the plant
is sectorial. The pathogenpathogen:
an organism, usually a microorganism, which causes disease by intimate association with its host
proceeds slowly downward from the infected young shoots to the branches and main limbs; when the trunk and roots become infected, the tree dies. When the fungus infects the base of a trunk or the roots, a phenomenon can occur known as – mal fulminante. In these cases, the tree usually dies quickly.

Host range

The principal hosthost:
an organism that is infected with or fed upon by a pathogenic or parasitic organism
of mal secco is lemon. The disease is highly destructive on lemon, and citron (C. medica L.), lime (C. latifolia Tan.), and bergamot (C. bergamia Risso) also have been reported to be very susceptible to natural infections. Different degrees of resistance are shown by other species and hybrids. Sweet orange (C. sinensis (L.) Osbeck), grapefruit (C. paradisi Macf.), clementine mandarin (C. clementina Hort.), tangerine (C. reticulate Blanco), and mandarin (C. deliciosa Tenore) are affected sporadically by the disease and are considered tolerant. Among rootstocks, sour orange (C. aurantium L.), the most widespread lemon rootstockrootstock:
a living plant, sometimes just the stump, with a healthy, established root system, used for grafting a cutting or budding from another plant; the grafted portion is called the scion
in Italy, Greece, and Turkey, rough lemon (C. jambiri Lush.), Volkamer lemon (C. volkameriana Ten. & Pasq.), and alemow (C. macrophilla Wester) are very susceptible. Reports on the degree of susceptibility of citrus species to mal secco are sometimes contradictory, especially for rootstocks. Sour orange, for example, is considered to be very susceptible in Italy, but only moderately affected in Israel. It can be stated that almost all citrus species are susceptible to P. tracheiphila when artificially inoculated by wounding.

Distribution

Mal secco is present in all the citrus producing countries in the Mediterranean and Black Sea areas with the exception of Spain, Portugal, Morocco, and some areas of the Arabian Peninsula. The disease is not known to occur in the citrus-growing countries of the Americas or Oceania, even though there is no obvious climatic or cultural factor limiting the establishment of mal secco disease in non infested areas.

Easily confused with

Citrus blast (Pseudomonas syringae pv. syringae)