Mal secco

Mal secco is caused by the wood-invading fungus Plenodermus tracheiphilus.

Symptoms

• Leaves and shoots turn yellow, wilt and fall, branches dieback.
• Fallen leaves may have reddish veins.
• The disease moves slowly downwards in the tree, eventually causing tree death.
• Infected bark on twigs and branches may become silver-grey then rupture, revealing black fruiting bodies of the fungus within.
• If the stem of the infected plant is cut open, the infected wood inside has an orange-reddish or salmon pink discolouration. The colour may be brown in older wood.
• Sometimes the plant responds to infection by sprouting new shoots at the base of infected branches or producing rootstock suckers.
• If the disease starts in the roots, the disease can progress rapidly and cause tree death.
• Chronic infections on mature trees, probably originating from the roots, may cause a brown discoloration of the heartwood without any initial external symptoms. However, when the fungus invades the water or food conducting tissue, infected trees collapse suddenly.

Plant stage and plant parts affected

It affects trees of any age, but is more severe on young trees. Symptoms may be worse in autumn or spring.

There are a number of citrus disorders that can cause some of the symptoms described for mal secco, but the combination of symptoms outlined above is characteristic of the disease.

Mal secco is found around the Mediterranean basin and in the Black Sea area of Europe. It is not known to occur in Australia.

Citrus, mainly lemons, and:

• susceptible rootstocks including rough lemon, trifoliate orange and Troyer and Carrizo citranges
• limes, citrons, bergamots, tangelos, tangors and some mandarin cultivars are also susceptible
• infection can occur in sweet orange (e.g. Newhall navel) and grapefruit, but is generally not severe.

The fungus enters the plant through wounds in leaves, branches or roots, and colonises plant tissue.

Incubation period can vary, depending on the season.

The fungal spores (conidia) can be produced on withered twigs or woody substrates on the tree on or from tree debris on the ground.

Citrus is an important crop in Australia, there are over 28,000 hectares of citrus planted and around 1,900 growers. For the year ending June 2017, citrus production was valued at $724.4 million. Queensland grows about 5,300 hectares of citrus, mainly mandarins, limes and lemons (source: Citrus Australia).

Mal secco is a serious disease of lemons where it occurs in the Mediterranean region. The disease significantly reduces quantity and quality of lemon production.

If the disease were to be found in Australia, it would have a significant impact on affected producers as a result of yield reductions and increased production costs to manage the disease. Market access could also be disrupted.

Home gardeners would also be affected as lemon and lime trees are a common backyard plant.

Illegal importation of infected plant material poses the greatest risk of introduction of this disease to Australia. The Australian Government closely regulates approved plant imports and monitors for illegal plant movement.

Etrog citron (Citrus medica) fruits are an important part of the Jewish cultural event known as Sukkot. Etrogs can be imported into Australia for this event, subject to strict Australian Government guidelines. Find out about importing Etrogs or other citrus items.

Mal secco fungal spores can spread short distances in water or with wind-driven rain and overhead irrigation.

The fungus can survive within infected twigs in the soil for over 4 months.

The spores can be carried by insects, possibly by birds or animals, or on contaminated equipment (vehicles, tools, gardening equipment) or people (hands, shoes and clothing).

Contaminated pruning tools can aid disease entry.

Mal secco is not spread in seed.

Regularly monitor common citrus plants, especially lemons, for yellowing leaves, wilt and dieback.

Check:

• fallen leaves for reddish veins
• bark for silver-grey discolouration and ruptures revealing black fruiting bodies of the fungus within
• inside affected stems or branches for orange-reddish or salmon pink discolouration. The colour may be brown in older wood.

Report suspected mal secco to Biosecurity Queensland immediately on 13 25 23 or contact the Exotic Plant Pest Hotline on 1800 084 881.

• Protect your farm from emergency plant pests
• Understand good farm biosecurity
• Visit the farm biosecurity website

Mal secco is prohibited matter under the Biosecurity Act 2014.

Report suspected mal secco to Biosecurity Queensland immediately on 13 25 23 or contact the Exotic Plant Pest Hotline on 1800 084 881.

If you think you have found mal secco, you must take all reasonable and practical steps under your control to minimise any associated risks. This is called a general biosecurity obligation (GBO).