The Phoma tracheiphila (Mal Secco disease) is a mycotic endemic disease that is very frequent and noxious for citrus fruit, especially for the lemon which, among all the citrus fruit, is the most susceptible to it and the most damaged.
The cause of the disease is the fungus Phoma tracheiphila, which settles down in the ligneous vessels of the plant (but it can penetrate through the stomas, too) causing a typical tracheomycosis. The infection starts through the cuts that appear in the roots or, more frequently, in the foliage and develops between autumn and winter, when the climate is wetter and the temperature is around 15˚ and 20˚ C.
The boughs turn yellow, the trees shed their leaves and dry up. The death of the plants that have been damaged by the fungus can happen in a variable lapse of time, from a few months to a year, depending on the part of the plant that has been damaged.
The natural remedies adopted by the agronomists are numerous, even though not totally decisive: actually, it is very important to avoid, in the lemon groves, a downpour-irrigation on the foliage, which would favour the spreading of the pathogenic, and to protect the plants from wind, cold and hail. Moreover, it is necessary to prune the tree often (but not too much, not to weaken the plant), to burn the infected parts, to cut the infected stumps and to limit the use of too azotized fertilizer: actually, the excess of azote makes the plant more accessible to the fungus.
All these strategies to cure the plants in a natural way reveal to be ineffective or not sufficient, thats why it is usually necessary to support them with chemical cures (artificial remedies): applications of Ziram or of cupric salts, that can prevent the infections penetrated through the foliage but are not effective against the infections which develop from the plant and the roots.
In spite of the satisfactory results obtained, these chemical remedies too show some defects in the practical application, because they have to be repeated more than once in autumn and in winter and, moreover, because they smear the fruit and can provoke phenomena of phytotoxicity.
It is possible to defend the plants from this disease, which is difficult to control, only using varieties resistant to the Phoma tracheiphila and not very susceptible to it. But this is not an utterly satisfactory solution either. As a matter of fact among the varieties of the lemon tree there are distinct levels of sensibility, and some cultivar (Monachello, Interdonato, Femminello S. Teresa, Femminello, Femminella Continello, etc.) show a certain resistance, but the disadvantage is that, for some of these varieties, the capability of resisting to the Phoma tracheiphila often doesnt match quality bio-agronomical characteristics.
Virus and bacteria
The lemon trees can be attacked by various virus: Exocortite, Ringspot, Psorosis, Tristeza, Impietratura and Cristacortis. The most dangerous bacteriosis is caused by the Pseudomonas syringae, which breaks out with dark lesions on the leaves, especially in spring.
To eliminate this pathogenic agent it is recommendable to prune very accurately the tree, in order to eliminate the infected boughs and better the airing of the foliage. In case of strong attacks it is advisable one intervention only, with cupric derivatives, between the end of October and the beginning of November
The lemons are also subjected to the attack of parasites of either vegetal or animal origin. Among the animal parasites we find insects, acari and nematodes. Among the insects we can remember the cochineal, that perforates the lamina of the leaf or the new trunks and sucks the sap. If the infestation is not so widespread it is possible to defeat this parasite removing it by hand, remembering to disinfect the holes left by the insects with a flock of cottonwool soaked with alcohol.
If the infestation should be widespread, it is necessary to use some white oil, activated with a pyrethroid or with malathion for a more effective action.
For an optimal result it is advisable to treat the plants with anti-coccids at the beginning of the spring or in autumn-winter, in order to destroy most of the eggs.
Beware of snails, rodents and voles, too.