How to combat the bad dryness of citrus fruits in a biological way
The “bad dryness” of the citrus fruit (Deuterophoma tracheiphila, sin. Phoma tracheiphila) is caused by an imperfect fungus and can represent under certain conditions one of the main phytopathological problems of the lemon. This mushroom also attacks mandarin, clementine and sweet orange, although it is particularly susceptible to bitter orange.
The symptoms of the manifestation of “bad dryness” occur with the discoloration of the primary and secondary ribs; then the leaves turn yellow and fall, disarticulating at the petioles. Subsequently there is a progressive basipeto desiccation of the twigs, branches and whole branches, until the death of the plant can be verified.
The diagnosis of the disease occurs through the manifestation of the salmon pink or reddish-yellow color that take the infected woody tissues.
If the infection occurs on the apical portion of the young branches, the disease takes a slow course, which the plant tries to stop with the emission of recoveries from the basal portion of the struck branches.
On the other hand, when the infection originates from the roots or basal parts of the trunk or the main branches, the disease takes instead a rapid course (aching fulminant). Another manifestation (bad black) is the one that occurs when the root infections reach the inner layers of the wood and extend into the heartwood, which takes on a brown color. Even here the chronic manifestation manifests itself with a gradual drying of the plant, and then worsens when the fungus reaches the outer circles of the wood. In this phase the symptomatology is not different from that of the “fulminating mal”.
The fight against this fungus is mainly agronomic and precautionary. The agronomic struggle essentially consists in trying to reduce the possibility of infection at the level of root infections and of the branches. For root infections it is absolutely necessary not to use rotary organs for tillage; some citrus roots are very superficial and during this type of processing they are damaged and in correspondence of these cuts or lesions the Deuterophoma tracheiphila can be introduced. For this reason it is useful to go up with the depth of processing and use tools and tools that avoid these injuries (light tiller, etc.). As for cutting and pruning operations on branches, always operate with sharp, sterilized and possibly impregnated with copper solutions. It is also advisable to carry out the interventions (transplants, workings and pruning) with more sunny and less humid days when the virulence conditions of the fungus are less. The fungus can enter the plant even after hailstorms.
At the end of the pruning operation, always burn the branches (moreover with this technique the removed mineral salts are recovered and can be used for mineral fertilizations based on ash). If despite these precautions and the use of resistant rootstocks and cultivars the infection occurs, it will be necessary to resort to spraying with products that hinder the growth of the fungus. The chemical fight essentially consists in spraying that must be carried out in autumn-winter with monthly cadences based on 1% mixture of Bordeaux mixture. Also interesting is the preventive treatment based on decoction of horsetail that in addition to performing fungicidal action reinforces the immune system defenses.