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Verticillium Dahliae is a disease provoked by a fungus that affects the vascular system. It obstructs the vascular bundles that transport sap and gradually causes whole branches to dry out and can even kill the olive tree.
The infection is produced underground. In the soil, the spore detects that it is close to a root through the substances that the root secretes. In this case, it germinates and penetrates into the root, and settles into the vascular bundels, feeding off the sap there. It colonizes complete sectors of phloematic vessels from the root up to the leaves.
The incidence of this disease is quite variable, depending very much on the variety of olive tree and the specific ambient conditions. Damp soils with high temperatures are especially favorable for the development of this disease, which is especially virulent in irrigated groves of clay soil in warm climates.
Once the disease has infested a plant there is no way to erradicate it. There are only palliative measures for infested plants, and preventive measures for those that are not infested.
Palliative measures include reducing watering and nitrate fertilizers, and invigorating the tree through pruning and balanced nutrition and preventive measures mainly consist of reducing the number of inoculations.
Verticillium is an obligated parasite. If the soil is rich in fungus and bacterias, they will undermine the verticillium and the chance of the disease appearing diminishes. If the soil is filled with puddles along with high temperatures, this is very favorable for infection. Therefore, in places with mild winter temperatures, clay soil, varietals that are susceptible, and the presence of infected trees close by, then in summer you should try to create a certain level of water stress and not have the water drips right on the trunk.
When you notice that a tree or a branch is infected, it is a good idea to destroy it by burning it, so that the disease does not further propogate, since the leaves and sprouts that have been destroyed by the disease are where the microsclerotia of the fungus are generated.