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Verticillium wilt is caused by a fungus that lives in the soil. The fungus penetrates the root system of susceptible plants, eventually blocking the plant’s water-conducting system.
The fungus affects more than 300 types of plants throughout the world - from raspberries and tomatoes, to maples and elms. Although the disease occurs in naturally forested areas, it is found mostly in landscape
plantings.
How It Spreads
The two fungal species, V. alboatrum and V. dahliae, can survive in the soil for decades lying in wait for new plants to move in. The fungus usually enters the roots through wounds, but if
the tree is weak, it can actually penetrate the root.
When a plant dies, the fungus enters a resting state, producing structures called “microsclerotia”. These structures can be easily transported from place to place when trees are transplanted. In dry conditions,
these microsclerotia can by carried by the wind to infect new areas.
Tools can also carry the fungus, so proper sanitation procedures should be followed.
How the Fungus Kills
Once inside the root, the fungus reproduces and spreads through the tree via the xylem, or water-conducting tissue. As it spreads, it causes tissue damage and clogs the xylem, preventing water
from reaching the outer branches. Without moisture and necessary nutrients, these outer limbs wilt and die.
Symptoms
The first sign of disease is a slight yellowing of the foliage (similar to symptoms of Dutch elm disease although with less extensive crown involvement). There is also discolouration of the
wood. Branches, stems and roots show a light to dark brown staining of the sapwood. Cankers may form on the branch and stem.
Trees with extensive infection show reduced growth rates and branch dieback. Younger trees usually succumb within one year. Older trees tend to deteriorate for a few years before finally dying.
Control
Fertilizing
Fertilizing with a balanced mixture light on nitrogen (5-10-10) may help to alleviate some of the symptoms. High nitrogen fertilizers, however, should be avoided. They promote new growth
that would be vulnerable to the fungus.
Natural Defenses
One area that does offer hope comes from the Forestry Department of the University of Toronto. A team led by Dr. Martin Hubbes has isolated a glycoprotein that can potentially boost the natural defenses
of a tree. Although research so far has focused on Dutch elm disease, stimulation of natural defenses could also be effective with Verticillium wilt.
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