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Oak Wilt

How is it caused?

Many people think that Oak Wilt is caused by the sap sucking bark-beetle (Nitidulidae species). But that is not strictly true. It is actually a fungus called Ceratocystis fagacearum, which is the cause of trouble. The beetles are just one of the means by which the fungus can be transferred from tree to tree. The beetle carries the fungal spores from a spore matt developed only on a red oak that has succumbed from Oak Wilt. It is similar to the honeybee effect of gathering pollen. The disease can trans-locate through the root systems at a rate of about a 100 feet per year. Live Oaks only spread the disease via intergrafted root systems between trees. Live Oaks propagate from ramets or buds that form on the root system. A grove or a Moot of Live oaks is actually a clone of one tree and survival is guaranteed because the new tree has the support of a fully developed root system from its clone parent so to speak.
During warm spells in the early spring the young beetles emerge and are able to fly several miles to find an oak tree and feed on the sap. If successful it gives off a scent to alert its little friends to the find. Incisions are made in the branches to reach the sap and this is the beginning of the end for the tree because the beetles infect the sap with the fungal spores. Ingestation takes about four to six months and there are no noticeable effects of the disease until you see the start of the foliar effects. The pathogen has to have a healthy living host to survive. It cannot live in the soil or dead wood tissue. Also during pruning, fresh cut wounds through molecular evaporation emit an oak sap odor, which is like a loud dinner bell to the beetle. The larger the wounds, the greater the molecular evaporation, when pruning oak trees these wounds should be painted immediately with a light application of a spray paint
The fungus is similar to yeast and this is carried along the vascular system in the tree sap. It excretes digestive substances, which are toxic to the tree. The cells in the wall of the sap-vessels react to it by producing gum-filled enlargements (tylosis-a white rubbery substance), which then clock the vessels. This possibly may be an effort of defense by the tree to control the spread of fungi, but in doing so it suffocates/ starves itself. (A defeatis attitude, kill yourself, kill the disease and the effect is like a serious cholesterol problem in humans). The flow of nutrients and water is stopped and soon the outwards signs of the disease can be seen: yellowing leaves, tips of the leaves turn brown, and venial necrosis (the main venial rib turns brown, the area between the veins remains green or yellow) within a few weeks a dying branch or the entire tree. There are different forms of the fungus, which may exist side by side. One is non-aggressive, whereas the other causes the death of the tree in a short time.