Scenic Hills Nursery -- Oak Wilt Specialists in the Texas Hill Country

THE OAK WILT DISEASE: All oak species native to North America are susceptible to oak wilt. The White oaks are somewhat resistant to the oak wilt fungus. Red oaks and the live oak are highly susceptible and this includes the Lacey oak, in spite of it being of the white oak family.

Many people think that Oak Wilt Disease is caused by the sap sucking bark-beetle (Nitidulidae species). But that is not strictly true. The beetles fly to healthy oaks and feeds in the small twigs crotches and upper branches. 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 mat developed only on a red oak that has succumbed from Oak Wilt and died in late summer and early fall. During the winter months the spore mat grows and develops. By mid February, as the mats grow, they push and break out the bark. The fruiting body has a sour oak sap odor that attracts the beetle and as he feeds, the spores collect of his body. A sort of honey bee or humming bird effect of gathering pollen.

Live oaks only, spread the disease via inter-grafted root systems between trees. The disease can trans-locate through the roots systems at a rate of about a 75 to 100 feet per year. Live oaks are very bad parents and do not want to compete from their syblings. So the tree produces a predisposed a soil borne fungus to kill the germination of the acorn. They depend on wild life to move the acorns to other locations for forest diversity. Live oaks propagate from ramets or buds that form on the root system. A grove or a Mott 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, mature root system from it's clone parent so to speak.

During warm spells in the early spring the young beetles emerge and are able to fly several furlongs or 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 picnic. 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 can not 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 water conducting vessels (the xylem) are exposed to the beetle-carried spores as the insect feeds through the bark. Once established, the fungus can move through the tree’s vascular and phloem systems, resulting in systemic infection. The initial fungus spread is relative slow and the developing infections are not apparent until the fungus moves into the larger branches and then rapid wilting will occur. The disease is also spread from the same species by interconnected root grafts. Death of a tree infected through root graft is much more rapid than caused by the beetle.

The fungus is similar to a yeast and this is carried systemicly 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, a balloon-like enlargement, protruding into the cavity of a cell), which then block the vessels. This most possibly is an effort of defense by the tree to control the spread of fungi, but in doing so it suffocates / starves itself. (A defeatist 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 noted: yellowing leaves, tips of the leaves turn brown, and veinal necrosis (the main veinal 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 very short time.

Tylosis, balloon-like enlargement, a white rubbery gum, protruding into the cavity of a cell and then block the vessels. When a limb is cut off or any trpe of wound, its' normal function is produced, to seal the wound and to protect the tree from infection.

The Fungicide "Propiconazole" 14.3 MEC How does it work?

A Sterol biosynthesis Inhibitor

Sterols are essential compounds in the cells of all living organisms, components of cell membranes and other important anatomical features.

The triazole fungicide, Propiconazole is effective in management of the oak wilt disease, it possesses systemic and some curative properties against certain fungal diseases. It is a highly systemic sterol inhibitor that penetrates and trans-locates, preventing fungal cell development, by interfering with cell wall formation and growth throughout the plant by inhibiting sterol biosynthesis.

Propoiconazole is absorbed into the fungus where its two modes of action attack fungal cells at several sites altering the cell, thereby inhibiting sulfur-containing enzymes and disrupting fungal energy production. It has preventive activity, and is primarily active on mycelium with some anti-sporulant activity, and prevents spore germination.The commercially available formulation of propiconazole 14.3% MEC (microencapsulated) is available but not limited to; KESTREL®, Quali-Pro®, and Alamo®,.

Propiconazole at the highest label rate (therapeutic treatment of 20 mils per diameter inch) may provide protection for multiple seasons. However there is no residual activity in the tree after 23-24 months. The preventive dosage rate is 10 mils per diameter inch for trees up to 20 inches in diameter and for trees greater than 20 inches diameter due to their value and size, the 20 mil dosage per diameter inch is highly recommended. Foliar toxicity of propiconazole is low, even at rates of up to the highest label rate, though severe phytotoxicity may occur with high dosage rates on small diameter trees when treated early in the growing season. Propiconazole does not require high dilution rates with water so treatment is considerably faster uptake, and there is less tissue injury at the injection site due to the fact that it has a near neutral pH.

The injection treatment is not a universal CURE, however it will extend the life of the tree, and is effective for inhibiting the disease in uninfected or newly infected trees. Designed for use on high-value trees in your landscape, trees should be selected for preventive treatment based on the risk of the disease pressure. The chemical residual in the tree is effective for about 24 months and preventive re-treatment should be considered and applied within the second and third year. Trees that were infected and are in a weakened condition should be retreated the following year.

Prevention is better than cure.