Oak root fungus, also known as Armillaria root rot, is a difficult to control disease which is difficult to control. In part, this is due to the fungus’s ability to survive on small scraps of infected root tissue in the soil. Attempts to fell and backhoe infected trees and vines can leave infected root debris distributed across a wide area at various depths, contributing to its survival and further distribution.
The difficulty of controlling oak root fungus is further compounded by the physical limits of conventional fumigation programs. Fumigants such as methyl bromide and carbon disulfide are limited to practical depths of injection, gaseous permeation, and control capabilities. In most cases, growers can achieve effective control to maximum depths of about 1.5 to 2. 5 feet. Furthermore, the thoroughness of soil permeation by the fumigant can be negatively impacted by heavy moisture in the soil. As a result, it is not only particularly deep roots and root pieces at depths of 7 to 12 feet that escape fumigation, but also root debris in zones of moist soil as well.
Oak root fungus and nematodes are difficult to effectively treat for similar reasons.
The tree and grape industries are also plagued by various, aggressive plant-parasitic nematode problems. To give one an appreciation for the magnitude of these infestations, many vineyards host around 4 to 7 different species of debilitating plant-parasitic nematodes. Growers who plant orchards in locations which previously hosted grapevines must institute a thorough fumigation program to avoid inheriting the typically dense populations of nematodes in the soil.
The same obstacles which prevent thorough control of oak root fungus also apply to nematodes. That is, many species of nematodes can be found in moist soil and at depths of soil well beyond the effective control range of conventional fumigation practices. Thorough control is necessary, as some of these nematode species can carry and transmit viral diseases of grapevines. All too often, growers will plant in fumigated areas previously hosting fanleaf virus disease, only for reinfection to occur due to surviving nematodes.
While these two sources of trouble for growers are extremely different in their basic characteristics, the similarity in treatment methodologies—and treatment difficulties—merits a joint examination.
Oak root fungus represents an instance of a symbiotic relationship becoming parasitic and destructive when encountering non-adapted species.
Armillarea mellea, also known as oak root fungus, is a member of the mushroom fungi. It is sometimes referred to as the honey mushroom, due to the shiny, honey-colored fruiting bodies produced by the species. With native trees, such as oak, aspen, fir and willow species, oak root fungus forms a sheath or crust of fungal tissue about the roots. From this surrounding sheath it extends fungal threads into the interior of the host’s roots. The fungus secures carbohydrates and other nutrients from the roots, but in return aids the tree roots in harvesting various minerals from the soil, such as phosphorus, potassium, calcium, nitrogen, and others.
The native tree species which have, over the eons of time, evolved this intimate association actually thrive and grow better in the presence of the fungus, thanks to the mutualistic association formed by the host and fungus. This association has also been given the name of symbiosis. Fungi in such relationships are referred to as ectotrophic mycorrhiza. Many such symbiotic associations exist in nature and are common to several pine species colonized by various mushroom fungi, many of which are edible species (e.g. Boletus, Lactarius, Suillus, Russula, etc.).
Oak root fungus falls into a group of fungi sometimes referred to as ‘white rot,’ because of their ability to digest cellulose and lignin (the brown, natural preservative and cementing agent found in wood). The fungus produces a white, punky product reminiscent of balsa wood.
Oftentimes, many indigenous tree species are cleared for commercial plantings. When trees such as the native oak are removed, however, many pieces of root carrying oak root fungus can remain in the soil. Upon introduction of a non-native tree or vine species, the fungus grows and attempts to establish a symbiotic association similar to that typically formed with native species. But the delicate physiological balance characteristic of the oak-fungus association, developed over millions of years, is absent with non-native species. Consequently, the attempts at establishing a symbiotic association result in parasitism, whereby the fungus becomes a pathogen.
Oak root fungus is devastating to commonly bred commercial plant strains which have the capacity for establishing symbiotic relationships with the fungus.
Commercial plantings of vines and trees often succumb to what is a devastating disease due to their lack of adaptation. The conditions for parasitism are further promoted, as farmed species are bred for commercial characteristics of yield and quality. This frequently involves breeding away from wild, native strains which hosted poor to mediocre agronomic characteristics, but could tolerate or resist the fungus.
Infections can be spread from vine to vine through root-to-root contact. The fungus also produces a resilient, thick shoestring-like growth, capable of growing through the soil and infecting encountered roots. Oak root fungus infections in a vineyard characteristically have an epicenter, from which the fungus radiates, causing further infections. The epicenter frequently represents the central point from which the native tree was removed. From this central point, the fungus can radiate outward as much as 6 to 9 feet per year. Flood irrigation and cultivation practices can also move pieces of the fungus to reestablish new infection centers.
Grapevines are also often attacked by plant parasitic nematodes, which are common disease vectors.
While many species of nematode attack grapevines, the dagger nematode (Xiphinema index) is particularly troublesome. Common hosts of the dagger nematode include rose, peach, fig, citrus, grapevines, sweet pea, oats and others. This nematode is one of the larger species found attacking plants, with adults reaching 3.3 mm in length. They are robust, and capable of inflicting significant damage while preying on external root tissues.
Typical symptoms include galling and severe root distortion. Feeding often results in cells which have several nuclei and are 3 to 10 times the volume of normal cells. These giant cells act as a sink to which many rich plant nutrients are directed, which are then preyed upon by the nematode. Thus, the initiation of feeding by the dagger nematode sets the stage for an enriched food source, with the nematode capable of securing a wealth of nutrients from a single feeding position for extended periods.
Complicating matters is the fact that X. index is a vector (transmitting agent) of the fanleaf virus in grapevines. The nematode acquires the virus during feeding. Proteins on the virus coat appear to be specifically designed for attachment to the inner walls of the esophagus and lower spear (through which the nematode feeds). When the nematode feeds upon an unafflicted plant, gland secretions pass through the esophagus and hollow lower spear, releasing the attached virus particles which are then injected into the host cells.
The nematode is known to be capable of storing virus particles for up to 9 months in the absence of a host. Shockingly, tests have shown that in the absence of a host, the virus appears to increase the lifespan of the nematode.
The virus disease itself can be devastating, reducing yields as much as 80%. Traditional fumigation can typically control nematodes to maximum depths of 1.5 to 2.5 feet. However, experienced grape growers know that X. index can be found in strata far below this range. It’s not unusual to find them in soils as deep as 7 feet and, in some cases, depths exceeding 11 feet. Thus, when soils with a history of fanleaf are fumigated and replanted, the chances for recurrence may just be a matter of time.
Tests have shown that it is possible to reduce the potential spread of oak root fungus and nematode-carried fanleaf virus with an innovative treatment.
Years ago, power companies explored alternative methods of extending the life of electrical poles. Although the wood was pressure heated and impregnated with creosote and other preservatives, extended exposure in the moist soil environment would eventually result in colonization by wood-rotting fungi. Tests were conducted whereby holes would be drilled at oblique angles into the poles above the soil level. Volatile fungicides were introduced into these drillings, the holes were capped, and the fungicide given sufficient time to permeate down through the wood. Many of these tests proved successful, mitigating the need for total replacement of power poles and saving millions of dollars for both the companies and their customers.
Two of the materials that proved quite successful were metam sodium (sold as Vapam), and methyl isothiocyanate (sold as Vorlex). The effectiveness of this system was best demonstrated with injections made on horizontal poles 50 to 70 feet in length. Holes were drilled at one end of the pole, fungicide introduced, and the holes capped and tarred to prevent loss to volatilization. Within 30 to 40 days, lethal concentrations of the fungicide (more than 15 ppm) could be found at the other end of the pole.
The scientists of Sunburst Plant Disease Clinic, upon learning of the success of this procedure, decided to see if it could be applied to trees infected with oak root fungus and nematodes. Tests were conducted on two groups of threes: trees heavily infected with oak root fungus, and trees marginally infected with oak root fungus but with significant infestations of nematodes.
Trees were cut to a short stump, and holes were drilled with ¾-inch diameter drill bits to a depth of 6 to 8 inches, the drill bit being moved up repeatedly to clear the hole of sawdust. Four-inch diameter drunks were drilled with 3 evenly spaced holes to form a triangle. Larger diameter trunks were drilled with holes positioned at least 1 to 1.5 inches from the perimeter and spaced evenly about the circumference, approximately 3 to 4 inches apart. A large space in the middle was drilled to distribute holes evenly throughout the surface. Metam sodium was carefully poured into each hole, leaving a minimum airspace of 1.5 to 2.0 inches to accommodate capping. After holes were filled, all openings were tightly capped with a rubber stopper. The entire trunk and stoppers were then sealed with tree tar.
The treatments were conducted in September and October. 30 to 40 days after treatment the roots of infected trees were excavated, and the root system divided into 3 sections: root tips, middle section, and upper roots. All were examined for viable oak root fungus. The trees infested with nematodes were excavated at 3 levels: top, middle and lower root system. Soil and roots were collected at each level and examined for nematode populations.
As a cross-reference, a set of untreated, healthy trees were comparably examined for nematodes. All treatments left the root system free of viable oak root fungus. This was true of roots visibly manifesting previous infections with the fungus. Nematode populations were also dramatically reduced at all 3 levels examined. Upon examination, this was understandable, as nematodes are obligate parasites, which must secure their food from living tissue. Thus, being near or within the roots exposed them to lethal doses of metam sodium.
As a check on the effectiveness of treatments, we did find large populations of nematodes in the healthy trees. Following backhoeing and removal of root pieces, a basin was formed at the surface of and about the perimeter of the 10-foot-wide by 4-foot-deep excavation site. One gallon of metam sodium was mixed in 200 gallons of water and delivered to each site. The entire 200 gallons were delivered evenly within the basin and allowed to permeate the soil. The depth of permeation was approximately 5 feet. After 60 days, the area was ready for planting. Trees subsequently planted in the treated location remained healthy, without reinfection of oak root fungus or buildup of nematode populations.
There are a few factors to consider when utilizing the metam sodium treatment to combat oak root fungus and nematodes.
Following are a list of rationale or considerations
- Conventional pulling and backhoeing of trees will leave many pieces of fungus-infected and/or nematode-infested root tissue, which can later provide a source of infection to successive plantings. This factor is minimized with pretreatment of trunks with the injection technique described.
- Conventional methods now available rely upon methyl bromide or carbon disulfide, both of which have relatively higher degrees of hazard, and even with unusual efforts, may not control ORF or PPN at lower depths of the soil. Control with this system is further exacerbated with wet or moist soil.
- Follow-up fumigation with metam sodium is possible even under wet or moist conditions. Use of methyl bromide or carbon disulfide requires that the soil be dry, a virtual impossibility for the depths required for effective control.
- There is a new wave of research being conducted on reintroduction to fumigated soil of beneficial microorganisms, which have proven to be quite effective. However, many growers have grown habituated to the use of highly effective chemicals, resulting in the expectation of quick fixes. The use of biologicals, however, requires specific characterizations of the condition at hand, and specially designed formulas to achieve the desired results for each unique situation.
- We believe that a highly viable, yet safe treatment with injection technology will involve development of a ‘pre-containerized’ cylinder of metam sodium Holes will be drilled to a specified depth, the cylinder dropped into the hole, and a catalyst (e.g. alcohol or water) added to initiate breakdown of the degradable cylinder coating.
- As regulatory agencies continue to advocate for targeted use of pesticides and minimal pesticide volumes in the environment, the imports of the injection system provide a close fit to these guidelines.
