Armillaria root disease is a chronic problem in California vineyards.
Commonly called oak root fungus, the disease is caused by
Armillaria mellea. This fungus is native to forest trees in California.
Its range includes the Central Valley, the San Gabriel Valley, the Coastal
Mountains, and elevations up to 6,000 ft. of the Sierra Nevada (Baumgartner
& Rizzo, 1998).
Though several Armillaria species are native to California, only
Armillaria mellea kills grapevines. Armillaria root disease has
been reported on grapevines in the southeastern U.S., Brazil, Central and
Eastern Europe, and Australia (Hood et al., 1991).
Armillaria infects vine roots and can eventually kill the vine. It may
spread to neighboring vines forming distinctive clusters of dead vines.
Attempts to replant within these clusters are usually ineffective. Currently,
there are no adequate controls for Armillaria root disease.
Armillaria causes above-ground symptoms typical of most root
diseases: dwarfed shoots, yellow or red leaves, and premature defoliation.
Symptoms are most obvious in late summer when vines may completely collapse and
die. Symptomatic vines can be positively diagnosed by examining their root
collars for below-ground disease characteristics. Unique to Armillaria
are: mycelial fans (white sheets of fungal tissue found beneath bark [see
photo]) and rhizomorphs (black, shoestring-like structures found on bark and
growing through soil). Armillaria may form mushrooms at or near the base
of infected vines in winter, but this may not occur on an annual basis.
Therefore, absence of mushrooms does not mean absence of infection.
Course of the disease
How long it takes symptoms to develop and for the vine to die after
infection is not predictable. This depends on a number of factors, such as
rootstock tolerance, amount of inoculum present, extent of infection, and soil
conditions. In vineyard and greenhouse studies, we are currently examining how
reliable disease symptoms are for predicting vine death. Our results to date
show that both symptom development and time until death after infection can
take from one to several years.
How infections spread
Armillaria may be present on a site before a vineyard is established.
The disease affects more than 500 species of woody plants, including most
native trees, such as oak, madrone, laurel, Douglas fir, and Ponderosa pine
(Raabe, 1962). After land is cleared of an oak woodland, any infected roots
that remain underground become inoculum sources.
Armillaria is a wood-decay fungus. It lives in soil but needs woody
tissue on which to survive. It can live in decaying roots for up to 50 years,
depending on their mass.
Rhizomorphs, the agents of infection, grow through the soil from an infected
root, but they die if they are separated from the roots they feed on.
Armillaria can attack any woody part of a grapevines root system.
Vines become infected when roots grow into contact with old
Armillaria-infected root pieces, or when rhizomorphs grow from these
inoculum sources and contact vine roots. In either case, mode of infection is
the same: once the fungus contacts a root, it bores through the bark with the
aid of lytic enzymes. Below the bark, Armillaria kills the cambium and a
mycelial fan forms. The mycelial fan expands beneath the root bark, and the
fungus decays the wood.
Armillaria does not live freely in the soil. It is present only on
infected root pieces. Vines planted in infected sites may not show signs of
root disease for several years, because it takes time for roots to grow into
contact with inoculum in the soil or vice versa.
Once a vine is infected, Armillaria can move to neighboring vines in two
ways: by direct vine root-to-root contact or via rhizomorphs. Hyphae (strands
of fungal tissue) grow from infected roots to healthy roots that are touching
them. Rhizomorphs grow from an infected root, through the soil, to the roots of
a nearby vine.
Vine-to-vine spread of Armillaria is usually quite slow. The rate of
spread depends on many factors, including soil moisture and temperature,
rootstock growth rate and tolerance, amount of inoculum, and vine-spacing.
Three things definitely hasten Armillaria infection and spread:
excessive soil moisture, large quantities of inoculum, and close vine-spacing.
Chemical infection controls
Pre-plant soil fumigation is more effective for control of Armillaria
root disease than post-infection spot fumigation. If soil moisture and texture
are optimal, methyl-bromide will kill inoculum to a depth of approximately one
meter. For spot fumigation, however, methyl-bromide is often only effective for
a few years, which is about how long it takes for roots of replants to meet
inoculum. Spot-fumigation only works if the fumigant penetrates deep enough to
kill Armillaria on the roots of the dead vine and the inoculum that
originally infected it.
Unfortunately, based on our observations of infected vineyards that were
fumigated before planting, we know that methyl-bromide doesnt always
provide permanent control. Without thorough land-clearing, infected roots below
a depth of one meter are not affected by methyl-bromide.
The use of Enzone (sodium tetrathiocarbonate or STTC) for control of
Armillaria root disease in grapevines was found to be ineffective in
limited experimental trials (Doug Gubler, Dept. of Plant Pathology, University
of California, Davis, personal communication), although some control was
achieved in almonds (Adaskaveg et al., 1999). We are currently testing its
efficacy in pears.
Enzone is either applied as a soil drench or injected into the soil near the
root collar of infected trees on a yearly basis as a preventative measure.
Enzone (and other chemical eradicants) must kill all inoculum to provide
permanent control. If inoculum remains, root collar applications of Enzone may
be required throughout the vines life.
Cultural controls are more promising for long-term control of
Armillaria than chemical controls, especially those that decrease soil
moisture at the base of the vine. If Armillaria is restricted to the
ends of vine roots, a healthy plant can make new roots to compensate for those
destroyed by infection. However, once infection surrounds the root collar, the
plant is girdled and will likely die.
Moving drip-line emitters to areas between vines as soon as possible after
planting may keep Armillaria away from the root collar. We have seen
severe cases of root disease in vineyards where emitters were left directly at
the base of vines over two years after planting.
In the absence of excessive moisture, many plants can restrict Armillaria
infections to tolerable levels. Living native trees bordering an orchard or
vineyard, even though they may be infected, contribute little in the way of
inoculum. Once they are cut, however, root wood is quickly converted to
One of the worst cases of Armillaria we studied was in a vineyard with a
freshly-cut laurel stump two meters from its edge. Vines directly adjacent to
this stump began showing symptoms only five years after planting. Now a cluster
of dead vines extends 20 meters into the vineyard radiating from the edge
nearest the stump. After the tree was cut, its root system became one huge
piece of inoculum.
Mycorrhizal fungi do not protect grapevine roots from Armillaria root
disease. These fungi infect root hairs, while Armillaria infects woody
roots. Mycorrhizae do contribute to the overall health of a grapevine though,
and healthy vines are less likely to die from root disease than stressed
Root-collar excavation is a common practice used to control Armillaria
root disease on infected ornamental and landscape trees. It often extends the
trees life and causes Armillaria to die back. Biological reasons
behind this methods success are unclear and results are primarily
anecdotal,however, it may be due to several effects. Root-collar excavation
helps keep bark dry and can offset the influence of excessive moisture. It may
restrict infections to peripheral roots, prevent initial root-collar infection
by rhizomorphs, and allow infected tissues to recover
Success of root-collar excavation depends on the extent of infection when the
treatment is applied. However, it is unlikely to harm an infected vine and is
costly only in terms of time spent digging.
The best way to avoid root disease is to plant on land with no
Armillaria on it. When clearing an orchard or vineyard, look for foliar
symptoms (most obvious in late summer) and clusters of dead plants. Look for
mycelial fans (present year-round) on symptomatic plants by removing
approximately five inches of soil from around the root collar and peeling back
the bark with a knife.
Unfortunately, Armillaria is harder to detect in oak woodlands because
above-ground symptoms are rare. Mycelial fans and mushrooms do form on infected
oaks, but all native Armillaria species make these fans and mushrooms,
and they look identical.
Whether you positively identify Armillaria on a site or not, we
recommend thorough land-clearing. Absence of symptoms and mycelial fans does
not ensure absence of root disease. Infections can exist below the root collar.
Ripping the soil in several directions after clearing will bring most large
roots to the surface and intensive root-picking will likely remove most
inoculum. This should be done even if you plan to fumigate.
We are currently screening 20 different grapevine rootstocks, selected by
Andrew Walker (Department of Viticulture & Enology, University of
California, Davis), for resistance to Armillaria root disease. Based on
the biology and frequency of Armillaria in native habitats commonly
cleared for vineyard establishment in California, a management program
including careful land-clearing, use of resistant rootstocks, and cultural
controls such as those described here will likely provide longer-lasting
control of root disease than a single chemical eradicant. n
Research is funded by the USDA Viticulture Consortium and the American
Vineyard Foundation. We thank Rex Geitner, Greg Bjornstad, and Don Gehring for
access to vineyard and oak woodland study sites. We also thank Rob Gross for
helpful comments on root collar excavation.
Adaskaveg, J. E., H. Forster, L. Wade, D. F. Thompson,
and J. H. Connell. 1999. Efficacy of sodium tetrathiocarbonate and
propiconazole in managing Armillaria root rot of almond on peach
rootstock. Plant Disease 83: 240-246. Baumgartner, K., and D. M. Rizzo.
1998. Armillaria root disease in California. Proceedings of the
46th Annual Western International Forest Disease Work Conference, Reno, Nevada,
Sept. 28-Oct. 2, 1998.
Hood, I. A., D. B. Redfern, and G. A. Kile. 1991. Armillaria in planted
hosts. In: Armillaria Root Disease. Washington, D.C., United States
Department of Agriculture Forest Service Agriculture Handbook No. 691. 122-149.
Raabe, R. D. 1962. Host list of the root rot fungus, Armillaria
mellea. Hilgardia 33: 25-88.