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Oak Root Fungus aka Honey Mushroom on Grape Vines
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 and other areas of the country. 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.
Disease symptoms
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 grapevine’s 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 doesn’t always
provide permanent control. Without thorough land-clearing, infected
roots below a depth of one meter are not affected by methyl-bromide.
Cultural controls
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 inoculum.
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 vines.
Root-collar excavation is a common practice used to control
Armillaria root disease on infected ornamental and landscape trees.
It often extends the tree’s life and causes Armillaria to die
back. Biological reasons behind this method’s 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.
Avoiding infection
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.