AS YOU raise your glass during the festive season, spare a thought for the
world鈥檚 vine growers. Many vineyard owners will be looking out over their
dormant vines and wondering whether the new year will bring good fortune or doom
in the guise of a mysterious killer nicknamed the black goo which stalks young
vines.
In California the stakes are especially high. Over the past five years, vine
growers in Napa and Sonoma Valleys alone have spent more than a quarter of a
billion dollars replacing plants struck by another scourge, the root-destroying
louse Phylloxera(鈥淐alifornia鈥檚 lousy vintage鈥�, New 杏吧原创,
17 April 1993, p 27). Now these new plants are falling prey to the botanical
equivalent of arteriosclerosis. Their normally clear sap turns dark amber or
pitch black, acquiring the consistency of thick molasses and choking up vital
arteries. Death may be prolonged in the temperate climates of France, Italy, New
Zealand and South Africa, but the stresses imposed by extreme temperatures in
California and Australia can kill a seemingly healthy plant within hours.
Californian growers who have already been plunged into debt by
Phylloxera say that another disease could bring bankruptcy. Given the
reluctance of banks to lend money to vineyards with Phylloxera, and the
suspicion of wineries, the reaction of the American wine industry to a new
threat was hardly surprising. In 1993 when Lucie Morton, a Virginia-based
vineyard consultant, made the rounds of trade shows with samples of diseased
plants she was branded an alarmist and dismissed out of hand.
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As Morton visited her clients to advise them on which rootstocks to plant in
vineyards wiped out by Phylloxera, she kept coming across plants
infected with the black goo. The first signs of disease are stunted growth,
yellowing and curling leaves: symptoms which mimic aspects of other vine
problems. Such vines might be suffering from Pierce鈥檚 disease, viral infections
or even poor cultivation. But cut them transversely below the place where the
grape-bearing limbs are grafted onto the roots and a tell-tale dark goo oozes
from the small vessels known as xylem that carry water and nourishment from the
roots to the stems, leaves and fruit.
What exactly is this goo? Chemical analysis reveals that it is made up of
several phenolic compounds. 鈥淣o bacterial cells, conidia [spores] or fragments
of fungal mycelium have been observed at a magnification of 400 times,鈥� says
Glenn Friebertshauser, owner of Agri-Analyis, an agricultural laboratory based
in Davis, California. 鈥淚t is our opinion that the black droplets of ooze are the
direct response of the host vine to the presence of an as yet incompletely
identified fungal pathogen or pathogens in or near the xylem vessels.鈥�
Sudden death
Friebertshauser鈥檚 team has shown that affected vines have a lower rate of
carbohydrate mobilisation in the spring, which contributes to their stunted
growth. His researchers have also found balloon-like swellings protruding from
xylem cell walls and gradually clogging up the vessels. A small number of these
could cause a gradual decline in the vine鈥檚 health while a severe attack coupled
with an extremely hot dry day might kill a vine in a matter of hours.
Other researchers think they can pinpoint the pathogen responsible for black
goo. At the Nietvoorbij Institute for Viticulture and Oenology in Stellenbosch,
South Africa, plant pathologist J. H. Strauss Ferreira has isolated a fungus,
Phialophora parasitica, from sick vines and used it to reproduce the
disease symptoms in healthy plants. He believes the pathogen is one of the fungi
associated with Esca (also called Black Measles) a disease which may date back
2000 years to Roman times. Nevertheless, Strauss Ferreira鈥檚 fungus looks so
unusual that a new genus, Phaeoacremonium, has been created for it.
Douglas Gubler, a plant pathologist at the University of California at Davis,
is also convinced that one or more species of Phaeoacremonium are
responsible for the disease. His team is now using PCR and other genetic
analysis to determine whether the suspect pathogens identified to date might all
be the same species. But speculation about the pathogen persists. Other fungi
such as Cylindrocarpon obtusisprorium can produce black goo symptoms.
鈥淲e don鈥檛 always get the same fungus from the same symptoms so we are inclined
to be very cautious about ascribing all the disease to Phaeoacremonium,鈥�
says Ian Pascoe, a mycologist at the Institute for Horticultural Development
in Melbourne, Australia. 鈥淚 am not sure if we are dealing with a single disease
or a complex of diseases producing similar symptoms.鈥�
Another mystery is exactly how the disease spreads. 鈥淣othing factual is known
of the method of transmission,鈥� says Pascoe, 鈥渂ut in theory,
Phaeoacremonium should be splash dispersed [by rain and irrigation
sprinklers] and possibly wind dispersed.鈥� Gubler believes the disease may be
soil-borne and transmitted into vine roots, or even carried by insects. Or
perhaps the fungus is present in 鈥渕other vines鈥� which are grown to provide a
source of plant material for commercial nurseries, as Morton suggests.
Friebert-shauser has had cases where more than 99 per cent of the vines in a
block are afflicted鈥攕o whatever the transmission mechanism, it seems to be
efficient.
High stakes
There is also disagreement over the extent of infection. Without any hard
statistics, experts are forced to generalise. Strauss Ferreira, for example,
thinks the disease is common in South Africa and Pascoe describes it as
鈥渨idespread鈥� in Australia. Morton, meanwhile, believes that the majority of
vineyards planted in California since 1985 are affected to some degree. 鈥淭here鈥檚
no way to know right now but my guess is that 30 to 50 per cent of vines may be
infected, but may not show symptoms for some time.鈥�
This confusion provides some justification for those who would rather ignore
black goo. 鈥淚f more people tested sick vines, they would find it,鈥� says Mike
Porter, who manages several vineyards in California鈥檚 Sonoma and Napa Counties.
鈥淯nfortunately most sick vines are burned without testing, which is the way some
nurserymen would prefer it.鈥� Porter would like a more open discussion of the
disease, but many viticulturists resist this idea.
鈥淣obody wants to admit they have the disease,鈥� says one large Californian
vineyard owner. 鈥淎fter the Phylloxera debacle, all we need is one more
excuse for the banks not to finance us, or for wineries to reject our grapes.
There鈥檚 a lot of money at stake here.鈥� Nevertheless, each of the 13 vineyard
managers interviewed for this article said they had seen black goo in new vines
in their own or someone else鈥檚 vineyard, and they all believe the problem is
increasing.
Californian vine growers face a greater threat from black goo than anyone
else. With over 15 000 acres of replantings following Phylloxera
infestation鈥攁nd about the same again still required鈥攁nd a general
expansion of vineyard acreage, nurseries are under pressure. Gubler and others
say replacement vines may be less resistant to the fungus, in part because a
shortage of new vines means that plants that would previously have been rejected
are now being used. 鈥淲e鈥檙e asking more of vines than ever before: they are being
planted later in the season and have stretched the usual nursery production
cycle,鈥� adds Ed Weber a farm adviser from Napa County. In addition, most new
vineyards are being planted with up to twice as many vines per acre as older
vineyards, which may help the pathogen to spread.
Researchers may not agree about the origins, severity or spread of the
disease, but one thing is certain鈥攁ny of the proposed transmission
mechanisms gives black goo the potential for a geometric increase that could
prove as devastating as Phylloxera and result in a multibillion-dollar
debacle for the global wine industry.
Wine lovers can only hope that techniques to prevent or treat the disease are
developed soon. If not, by next year your Christmas tipple could be considerably
more expensive.
Lucie Morton is organising the International Council on Grapevine Trunk
Diseases to be held in California next summer. She can be contacted by e-mail
at: mortonvit@aol.com.