Managing Apple Replant Disease in Tasmania, Australia, Without MeBr
Managing Apple Replant Disease in Tasmania, Australia,
Without MeBr
Apple replant disease is a significant problem in Australia and is currently
held in check there with the use of methyl bromide. Apples are a major crop in
the island state of Tasmania, with 1.5 million trees producing more than 47,000
tons of apples a year.
Orchards in Tasmania experience significant levels of apple replant disease.
There 60 ha of orchard are replanted and of these, 35 ha are treated with
methyl bromide before planting. This use accounts for 3045 percent of
Australia's methyl bromide use. With the 2005 deadline fast approaching for the
methyl bromide phaseout, the need to find alternatives to methyl bromide cannot
be overstated for this industry.
Gordon S. Brown, a research fellow at the
Tasmanian Institute of Agricultural
Research, conducted studies to determine the extent and severity of apple
replant disease, the effect of the disease on fruit yield, and the role of
possible antagonists to the disease.
"The focus was on nonchemical means of overcoming apple replant
syndrome," said Brown. "In the absence of methyl bromide, it is
probable the role of biological and cultural control practices will increase in
managing replanting problems."
To determine the extent and severity of replant disease, a pot experiment
was conducted using 11 soil samples, 10 from orchards and one from a nonorchard
source. Each of the soils showed an improvement in shoot growth using soil
sterilization, with the exception of the nonorchard soil. The severity of the
disease varied among the sites, with the majority of plants showing more than a
50 percent reduction in growth due to apple replant disease.
The impact of replant disease on fruit yields was examined in two field
trials started in 1997. Foliar area was measured at the end of the first season
and fruit yield was measured in the second season. Foliar growth was reduced by
40 percent in nontreated soil, compared to soil treated with methyl bromide.
Fruit yield showed some small reduction between the untreated fields and
those treated with methyl bromide, but this amount was not statistically
significant.
Possible Causes of Apple Replant Disease
Nematodes, fungi, and bacteria were each investigated for their contribution
to apple replant disease. In a pot trial, it was determined that nematodes are
not the principal cause of replant disease in Tasmania. However, it
seems the use of nematicides had some positive effect in 40 percent of the
orchards. In four trials using fungicides, fungi were eliminated as causes of
replant disease.
Bacteria, on the other hand, seem to be a primary cause of apple replant
disease in Tasmania. In two sets of pot trials, the antibiotic streptomycin was
incorporated into the soil prior to planting. In both sets of trials, growth of
young plants significantly improved. But, streptomycin is expensive and is not
registered for any agricultural crop in Australia, so it is not a viable
alternative treatment for Tasmanian orchardists.
According to Brown, in the long term biological control, along with cultural
and chemical control practices, will be important tools in the management of
this problem. He also notes, "it is important, however, to develop a
replanting system that is cost-effective, reliable, and maximizes early returns
of high-quality fruit."
Potential Biological Controls
A comparative study of Trichopel (selected strains of Trichoderma
spp., Agrimm Technologies, New Zealand) and Vaminoc (selected strains of
Glomus, Agrimm Technologies) indicates Trichopel may help apple trees
overcome replant disease. Vaminoc, applied at planting time, did little to
overcome the disease, but it may prove more effective if applied prior to
planting, which would give it the opportunity to establish mycorrhiza
before exposure to replant disease. Field trials are currently under way
to study Trichopel and Vaminoc further.
Calcium hydroxide was also investigated as an agent to combat replant
disease, but its effects seemed to vary with soil pH levels. In two trials it
was applied to three replant soils. The first pot trial was administered on an
acidic soil (pH 4.5) where replant disease was not severe. This calcium
hydroxide application eliminated the growth-retarding effect of apple replant
disease. While the general trend of lowered growth retardation was seen in the
second trial, the results were not statistically significant. This treatment is
under further review in field studies.
The effects of mono ammonium phosphate (MAP) fertilizer was studied in pot
trials, and the data showed it increased shoot growth. Using two orchard soils,
the addition of MAP fertilizer to sterile soil resulted in a 20 percent
increase in shoot length, while in nonsterile soil, shoot length increased 120
percent. These data imply that the addition of MAP fertilizer to nonsterile
soil resulted in a more pronounced shoot growth response than that expected by
nutrients alone. Additional studies are being conducted.
In the final set of pot trials, improved irrigation techniques could not
overcome the symptoms of apple replant disease.
"A problem for the Australian apple industry," said Brown,
"is the potential size of the market for a product against replant
disease. Each year only about 100 ha of orchard are fumigated in all of
Australia (including Tasmania), and this makes the cost of registration
unattractive for chemical companies."
[January 2000 Table of Contents]
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Last Updated: February 24, 2000
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