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 30–45 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."

Last Updated: February 24, 2000